# Discovery of a mid-infrared protostellar outburst of exceptional amplitude

P. W. Lucas1, J. Elias2, S. Points3, Z. Guo1, L.C. Smith4, B. Stecklum5, E.
Vorobyov6,7,8, C. Morris1, J. Borissova9,10, R. Kurtev9,10, C. Contreras
Peña11, N. Medina9,10, D. Minniti12,13, V.D. Ivanov14, R.K. Saito15
1Centre for Astrophysics, University of Hertfordshire, College Lane, Hatfield,
AL10 9AB, UK
2SOAR Telescope/NSF s NOIRLab, Casilla 603, La Serena, Chile
3Cerro Tololo Inter-American Observatory/NSF s NOIRLab, Casilla 603, La
Serena, Chile
4Institute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3
0HA, UK
5Thüringer Landessternwarte Tautenburg, Sternwarte 5, D-07778 Tautenburg,
Germany
6University of Vienna, Department of Astrophysics, Vienna 1180, Austria
7Ural Federal University, 51 Lenin Str., 620051 Ekaterinburg, Russia
8Research Institute of Physics, Southern Federal University, Rostov-on-Don,
344090, Russia
9Instituto de Física y Astronomía, Universidad de Valparaíso, ave. Gran
Bretaña, 1111, Casilla 5030, Valparaíso, Chile
10Millennium Institute of Astrophysics, Av. Vicuna Mackenna 4860, 782-0436,
Macul, Santiago, Chile
11School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
12Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha
700, Las Condes, Santiago, Chile
13Vatican Observatory, V00120 Vatican City State, Italy
14European Southern Observatory, Karl-Schwarszchild-Str 2, D-85748 Garching
bei Muenchen, Germany
15Departamento de Física, Universidade Federal de Santa Catarina, Trindade
88040-900, Florianopolis, SC, Brazil

(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We report the discovery of a mid-infrared outburst in a Young Stellar Object
(YSO) with an amplitude close to 8 mag at $\lambda\approx 4.6~{}\mu$m. WISEA
J142238.82-611553.7 is one of 23 highly variable Wide-field Infrared Survey
Explorer (WISE) sources discovered in a search of Infrared Dark Clouds
(IRDCs). It lies within the small IRDC G313.671-0.309 ($d$$\approx$2.6 kpc),
seen by the Herschel/Hi-Gal survey as a compact, massive cloud core that may
have been measurably warmed by the event. Pre-outburst data from Spitzer in
2004 suggest it is a class I YSO, a view supported by observation of weak 2.12
$\mu$m $H_{2}$ emission in an otherwise featureless red continuum spectrum in
2019 (6 mag below the peak in $K_{s}$). Spitzer, WISE and VISTA Variables in
the Via Lactea (VVV) data show that the outburst began by 2006 and has a
duration $>13$ yr, with a fairly flat peak from 2010–2014. The low pre-
outburst luminosity implies a low mass progenitor. The outburst luminosity of
a few $\times 10^{2}$ L⊙ is consistent with an accretion rate $\dot{M}\approx
10^{-4}$ M⊙yr-1, comparable to a classical FU Orionis event. The 4.6 $\mu$m
peak in 2010 implies $T$ = 800-1000 K and a disc radial location $R\approx
4.5$ au for the emitting region. The colour evolution suggests subsequent
progression outward. The apparent absence of the hotter matter expected in
thermal instability or MRI models may be due to complete obscuration of the
innermost disc, e.g. by an edge-on disc view. Alternatively, disc
fragmentation/infalling fragment models might more naturally explain a mid-
infrared peak, though this is not yet clear.

###### keywords:

stars: pre-main sequence, stars: protostars, stars: variables: T Tauri, Herbig
Ae/Be, infrared: stars

††pubyear: 2015††pagerange: Discovery of a mid-infrared protostellar outburst
of exceptional amplitude–A

##  1 Introduction

Eruptive variable YSOs are newborn star sytems wherein a sudden dramatic
increase in the accretion rate causes a large increase in flux, with a
duration ranging from a few months up to a century (Herbig, 1977, 1989;
Hartmann & Kenyon, 1996; Audard et al., 2014; Contreras Peña et al., 2017a).
The prototypical outburst was that of FU Ori (Herbig, 1977) with a rise of 6
mag in the visible, thought to be associated with a thousand-fold increase in
accretion rate for reasons not yet established (Hartmann & Kenyon 1996). For a
long time, this was the largest photometric outburst known but optical
variations of 7 mag were seen in the atypical “hot FUor" PTF14jg (Hillenbrand
et al., 2019), identified by the Palomar Transient Factory (Law et al., 2009).
An initial 1 mag dip was followed by a 7 mag rise over 150 days. Detection of
such very high amplitude events may make it possible to rule out some of the
physical mechanisms that have been proposed to explain them. There may however
be more than one such mechanism, leading to events with different amplitudes
and durations.

Traditionally, most high amplitude outbursts were detected in visible light
but there has been a recent trend towards detection of events in embedded YSOs
in the near infrared (Contreras Peña et al., 2017a), in the submm/mm continuum
(Safron et al., 2015; Hunter et al., 2017) and via methanol maser emission
(Caratti o Garatti et al., 2017).

In this paper we report the mid-infrared detection of a YSO outburst with a
rise of $\sim$8 mag, higher than any previously known. This was a somewhat
serendipitous discovery, using data from the WISE satellite (Wright et al.,
2010), with pre-outburst data from the Galactic Legacy Infrared Mid-Plane
Survey Extraordinaire (GLIMPSE, Benjamin et al., 2003), by the Spitzer Space
Telescope, (Werner et al., 2004; Gehrz et al., 2007). It followed from our
investigation of the unrelated infrared transient VVV-WIT-01 (Lucas et al.,
2020), first detected by the VVV survey (Minniti et al., 2010). However, 22
additional high amplitude mid-infrared variables were found in the same search
(see section 2.1).

While there have been past mid-infrared investigations of YSO variability (see
e.g. Rebull et al., 2014; Contreras Peña et al., 2020) there have been only a
few previous discoveries of outbursts in this waveband. Scholz, Froebrich &
Wood (2013) found two likely eruptive YSOs (and a few less likely candidates)
by comparing GLIMPSE and WISE data for known YSO candidates. Antoniucci et al.
(2014) found a few candidate short duration outbursts of the EX Lupi type (see
Herbig, 1977), often referred to as EXors. by a similar comparison. These
previous mid-infrared discoveries all had amplitudes $\la$2 mag. By contrast,
most of our discoveries have mid-infrared amplitudes $\ga$2 mag. These higher
amplitude findings demonstrate the potential of the ongoing WISE/NEOWISE
mission for eruptive YSO science and the mid-infrared variable sky more
generally. Comparable high amplitude mid-infrared YSO outbursts have
previously been detected only after discovery in another waveband. E.g. in the
optically detected case of Gaia 17bpi (Hillenbrand et al., 2018), an optical
and mid-infrared outburst was found to have been preceded by an earlier mid-
infrared event with no optical counterpart. Stecklum (2020) recently
discovered an outbursting YSO in WISE data by following up a report of a new
optical nebula (Borisov & Denisenko, 2020).

The layout of this paper is as follows. In section 2 we describe our search
for highly variable sources in the WISE data and briefly summarise the
discoveries. In section 3 we present details of the extreme outbursting YSO,
including multi-wavelength public survey data and follow-up spectroscopy. We
then discuss what we can learn from the event in section 4, before giving our
conclusions in section 5.

##  2 Variable star search

###  2.1 Method

We searched the WISE multi-epoch photometric catalogues for highly variable
sources and transients projected within two arcminutes of any of the 7139
IRDCs listed by Peretto et al. (2016) as bona fide dense molecular clouds.
These IRDCs showed evidence for 250 $\mu$m emission in their automated
analysis of images from the Herschel/Hi-Gal survey (Molinari et al., 2010).
All the IRDCs are located in the Spitzer/GLIMPSE fields at Galactic lattitudes
$|b|<1^{\circ}$ and longitudes $295^{\circ}<l<350^{\circ}$ or
$10^{\circ}<l<65^{\circ}$.

The search was undertaken during an investigation of a very red transient,
VVV-WIT-01, which was projected against an IRDC from the list of Peretto &
Fuller (2009), see Lucas et al. (2020). The covering fraction of IRDCs in the
GLIMPSE region is only 1–2% (depending on whether unconfirmed IRDCs are
included) so it was thought that VVV-WIT-01 might be a pre-main sequence
event. A search for additional transients in IRDCs therefore seemed logical,
also offering the chance to detect eruptive YSOs and other high amplitude
variable stars. It would be preferable to search the available WISE datasets
more fully (e.g. using the unWISE images, Lang 2014; Meisner, Lang & Schlegel
2018; Schlafly, Meisner & Green 2019) but this would be a computationally
demanding task. The two arcminute search radius was chosen because it provides
high completeness for any sources inside IRDCs (98% of the 7139 confirmed
IRDCs have a smaller equivalent radius). There are typically many WISE sources
within a two arcminute radius in the Galactic plane, even at the depth of
individual WISE scans. The total area searched is $\sim$2 deg2 which is only
1% of the GLIMPSE area. As part of its all-sky survey, the WISE satellite
initially observed the GLIMPSE fields in 2010. The satellite was later
reactivated as the NEOWISE mission and it observed the GLIMPSE fields from
2014 onward.

To construct a time domain catalogue in the WISE $W1$ (3.3 $\mu$m) and $W2$
(4.6 $\mu$m) passbands, we downloaded from IPAC/IRSA all catalogue entries in
the WISE All-Sky Single Exposure (L1b) Source Table located within the 7139
areas. We performed similar downloads from the WISE 3-Band Cryo Single
Exposure (L1b) Source Table, the WISE Post-Cryo Single Exposure (L1b) Source
Table and the NEOWISE-R Single Exposure (L1b) Source Table. Together these
comprise data taken throughout 2010 and in the period from 2013 December to
2017 December. Since the WISE satellite scans the full sky every six months,
each IRDC area typically has 10 epochs of observation (though more epochs have
been added to the NEOWISE-R database since our download). Each epoch is
subdivided into several individual scans made over a $\sim$1 day interval, for
which photometry is recorded separately in the L1b source tables listed above.

We used the stilts software (Taylor, 2006) to perform initial matching of the
catalogue entries, grouping them into time sequences for individual sources
with a matching radius of 2″ (about a third of the WISE beam width).
Duplicates of catalogue entries that were associated with multiple adjacent
IRDCs were removed. Using a custom-written code, the per-scan magnitudes for
detections in the $W1$ and $W2$ passbands and all the other associated
catalogue parameters were then averaged to give values for each epoch. Then,
starting with the epoch-averaged data for the 4-year NEOWISE-R dataset, the
code cross-matched the sources associated with each IRDC against the epoch-
averaged data for the same IRDC that were drawn from the three WISE datasets
covering calendar year 2010. This provided a reasonably complete time domain
catalogue for variable stars. To improve completeness for transients, we
repeated the cross-matching process using each of the three 2010 WISE datasets
in turn to provide the initial source list.

In total, the resulting time domain catalogue contains a little over half a
million sources. We investigated only the most promising high amplitude
candidates. To search for variable stars, we selected sources that varied by
at least two magnitudes (maximum minus minimum) in at least one of the two
passbands ($W1$ and $W2$). (We later applied saturation corrections, see
section 2.2, that reduced the amplitudes below 2 mag for a few of the brighter
discoveries.) We then set quality criteria, to be satisfied in at least one
passband, using parameters in the WISE L1b source tables. The criteria were
designed to select unblended sources that were relatively bright and detected
with a signal to noise ratio, $snr>5$ at one or more epochs. An additional
criterion related to the extent to which each source was well fitted by a
point source model, based on relations between the fit quality parameter,
$chi$, and signal to noise ratio, $snr$, as described in Koenig & Leisawitz
(2014). For this last criterion we averaged the $chi$ parameter in each
passband over all epochs with a detection and used the maximum value of the
$snr$ parameter (after averaging these parameters over the scans within each
separate epoch) in order to not miss any bona fide detections.

The quality criteria can be written as: $\overline{NB}<1.1$ (i.e. the number
of deblended sources contributing to the detection is $<1.1$, after averaging
over all epochs) and the following conditions for the two passbands,

$\displaystyle
w1mpro<9~{}\mathrm{AND}~{}w1snr>5~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}\mathrm{at~{}one~{}or~{}more~{}epochs}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}$
$\displaystyle\overline{w1chi}<5~{}\mathrm{OR}~{}\overline{w1chi}<\frac{\left(\mathrm{max}\left(w1snr\right)-3\right)}{7}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}$
$\displaystyle
w2mpro<9~{}\mathrm{AND}~{}w2snr>5~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}\mathrm{at~{}one~{}or~{}more~{}epochs}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}$
$\displaystyle\overline{w2chi}<5~{}\mathrm{OR}~{}\overline{w2chi}<\left(0.1~{}\mathrm{max}\left(w2snr\right)-0.3\right)~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}~{}$

Here $w1mpro$ and $w2mpro$ are the $W1$ and $W2$ magnitudes, $w1snr$ and
$w2snr$ are the signal to noise ratios and $w1chi$ and $w2chi$ are the $\chi$
parameters, in the indicated passband.

This selection yielded 177 candidates, all of which were visually inspected
using the multi-epoch images of the individual scans available at IPAC/IRSA.
Of these, 14 were confirmed as bona fide high amplitude variable stars. A
further nine bona fide variables were found while testing different quality
selections. These nine failed the quality cuts on $\overline{NB}$ or the mean
(w1/w2)chi parameters due to poor data quality at one or two epochs. The 23
real high amplitude variable stars are listed in Table 1. This search is not
100% complete but merely a first exploration.

To search for transients we required similar quality criteria to be satisfied
in at least one passband. We selected sources detected at only one epoch for
which the number of deblended sources contributing to the detection, $NB$,
satisfies $NB<1.5$. We applied essentially the same conditions on magnitude,
$chi$ and $snr$ as above. (There was no need to average or maximise $NB$,
$chi$ and $snr$ over multiple epochs, for these single epoch detections,
though these quantities are averaged over the individual scans.)

This selection yielded 104 transient candidates, of which only one, VVV-
WIT-01, passed visual inspection. This transient search is likely to be more
complete than our variable star search, being less affected by data quality
issues that affect only one or two epochs.

###  2.2 Summary of variable star findings

In Table 1 we give the coordinates and mean $W1$ and $W2$ magnitudes of the 23
variable stars, together with the amplitude in each passband (maximum minus
minimum, after applying saturation corrections if needed111For saturation
correction in $W1$ and $W2$ we used the tables at
http://wise2.ipac.caltech.edu/docs/release/neowise/expsup/sec2_1civa.html). We
also give an initial classification of each star as either a YSO or a
candidate pulsating Asymptotic Giant Branch (AGB) star, based mainly on the
WISE light curves and the VVV/VVVX222VVV (Minniti et al., 2010; Saito et al.,
2012) is a near infrared time domain survey of the Galactic bulge and the
adjacent southern Galactic plane that ran from 2010–2015, using the 4-m
Visible and Infrared Telescope for Astronomy (Sutherland et al., 2015). Time
series data were taken in the $K_{s}$ bandpass. Multi-colour data was taken
for each field in the $Z$, $Y$, $J$, $H$ and $K_{s}$ filters at two epochs, in
2010 and 2015\. VVVX is the continuing extension of the VVV survey, in
duration and area (Minniti, 2016) light curves that are available for 20 of
the 23. The candidate AGB stars are Long Period Variables, with periods of
several hundred days. The very red colours (typically $1<W1-W2<4$) of the 23
variable stars imply that warm circumstellar matter is present in all cases,
consistent with either a YSO or an AGB star with a high mass loss rate. While
extinction in the IRDCs also reddens the colours, this cannot be the dominant
effect because the mid-infrared extinction law is quite shallow in the WISE
passbands (Koenig & Leisawitz, 2014) and the measured optical depth of the
IRDCs has a mean of only 0.5 at 8 $\mu$m (Peretto & Fuller, 2009). This
optical depth corresponds to a colour excess of only $E(W1-W2)=0.17$, using
the infrared extinction data from McClure (2009) and Koenig & Leisawitz
(2014).

To search for periodic variability we included the additional time series data
added recently to the NEOWISE-R database for calendar years 2018 and 2019, so
that the full WISE time series covers 2010 and 2014-2019. A linear trend was
then subtracted from the time series. (Slow trends are often present in mass
losing AGB variables due to changing extinction by the circumstellar shell.)
Then the IDL implementation of the Lomb-Scargle normalized periodogram
(Scargle, 1982) was used to determine the frequency of the peak frequency.
Finally, a least-squares sine fit was used to refine it and provide amplitude
and phase information (not shown). The peak to trough amplitudes of the sine
fits to periodic sources range from 0.74 to 2.12 mag in $W1$ and similar
values in $W2$. We find that 13/23 stars are YSO candidates and 10/23 are AGB
star candidates, the latter group defined by the detection of a long period
and supported by their red colours. It is evident that the WISE mission
provides a trove of valuable data for detection of highly variable

Table 1: Visually confirmed high amplitude WISE variable stars in IRDCs No. | Name | RAa | Deca | $\overline{W1}$ | $\overline{W2}$ | $\Delta W1$ | $\Delta W2$ | Type | Period (d) | Notes and other identifications
---|---|---|---|---|---|---|---|---|---|---
1 | WISEA J134444.02$-$623127.4 | 206.1835 | -62.5244 | 9.39 | 6.51 | 2.32 | 2.22 | AGB | 634.1$\pm$3.1 | [RMB2008] G309.0355-00.2858
2 | WISEA J142238.82$-$611553.7 | 215.6620 | -61.2650 | 8.05 | 5.80 | 1.87 | 1.54 | YSO |  | Main subject of this work
3 | WISE J142345.85$-$612540.7b | 215.9411 | -61.4282 | 13.01 | 11.44 | 3.74 | 3.67 | YSO |  |
4 | WISEA J154914.33$-$543423.6 | 237.3096 | -54.5733 | 11.41 | 9.53 | 2.86 | 2.18 | YSO |  |
5 | WISEA J163957.05$-$462614.2 | 249.9877 | -46.4374 | 8.69 | 6.10 | 1.88 | 1.76 | AGB | 639.7$\pm$5.0 |
6 | WISEA J165035.47$-$444959.5 | 252.6479 | -44.8332 | 10.02 | 6.91 | 2.47 | 1.87 | AGB | 689.4$\pm$3.6 | [RMB2008] G340.7273-00.2234
7 | WISEA J165250.41$-$443908.4 | 253.2103 | -44.6524 | 10.59 | 7.02 | 1.83 | 1.85 | AGB | 760.9$\pm$6.4 | [RMB2008] G341.1209-00.4163
8 | WISEA J165344.39$-$432819.2 | 253.4351 | -43.4720 | 12.96 | 10.51 | 5.26 | 3.28 | YSO |  | VVVv746,
|  |  |  |  |  |  |  |  |  | [RMB2008] G342.1371+00.2054
9 | WISEA J170547.35$-$411307.5 | 256.4473 | -41.2187 | 12.57 | 9.79 | 3.21 | 2.26 | YSO |  |
10 | WISEA J171910.90$-$390226.9 | 259.7952 | -39.0409 | 12.05 | 9.83 | 3.28 | 2.16 | YSO |  | VVVv422
11 | WISEA J172258.05$-$370309.6 | 260.7421 | -37.0526 | 7.95 | 6.64 | 1.99 | 1.50 | AGB | 514.8$\pm$3.4 | IRAS 17195-3700
12 | WISEA J181041.21$-$191040.2 | 272.6718 | -19.1778 | 11.09 | 9.34 | 2.97 | 2.31 | AGB | 371.6$\pm$4.8 | [RMB2008] G011.3064-00.0637
13 | WISEA J181426.60$-$172921.9 | 273.6110 | -17.4894 | 8.59 | 6.34 | 1.66 | 1.95 | YSO |  | [RMB2008] G013.2154-00.0350,
|  |  |  |  |  |  |  |  |  | YSO candidate in Marton et al.(2016).
14 | WISEA J181704.22$-$162554.0 | 274.2676 | -16.4318 | 9.72 | 6.69 | 2.01 | 1.80 | AGB | 891.0$\pm$7.5 |
15 | WISEA J181725.67$-$170211.7 | 274.3572 | -17.0366 | 12.24 | 8.61 | 3.25 | 2.09 | YSO |  | [RMB2008] G013.9529-00.4460
16 | WISEA J181736.79$-$165006.2 | 274.4031 | -16.8351 | 8.37 | 7.00 | 2.50 | 1.73 | AGB | 501.7$\pm$7.3 |
17 | WISEA J181832.84$-$133239.3 | 274.6368 | -13.5443 | 11.54 | 9.23 | 2.61 | 2.60 | YSO |  | [RMB2008] G017.1562+00.9715
18 | WISEA J181849.10$-$140818.3 | 274.7048 | -14.1384 | 7.93 | 5.81 | 1.42 | 1.59 | AGB | 235.0$\pm$0.6 | [RMB2008] G016.6638+00.6324
19 | WISEA J182025.44$-$163608.8 | 275.1059 | -16.6024 | 10.61 | 9.43 | 2.60 | 2.28 | YSO |  | [RMB2008] G014.6746-00.8724
20 | WISEA J182712.94$-$124904.8 | 276.8040 | -12.8180 | 7.57 | 5.89 | 1.33 | 1.62 | AGB | 788.8$\pm$8.3 | [RMB2008] G018.7877-00.5509
21 | WISEA J185720.27+015711.8 | 284.3344 | 1.9534 | 7.88 | 6.96 | 3.47 | 3.23 | YSO |  | [RMB2008] G035.3429-00.4212,
|  |  |  |  |  |  |  |  |  | 4.6 mag fainter in GLIMPSE.
22 | WISEA J190424.69+054106.8 | 286.1031 | 5.6853 | 10.03 | 8.59 | 2.03 | 2.01 | YSO |  | Source 245 in Lucas et al.(2017),
|  |  |  |  |  |  |  |  |  | YSO candidate in Marton et al.(2016)
23 | WISEA J195146.18+272458.7 | 297.9423 | 27.4163 | 9.59 | 7.43 | 2.04 | 1.20 | YSO |  | [RMB2008] G063.9380+00.2509,
|  |  |  |  |  |  |  |  |  | YSO candidate in Marton et al.(2016).
Notes
$a:$ The coordinates of source 21 given in decimal degrees are taken from
UKIDSS, in order to distinguish this star from an optically brighter neighbour
(see
main text). All other sources have WISE-based coordinates from our time domain
catalogue. All are equinox J2000 values.
$b:$ Source 3 is not included in the AllWISE catalogue so the name is taken
from the WISE All-Sky release.

Figure 1: Multiwavelength light curves for WISE 1422-6115. The outburst
appears to have begun between the GLIMPSE epoch in March 2004 and the MIPSGAL
epoch in April 2006 (see main text). In 2019 the 3.4 $\mu$m, 4.6 $\mu$m and
$K_{s}$ magnitudes remain brighter than pre-outburst measurements and the
2MASS upper limit, implying that the outburst duration exceeds 13 yr. The 3.5
yr gap in the WISE data is due to the inactive period of the satellite between
the initial WISE mission in 2010 and the subsequent NEOWISE mission.

YSOs in the Galactic plane. All the YSO candidates lie in the “YSO region" of
the $W1-W2$ vs. $W3-W4$ diagram that distinguishes YSOs from AGB stars, as
defined separately by Koenig & Leisawitz (2014) and Lucas et al. (2017), with
the exception of source 2 in Table 1, see section 3.1. (We used the WISE All-
Sky database to provide contemporaneous colours for this assessment).

YSOs dominate the Galactic population of high amplitude near infrared variable
stars (Contreras Peña et al., 2014, 2017a; Lucas et al., 2017). Given that our
search was focussed on IRDCs we would therefore expect most of the 23 variable
stars in Table 1 to be YSOs. However, this is a bright sample, which causes
the proportion of AGB stars to be relatively high. It is sometimes difficult
to distinguish them from YSOs with periodic variability (see Contreras Peña et
al., 2017a, Guo et al., in prep.) but periodic YSOs are usually fainter.

Many of the sources were previously identified as YSO candidates in the
infrared colour and SED-based searches of Robitaille et al. (2008, sources
designated “RMB2008"), Marton et al. (2016) or by the time domain searches of
(Contreras Peña et al., 2017a; Lucas et al., 2017) using VVV and the UKIDSS
Galactic Plane Survey (Lawrence et al., 2007; Lucas et al., 2008). Sources 13
and 23 in Table 1 were listed as possible AGB stars by Robitaille et al.
(2008) but classified as YSOs by Marton et al. (2016). With the benefit of
time domain data, we prefer the YSO classification for these aperiodic
sources.

Source 2 in Table 1 is the main subject of this paper due to the $\sim$8 mag
difference between the Spitzer/GLIMPSE and WISE magnitudes (see section 3.1).
However, one other star also showed a large change between the two surveys:
source 21 brightened by 4.6 mag at 3.4 $\mu$m and 4.2 mag at 4.6 $\mu$m
between 2004 and 2014, after transforming the Spitzer photometry to the WISE
passbands with the photometric transformations derived for YSOs in Antoniucci
et al. (2014). Most of this change occurred between 2010 and 2014 and the star
has remained in a bright state since then. This object (SIMBAD identification
[RMB2008] G035.3429-00.4212) was listed as a YSO candidate in Robitaille et
al. (2008). In WISE, GLIMPSE and 2MASS (Skrutskie et al., 2006) it is blended
with an unrelated blue foreground star (which was listed in the catalogue of
Vioque et al. 2020 as a low-probability YSO candidate due to this source
confusion). The two stars are clearly resolved in UKIDSS $K$-bandpass images
from 2006, with a 0.9″ separation and similar brightness. They are not
resolved in the second epoch UKIDSS $K$ images from 2011 because source 21 had
become much brighter.

Separately, source 8, an eruptive YSO candidate listed previously by Contreras
Peña et al. (2017a) as VVVv746, showed a $\sim$5.3 mag variation in $W1$
within the WISE data. This will be discussed in a forthcoming paper (Lucas et
al., in prep., as a member of the very high amplitude VVV YSO sample). The
$\sim$5 mag infrared variations of sources 8 and 21 are towards the high end
of what has been reported in classical FU Orionis events (FUors) and EXors
(usually at optical wavelengths). We do not discuss these sources further
here.

Table 2: Time series photometry for WISE 1422-6115 MJD | $W1$ | $\sigma_{W1}$ | $W2$ | $\sigma_{W2}$ | $W1-W2$ | $\sigma_{\mathrm{col}}$
---|---|---|---|---|---|---
53075.5 | 14.41 | 0.13 | 13.08 | 0.15 | 1.33 | 0.20
55243.6 | 7.38 | 0.04 | 5.70 | 0.08 | 1.86 | 0.09
55422.2 | 7.30 | 0.04 | 5.52 | 0.13 | 1.78 | 0.14
56705.7 | 7.16 | 0.08 | 5.07 | 0.15 | 2.09 | 0.17
56886.9 | 7.58 | 0.07 | 5.37 | 0.14 | 2.21 | 0.16
57070.4 | 7.98 | 0.03 | 5.42 | 0.14 | 2.56 | 0.14
57245.7 | 8.23 | 0.03 | 5.75 | 0.11 | 2.48 | 0.11
57434.6 | 8.44 | 0.03 | 5.97 | 0.10 | 2.47 | 0.10
57604.7 | 8.62 | 0.03 | 6.24 | 0.08 | 2.38 | 0.09
57798.8 | 8.73 | 0.03 | 6.29 | 0.08 | 2.44 | 0.09
57967.3 | 8.97 | 0.03 | 6.55 | 0.07 | 2.42 | 0.08
58165.9 | 9.18 | 0.01 | 6.69 | 0.06 | 2.49 | 0.06
58328.5 | 9.44 | 0.01 | 6.93 | 0.05 | 2.51 | 0.05
58530.1 | 9.65 | 0.01 | 7.07 | 0.04 | 2.58 | 0.04
58692.7 | 9.94 | 0.01 | 7.31 | 0.04 | 2.63 | 0.04
MJD | $W3$ | $\sigma_{W3}$ | $W4$ | $\sigma_{W4}$ | $W3-W4$ | $\sigma_{\mathrm{col}}$
55243.6 | 4.35 | 0.01 | 3.01 | 0.02 | 1.34 | 0.02
55422.2 | 4.24 | 0.03 |  |  |  |
Note. Data in italics are from GLIMPSE, in $I1$ and $I2$. The
final column gives the uncertainty on the colour.

##  3 WISEA J142238.82-611553.7

###  3.1 Discovery and light curve

Figure 2: VVV near infrared Images WISE 1422-6115. All panels have the same
spatial scale and equatorial orientation. The upper left panel is a three-
colour image (red is $K_{s}$, green is $H$ and blue is $J$). This image taken
in 2010, just below maximum brightness, shows the bright central flux peak and
the faint cometary nebula extending to the southeast. The other five panels
are false-colour images in the $K_{s}$ and $H$ passbands, scaled to show the
changing morphology as the outburst faded (see main text). The $K_{s}$ and $H$
images use a colour map that emphasises faint features (see the vertical
colour bars between the the centre and right panels). The images have a linear
scaling from the sky background level to approximately the brightest count
level in each panel.

Source 2 in Table 1 was picked out as an object of special interest after
cross-matching the 23 variable stars with the Spitzer/GLIMPSE and GLIMPSE-II
Point Source Archives (Churchwell et al., 2009). Most sources have similar
brightness (within 1 mag) in the $I2$ (4.5 $\mu$m) passband of the Infrared
Array Camera (IRAC, Fazio et al. 2004) and the W2 (4.6 $\mu$m) passband but
source 2 (=WISEA J142238.82-611553.7, hereafter WISE 1422-6115) was brighter
by 8.0 mag at the brightest WISE epoch (2014 February 17) than it was at the
GLIMPSE epoch (2004 March 11), see Table 2. At $\lambda\approx 3.5~{}\mu$m the
rise in brightness from 2004 to 2014 was slightly smaller at 7.3 mag, based on
a simple comparison of the $I1$ (3.6 $\mu$m) and W1 (3.4 $\mu$m) magnitudes.
If we transform the Spitzer $I1$ and $I2$ magnitudes to the WISE photometric
system using the photometric transformations derived for YSOs in Antoniucci et
al. (2014) then the $I2$ value becomes 0.20 mag brighter ($I2$=12.88) and the
$I1$ value becomes 0.44 mag fainter ($I1=14.85$). The amplitudes at 3.4 $\mu$m
and 4.6 $\mu$m are then more equal: approximately 7.7 mag and 7.8 mag
respectively.

Table 2 lists the saturation-corrected333The saturation corrections (see
section 2.2) made the brightest $W1$ and $W2$ data points become fainter by
0.28 mag and 0.47 mag, respectively. We give the error as the larger of the
uncertainty on the saturation correction and the standard error on the mean
derived from the individual scans at each epoch, the latter being an
approximation to the standard error on the median. One highly discrepant scan
was removed from the second $W3$ measurement. time series of multi-filter WISE
photometry and the Spitzer/GLIMPSE $I1$ and $I2$ data, the latter given in
italics using the original values from the GLIMPSE Point Source Archive
because we do not know the uncertainty on the photometric transformation. For
the WISE data in each passband we calculated the median magnitude of the
several scans at each epoch, to minimise the effect of occasional outlying
data points.

WISE 1422-6115 is projected within the small IRDC (SDC 313.671-0.309,
equivalent radius 8.9″) from the list of (Peretto & Fuller, 2009), at a
distance 5.7″ from the centre. This IRDC has a compact far infrared and
submillimetre counterpart in the form of a cold molecular cloud core, HIGALBM
313.6699-0.3092. The core was detected by the Herschel/Hi-Gal far infrared
survey of the Galactic plane (Molinari et al., 2010) and listed in the
catalogue of Elia et al. (2017) as having a temperature of 15.8 K and a mass
of 53 M⊙. The presence of a large mass of cold matter centred within 4″ of the
WISE source (based on the Hi-Gal 250 $\mu$m coordinates given by Molinari et
al., 2016) helps to verify that this is a pre-main sequence event rather than
an unlikely chance projection. There are 62,438 Hi-Gal sources in the ”high
reliability“ catalogue of Elia et al. (2017), widely spread across an area of
276 deg2 at $-71^{\circ}<l<67^{\circ}$, $|b|<1^{\circ}$. The probability of a
chance projection of an exceptional but unrelated event within 4″ of the
listed position any of these sources is only $9\times 10^{-4}$. This
conclusion is supported by other local star formation indicators (see section
3.5) and the YSO-like morphology and spectrum (sections 3.2 and 3.3). As noted
in section 2.2 some of the WISE colours of WISE 1422-6115 (specifically the
relatively blue $W3$-$W4$ value) resemble a mass-losing AGB star more than a
YSO, according to the WISE two-colour diagram of Koenig & Leisawitz (2014,
their figure 7). However, it lies in the YSO region of the $W1$ vs. $W1$-$W2$
diagram in that work (their figure 9) and such colour-magnitude selections are
known to be unreliable, as those authors acknowledge. More importantly, the
time domain behaviour (see Figure 1) does not resemble a pulsating AGB star.
The longest period Mira variables have periods, $P<6$ yr (Groenewegen et al.,
2020; Navarro Molina et al., 2016), typically with smooth, sinusoidal light
curves. Unbiased searches of the near infrared variable sky with UKIDSS and
VVV find that their peak to trough amplitudes very rarely exceed $\Delta
K_{s}=4$ mag (Contreras Peña et al. 2017a; Lucas et al. 2017, Lucas et al., in
prep.), whilst the mid-infrared amplitudes of the candidate AGB variables in
Table 1 do not exceed 3 mag.

In Figure 1 we plot the time series photometry and upper limits from Spitzer,
WISE and the 2.15 $\mu$m data from VVV/VVVX, 2MASS and the Deep Near Infrared
Southern Sky Survey (DENIS, Epchtein et al., 1999). These additional
photometry are given in Table 3, along with mid-infrared photometry from the
Akari all-sky survey (Ishihara et al., 2010) and in Table 6 the far
infrared/submillimetre photometry from Herschel/Hi-Gal. The 2MASS image from
2000 March 12 provides an upper limit of $K_{s}=15.0$ (determined by
inspecting the image and noting the faintest sources detected in the field in
the 2MASS Point Source Catalogue and the 2MASS Survey Point Source Reject
Table).

VVV/VVVX covers the 2010–2019 time period, consisting of 157 $K_{s}$ data
points based on VVV pawprint image stacks. WISE 1422-6115 is spatially
resolved in VVV/VVVX images, showing a cometary morphology on a scale of
several arc seconds, see Figure 2. The source was also saturated near maximum
brightness but fortunately the flux profile is strongly peaked so for
comparison with 2MASS we were able apply a simple saturation correction
procedure to measure the brightness of the central flux peak, with $\sim$0.2
mag uncertainty, see Appendix A. The brightest VVV measurements were
$K_{s}\approx 8.3$ on 2011 September 5. We also calculated the magnitude in an
8″ diameter aperture on this date (see Appendix A) to give a best estimate of
the integrated flux: the integrated $K_{s}$ magnitude is only 0.05 mag
brighter than the value quoted above. However, the cometary nebula makes a
larger contribution to the integrated $J$ and $H$ magnitudes (measured in
2010) which is relevant for SED modelling, see section 3.5.

Inspection of Figure 1 indicates that the outburst began between the GLIMPSE
epoch in March 2004 and the epoch of the far infrared Spitzer/MIPSGAL survey
of the inner Galaxy in April 2006 (Rieke et al., 2004; Carey et al., 2009).
The [8.0]-[24] colour is [8.0]-[24]=6.83 and the Robitaille et al. (2008)
catalogue of intrinsically red Spitzer sources shows that this would be an
extreme outlier for a non-variable YSO. Observation dates do not appear to be
available for the Akari fluxes but the range of possible dates (May 2006 to
August 2007) lies slightly after the MIPSGAL datum. The Akari data do not help
us to determine the date of the outburst but the magnitude [9]=5.90 is 6.67
magnitudes brighter than the GLIMPSE 8 $\mu$m measurement, verifying that the
outburst had a very high amplitude at $\lambda=8$–9 $\mu$m. In 2019 the WISE
and $K_{s}$ magnitudes remain brighter than pre-outburst GLIMPSE measurements
and the 2MASS upper limit, implying that the outburst duration exceeds 13 yr.
The amplitude of the post-outburst decline is generally greater at shorter
wavelengths. The general trend towards higher amplitude at shorter wavelengths
suggests that the amplitude in $K_{s}$ may therefore have been greater than
the $\sim$8 mag change measured at $\lambda\approx 4.6~{}\mu$m. The 2MASS
upper limit tells us only that $\Delta K_{s}>6.7$ mag.

The peak of the outburst was fairly flat from 2010–2013. However, fairly rapid
$\sim$0.4 mag changes in the $K_{s}$ magnitude of the central peak (confirmed
by multiple measurements) did occur: over a one day interval in 2013
(MJD=56374–56375) and over a 23 day interval in 2011 leading up to the maximum
observed brightness at MJD=55809.0. Variations at this level are common in
embedded YSOs due to changes in extinction or accretion rate (if accretion is
magnetically controlled, see Romanova, Kulkarni & Lovelace 2008).

###  3.2 Spatially resolved structure

Figure 2 shows that most of the extended nebulosity faded at approximately the
same rate as the central flux peak, as expected for a reflection nebula. Near
maximum brightness (in 2010–2013) the cometary nebula extended $\sim$10″ to
the southeast in $K_{s}$ and $H$. As the outburst faded in later years, the
morphology changed. A faint arc of nebulosity curving to the south of the
central source is seen in the 2015 $H$ image and the 2019 $K_{s}$ image
stack444Most of the single-band images in Figure 2 are cut-outs from VVV tile
stack images, with 80 s time on source, taken during multi-colour observing
blocks in 2010 and 2015 (Saito et al., 2012). Other VVV/VVVX observing blocks
have shallower $K_{s}$ images so the “2019” image was constructed from a stack
of the 12 (8 s) $K_{s}$ pawprint stack images taken on three separate nights
in 2019, giving a combined exposure time of 96 s., extending as far as the
faint star labelled “S3" in the 2015 $H$ image. This arc gradually becomes
more visible in the $K_{s}$ images from 2015 onwards, as the rest of the
nebula fades. Other faint stars projected within the nebula become visible as
the outburst fades, e.g. the star labelled “S2" adjacent to the central flux
peak. (This is a fairly crowded Galactic star field and most stars in the
images are probably in the foreground since they have relatively blue colours,
see section 3.5). The arc of nebulosity appears to have faded only slightly
between 2015 and 2019 and it may have been present in 2010, simply hidden by
the bright reflection nebula. Classical FUors are often associated with rings
or arcs of optical or infrared reflection nebulosity (Goodrich, 1987; Liu et
al., 2016; Takami et al., 2018). The latter two studies interpreted these
features as evidence that YSO outbursts are triggered by gravitational
instability, noting that long tail-like structures might be attributed to
ejection of a massive clump of matter during a previous outburst. Vorobyov et
al. (2020) modelled tail-like structures produced by ejected clumps or by
dynamical encounters with passing stars, see also Cuello et al. (2020), though
the spatial scale of the modelled features was an order of magnitude smaller
than that seen here.

If the arc of nebulosity lies at the rear of the cloud core, e.g. 0.15 pc
behind the central source, then a light travel delay of $\sim$1 yr might
explain why a dense filament of reflection nebulosity (for example) appears
relatively bright when the rest of the nebula has faded. Alternatively, this
arc might possibly arise in shocked gas, in a situation where shocks created
by interaction of high velocity and low velocity gas in an outflow produce UV
line emission that is then reprocessed to lower frequencies. Its greater
prominence in $H$ than in $K_{s}$ in the 2015 image suggests that it suffers
less reddening than the rest of the nebula, tending to argue against the time
delay interpretation.

The overall size of the circumstellar structure is surprising. The 2010 and
2015 $K_{s}$ images and the 2010 $H$ image show a cometary nebula of the kind
commonly seen in nearby class I YSOs when light is scattered from the walls of
a cavity in the circumstellar envelope, attributable to a molecular outflow
(Whitney & Hartmann, 1993). However, the distance of the system is $d\approx
2.6$ kpc (see section 3.5) so the $\sim$10″ extent of the nebula to the
southeast of the central source corresponds to 26 000 au (0.13 pc). This is
unrealistically large for a circumstellar envelope but not unusual for either
a molecular cloud core or a molecular outflow. The fainter extremities of the
nebula might arise from scattering by dust in an outflow (Lucas & Roche, 1997;
Chrysostomou, Lucas & Hough, 2007). Alternatively, the outer portions of the
Hi-Gal submm cloud core may be dense enough to be observable in scattered
light. The 2010 images and the 2015 $K_{s}$ image have a sudden drop in
surface brightness 4 to 4.5″ southeast of the central peak. Elia et al. (2017)
measured a beam-deconvolved radius of 4.5″ at 250 $\mu$m for the cloud core so
the drop in $K_{s}$ surface brightness at a similar radius might correspond a
drop in the density of the cloud core. The alternative is that the drop in
flux occurs at the edge of a rather large YSO envelope (10 000–12 000 au
radius). The Hi-Gal radius measurement would appear to be uncertain due to the
known issue of variation in beam-deconvolved source sizes as a function of
wavelength (Elia et al., 2017) and the fact that the size is much less than
the Herschel beam size in this case. Nonetheless, the spatial scale resembles
that of a cloud core better than a YSO envelope (see Pokhrel et al., 2018, for
an illustration of the hierarchy of size scales in star formation).

Figure 3: SOAR/ARCoIRIS spectrum of WISE 1422-6115. (Upper panel): spectrum
with the continuum fit by a cubic polynomial. (Lower panel): residual after
subtracting the continuum fit. The (1-0) S(1) emission line of $H_{2}$ is
weakly detected at 2.12 $\mu$m.

###  3.3 Spectrum

Figure 4: SED of WISE 1422-6115 and the surrounding cold cloud core (black
points in outburst, blue points pre-outburst). The VVV, WISE and Hi-Gal in-
outburst data are roughly contemporaneous, all taken in 2010, when the
outburst was slightly below the peak. We also include 870 $\mu$m ATLASGAL
photometry from 2008. The form of this double peaked SED does not resemble the
SED of a normal embedded YSO: the near to mid-infrared peak is most likely due
to a dramatic episodic accretion event in the inner disc whilst the far
infrared/submm peak is due to emission from a massive cold cloud core on a
larger spatial scale. The latter peak is probably enhanced a little by the
warming effect of the outburst on the whole cloud core.

An infrared spectrum of WISE 1422-6115 was obtained by co-author JE on the
night of 2019 July 18/19 with the TripleSpec 4.1 infrared spectrograph on the
4-m Southern Astrophysical Research (SOAR) Telescope. TripleSpec 4.1 is a
near-infrared cross-dispersed spectrograph (Schlawin et al., 2014) providing a
spectral resolution R=3500 across the 0.97-2.5 $\mu$m wavelength region, with
a fixed slit width of 1.1 arcsec. Target acquisition with the instrument uses
a slit-viewing system operating in the J-band where WISE 1422-6115 is too
faint to detect. Consequently, the target was positioned on the slit by
offsetting from the nearby field star.

The integration time was 48 minutes on source, broken down into 16 three
minute exposures. The spectra were reduced with the Spextool 4.1.0 pipeline,
with independent reductions by co-authors SP and PWL providing essentially
identical results. The A0V star HIP 76244 was used for telluric correction.
The Spextool pipeline fits and removes the sky background in the dispersed
images without the need for nodding the telescope between two positions and
subtracting spectra. However, two separate nod positions were used in an ABBA
pattern, enabling quick subtraction during the observations to confirm target
acquisition and helping to guard against array defects.

The spectrum is shown in Figure 3, where we see a continuum that rises rapidly
with increasing wavelength. There is a weak detection of the (1-0) S(1)
emission line of H2 at wavelength $\lambda=2.1215~{}\mu$m, slightly blue-
shifted from the rest wavelength of $\lambda=2.1218~{}\mu$m. While the line is
weak (the equivalent width is $-1.0\pm 0.4$ Å) it is detected in variously
selected subsets of half of the data: position A observations, position B
observations, the first eight exposures (i.e. the first two ABBA nod cycles)
and the last eight exposures. Therefore it appears to be a genuine feature.
There is no sign of the usual spectroscopic features associated with FUor or
EXor outbursts (CO and H2O in absorption in the former type, CO emission and
Br$\gamma$ emission in the latter). This is not surprising because by 2019 the
outburst had faded by 6 mag from its peak brightness in $K_{s}$.

The detection of weak 2.12 $\mu$m $H_{2}$ emission serves to support the
interpretation of the system as a YSO because this line is typical of class I
YSOs (e.g. Greene & Lada, 1996; Doppmann et al., 2005) but rarely seen in
other types of star. Other than YSOs, this emission line is seen in
protoplanetary nebulae or other sites of shocked gas such as supernova
remnants but the former are not associated with high amplitude variability and
this was clearly not a fast transient event. Amongst YSOs, the 2.12 $\mu$m
line is much more commonly seen in class I YSOs than class II systems (Greene
& Lada, 1996). The feature typically arises in an outflow on a fairly large
spatial scale so it tends not to correlate with accretion-driven photometric
variations on timescales of only 1 or 2 years (Guo et al., 2020).

The radial velocity of the $H_{2}$ emission is estimated as
$v_{\mathrm{LSR}}=-66\pm 30$ km s-1, after applying the heliocentric
correction and correcting for the sun’s motion relative to the Local Standard
of Rest (Schönrich, Binney & Dehnen, 2010). The uncertainty includes the line
profile measurement error and the wavelength calibration uncertainty, with an
allowance for an estimated 0.25″ uncertainty in the spatial location of the
source within the 1.1″ slit. This radial velocity is broadly consistent the
system velocity of $v_{\mathrm{LSR}}\approx-40$ km s-1 that we expect, based
on our our preferred distance, $d\approx 2.6$ kpc see section 3.5). However,
we note that $H_{2}$ emission from a YSO outflow can have a velocity that is
significantly offset from the system’s reference frame (e.g. Guo et al.,
2020).

###  3.4 SED

In Figure 4, we show the SED of WISE 1422-6115 and the surrounding cold cloud
core from 1 $\mu$m to 1 mm. Blue points are the data from GLIMPSE and the
2MASS upper limit, all taken pre-outburst. Black points are based on data
taken during the bright part of outburst: VVV, WISE and Herschel/Hi-Gal data
were all taken in 2010, when VVV $K_{s}$ and WISE $W2$ fluxes were only a
factor of $\sim$2 lower than the highest values measured. The VVV and WISE
data points were taken only a month apart (February to March 2010) but the Hi-
Gal data from Elia et al. (2017), for the core designated HIGALBM
313.6699-0.3092, are an average of several observations at more separated
dates in the year. The 870 $\mu$m datum was taken in 2008, also during the
bright stage of the outburst (see Figure 1) by the 12-m Atacama Pathfinder
Experiment (APEX) Telescope (Güsten et al., 2006) as part of the APEX
Telescope Large Area Survey of the Galaxy (ATLASGAL, Schuller et al. 2009).
The Hi-Gal and ATLASGAL fluxes at $\lambda>250~{}\mu$m were scaled to the
measured size of the 250 $\mu$m detection, a procedure adopted by Elia et al.
(2017) for SED fitting for all the molecular clumps and cores in their
catalogue (see that work and references therein). The VVV $J$, $H$ and $K_{s}$
photometry shown here are integrated fluxes measured in an 8″ diameter
aperture, for better comparison with the lower resolution WISE
photometry.555The integrated VVV magnitudes ($J$=15.31, $H$=12.18,
$K_{s}$=9.11, based on data from 2010 March 19) are significantly brighter in
$J$ and $H$ than the point source magnitudes given in Table 3 ($J$=16.06,
$H$=12.65, $K_{s}$=9.17) because the cometary nebula makes a substantial
contribution up to a 4″ radius, though not beyond that.

####  3.4.1 Pre-outburst SED

The pre-outburst GLIMPSE colours suggest that WISE 1422-6115 is a class I YSO:
$I1$-$I2$=1.33, $I3$-$I4$=0.87 and $I2$-$I4$=1.51 are typical of class I YSOs
in Gutermuth et al. (2009). The $I2$-$I3$=0.64$\pm$0.23 measurement is blue
for a class I system, but the uncertainty allows it to be consistent with the
colour space adopted by those authors. These colours add to the evidence for
class I status provided by the 2.12 $\mu$m H2 emission line mentioned earlier.
Moreover, the location within a dense cloud core provides circumstantial
evidence that this is a protostar rather than a more evolved pre-main sequence
star.

####  3.4.2 In-outburst 1–22 $\mu$m SED

In outburst, the 1–22 $\mu$m SED can be fitted by a range of stage 2
(envelope-free) YSO models with the Robitaille et al. (2007) model grid
(Python sedfitter version) but the far infrared signature of any envelope that
may be present is obscured by the cloud core. The decline in the (in-outburst)
flux from 4.6 $\mu$m to 22 $\mu$m indicates that the outburst takes place in
the inner disc and suggests that radiation is not very efficiently transferred
to larger disc radii. Alternatively, there might be a gap in the disc. Using
$\lambda T=3.67$ mm K at the peak of a Planckian SED, the infrared peak at 4.6
$\mu$m implies $T\sim 800$ K. The in-outburst infrared luminosity measured in
2010 from 1.25 $\mu$m to 22 $\mu$m was $L=178(d/2.6\mathrm{~{}kpc})^{2}$ L⊙.
We calculated this very simply by the Trapezium Rule, i.e. treating the flux
in the wavelength interval between each pair of points in the SED as the
average of the two flux densities, multiplied by the wavelength difference.
Using these values for $L$ and $T$, the equation $L=2\pi R^{2}\sigma T^{4}$
yields a radius $R~{}=~{}4.6$ au for the outburst.

We might expect there to be significant infrared extinction towards the inner
disc of a class I YSO that is located inside an IRDC/dense cloud core. An
experiment with extinguished single-temperature black body fits to the 1.25–22
$\mu$m SED (using $\chi^{2}$ fitting) found that an 800 K source with zero
extinction is the best fit (with 40–60% residuals at some wavelengths) but
models with extinction up to $A(K_{s})=1.5$ mag and temperatures up to 1000 K
are only slightly worse, raising the $\chi^{2}$ parameter by only 60%. (We
used the near infrared extinction law of Cardelli et al. 1989 for the VVV data
and the mid-infrared extinction law given by Koenig & Leisawitz 2014 for WISE
data in fields with high extinction.) Adopting $T$=800–1000 K and
$0<A(K_{s})<1.5$ implies $L$=178–418 $(d/2.6\mathrm{~{}kpc})^{2}$ L⊙, from
1.25–22 $\mu$m, and the radius of the emitting region remains tightly
constrained at $R$=4.5–4.6 au. The fits to the Robitaille et al. (2007) model
grid provide some guidance here, even though these models were not designed to
accommodate YSO outbursts. In the best-fit models, the foreground extinction
is $15<A(V)<26$ mag and the disc is observed edge-on (system inclination
$i=87^{\circ}$) such that the star is entirely hidden from view by the disc
and only the disc contributes directly to the observed SED. (In these models,
the stellar photosphere has a temperature $5700<T<7600$ K, which happens to be
similar of the emitting region of the disc measured in optical spectra of
classical FUors.)

####  3.4.3 The Hi-Gal cloud core

The Hi-Gal cloud core appears very compact in all the Hi-Gal 70 to 500 $\mu$m
images (see Figure 5). Elia et al. (2017) fit a beam-deconvolved diameter of
9.09″ at 250 $\mu$m. Their grey-body SED fit to the 170 to 870 $\mu$m Hi-Gal
and ATLASGAL data gives a mass $M=53~{}M_{\odot}$, temperature $T=15.8\pm 0.2$
K and optical depth unity at wavelength $\lambda_{0}=81.5~{}\mu$m. (Their
adopted distance, 3.13 kpc, is comparable to our preferred distance of 2.6
kpc). This high mass and cold temperature make it clear that at wavelengths
$\lambda>100~{}\mu$m the SED is dominated by a cloud core rather than a
circumstellar envelope. The double-peaked SED, with the higher peak in the
near to mid-infrared, does not resemble a normal class I or class 0 YSO: we
confirmed this by trying unsuccessfully to fit it with the (Robitaille et al.,
2007) SED fitting tool. However, as discussed below (section 3.4.4), heating
by the outburst may well have increased the Hi-Gal and ATLASGAL cloud core
fluxes somewhat.

We regard the beam-deconvolved diameter of the core as uncertain since it is
much smaller than the Herschel/Hi-Gal beam at 250 $\mu$m (see 3.2). (The
published value is consistent with the size of the darkest portion of the IRDC
as it appears in the GLIMPSE $I4$ image but smaller by a factor of two than
the IRDC equivalent radius given by Peretto et al. 2016). Therefore we
performed our own grey-body fits of the 170 to 870 $\mu$m SED to check the
published parameters, in a similar manner to Ward-Thompson & Robson (1990). We
initially added the core’s solid angle on the sky, $\Omega$, and then also the
opacity power law index $\beta$ as free parameters ($\beta=2$ was adopted by
Elia et al., 2017). We found that $\beta$, $\Omega$ and $\lambda_{0}$ were not
well constrained by the data but the temperature is well constrained, with
minimum $\chi^{2}$ values corresponding to $T=15$ K for a 4 parameter fit, or
$T=16$ K if we set $\beta=1.8$. These temperatures are in close agreement with
Elia et al. (2017). Temperature is the key parameter that determines the
derived mass (when distance is known) so our mass estimates were of the same
order as that of Elia et al. (2017).

The mid-infrared decline in the SED is unusual for Hi-Gal protostellar cores
with AllWISE counterparts in the Elia et al. (2017) catalogue. Using their
definition of cores as bodies with radii $r<0.1$ pc, we found that during
outburst WISE 1422-6115 was a blue outlier with $W2-W4=2.68$, compared to
$4<W2-W4<9$ for most other cores. Unsurprisingly, it is one of the brightest
sources in $W2$ (4.6 $\mu$m).666Our cross-match to AllWISE found eight Hi-Gal
“protostellar cores” with much brighter and somewhat bluer WISE counterparts
than WISE 1422-6115 ($W2<0$, $W2-W4<2.5$). However, seven of these are listed
in the SIMBAD database as evolved stars, e.g. OH/IR stars. Only one, CRL 2136,
is a protostellar object.

####  3.4.4 Reprocessing

The higher luminosity of the shorter wavelength peak is consistent with YSO
outburst models (Johnstone et al., 2013; MacFarlane et al., 2019), wherein an
outburst in the inner disc greatly raises the near and mid-infrared flux but
has a gradually decreasing effect at longer wavelengths. In their model,
radiation is reprocessed in the circumstellar envelope (assuming there is one)
with a warming effect. However, the Rayleigh-Jeans tail of a Planckian SED
flux scales only linearly with temperature, leading to a smaller effect at
submm wavelengths. In WISE 1422-6115, the pre-outburst infrared luminosity
(blue points in Figure 4) is negligible compared to the luminosity of the
massive cold cloud core. The in-outburst infrared luminosity ($L$=178–418
$(d/2.6\mathrm{~{}kpc})^{2}$ L⊙ from 1.25–22 $\mu$m measured in 2010) exceeds
the cold core’s luminosity from 70–870 $\mu$m
($L=59(d/2.6\mathrm{~{}kpc})^{2}$ L⊙) by a factor of three to seven, perhaps
even more at the $K_{s}$ peak in 2011. The core is therefore likely to have
been heated by a few kelvins but this would not be expected to cause
significant chemical changes. The heating efficiency would depend somewhat on
whether the radiation is first reprocessed by a smaller circumstellar envelope
and re-emitted at submm wavelengths where the optical depth of the core is
low; future measurements of the core’s SED would should reveal the size of the
effect.

The 2010 luminosity measured from the 1.25 $\mu$m to 870 $\mu$m SED is
$L$=263–503$~{}(d/2.6\mathrm{~{}kpc})^{2}$ L⊙, adopting $0<A(K_{s})<1.5$. The
ratio of pre-outburst to in-outburst luminosity in the 3–5 $\mu$m region is
$\sim$850 (comparing the WISE $W1$ and $W2$ data in March 2010 to the GLIMPSE
magnitudes given in section 3.1, after transforming to the WISE photometric
system). If this flux ratio is applied to the whole SED, the implied quiescent
luminosity is only 0.3–0.6 L⊙. The ratio of far infrared to mid-infrared
luminosity during quiescence may have been an order of magnitude higher than
in 2010, assuming a class I SED, giving a quiescent luminosity of a few L⊙.
Even so, it is clear that the progenitor is a low mass YSO.

Figure 5: The wider environment. All images have equatorial orientation.
(top:) GLIMPSE 8 $\mu$m 6$\times$5 arcminute image taken pre-outburst, in
2004. WISE 1422-6115 is faintly seen, between the vertical white markers. The
IRDC (G313.671-0.309) is the adjacent small dark patch and the Planck cloud
PGCC G313.73-00.31 dominates the image as a dark region. (middle:) WISE
$16\times 12$ arcminute three colour image (blue: 3.4 $\mu$m, green: 4.6
$\mu$m, red: 12 $\mu$m) taken in 2010. The erupting YSO (marked with an arrow
and a blue circle) was prominent. (bottom:) Hi-Gal 160 $\mu$m $16\times 12$
arcminute false colour image, with a linear stretch (see colour bar below).
The IRAS 14188-6054 HII region and the cold cloud core surrounding WISE
1422-6115 are marked. Figure 6: Red clump giant extinction-based distance to
the Planck cloud PGCC G313.73-00.31. (left:) 2MASS map of sources in a 16
arcminute radius area centred on the region of highest extinction. The colour
coding according to $J-K_{s}$ colour shows that very few red stars are visible
near the centre, where the cloud is opaque. Stars are plotted with open
squares in the selected opaque (blue-ish) region, where foreground stars
predominate. The location of WISE 1422-6115 is marked with a black square.
(right:) 2MASS $K_{s}$ vs. $J-K_{s}$ CMD for the area of the left panel. Stars
in the selected opaque (blue-ish) region are plotted in red and the rest in
grey, providing a large control area. The giant branch is well defined by the
grey points but the distribution of red points in the giant branch cuts off
abruptly near $K_{s}$=10.5. We infer a red clump giant distance $d\approx 2.6$
kpc to the dark cloud and apply this distance to WISE 1422-6115 also.

###  3.5 Wider environment and distance

In Figure 5 we illustrate the star forming environment around WISE 1422-6115.
In the upper panel (a GLIMPSE 8 $\mu$m image taken pre-outburst), WISE
1422-6115 is seen as a very faint source at the edge of the IRDC
G313.671-0.309, which in turn appears as an unremarkable small patch at the
edge of a much larger dark cloud. We identify this large cloud as PGCC
G313.73-00.31 from the list of “Planck Galactic Cold Clumps" (Ade et al.,
2016). Those authors calculate distances to the molecular clouds in their list
using several methods, finding $d=2.72\pm 0.82$ kpc for PGCC G313.73-00.31.
That was an extinction-based distance derived from a comparison of the colours
and magnitudes of all stars in the region with the prediction of the Besançon
Galactic stellar population models (Robin et al., 2003, see https://model.obs-
besancon.fr). They noted that the result was consistent with a near kinematic
distance of 3.15 kpc for the IRDCs in the vicinity (Jackson et al., 2008): in
fact the latter authors find distances ranging from 2.76 to 3.15 kpc for
different components of the G313.70-00.31 IRDC, with CS (2-1) velocities
$-44.3<v_{LSR}<-39.4$ km s-1. Other authors have measured similar radial
velocities for cold cloud cores within the Planck clump (Vasyunina et al.,
2011; Purcell et al., 2012), and the former work specifies a distance
$d\approx 3.3$ kpc.

The middle and lower panels of Figure 5 give a wider field view (16 arcminute
width). The dark cloud is seen to extend further to the northeast in WISE
three colour image (the whole dark area corresponding to dark cloud no. 6082
of Dobashi 2011). WISE 1422-6115 appears bright in the WISE image and the Hi-
Gal cold cloud core (HIGALBM 313.6699-0.3092) is seen at the same location in
the lower panel.

Other bright regions in the middle and lower panels of Figure 5 correspond to
star formation activity at a kinematic distance similar to those mentioned
above. E.g. IRAS 14188-6054 is an HII region containing a 6.7 GHz methanol
maser, MMB G313.705-00.190 (Green et al., 2012), and the associated ATLASGAL
cloud core AGAL 313.706-00.191. The ATLASGAL team preferred a near kinematic
distance of $3.0^{+0.6}_{-0.5}$ kpc (Wienen et al., 2015) whereas Whitaker et
al. (2017) preferred the far kinematic distance of 8.5 kpc.

Figure 6 illustrates our own extinction-based distance calculation, a red
clump giant distance (López-Corredoira et al., 2002) based on the giant branch
as measured in the $K_{s}$ vs $J-K_{s}$ colour magnitude diagram (CMD)
constructed from the 2MASS Point Source Catalogue. (VVV data could not be used
for this because the cloud is too nearby, causing foreground stars at the
extinction discontinuity to saturate). We plotted a map of three-filter
detections ($J$, $H$, $K_{s}$), shown in the left panel of Figure 6, and found
that the dark cloud area seen in the middle panel of Figure 5 corresponds to
an opaque region where very few red stars are present. This region appears
blue in the detection map due to the predominance of foreground stars with
relatively blue colours. We traced this “blue region" by hand due to its
irregular shape (in fact it is split into two adjacent areas separated by a
narrow, slightly less opaque region populated by red stars). Stars in the
selected blue region are plotted with open squares in the detection map. In
the right panel of Figure 6, stars in the selected region are plotted as red
circles. We see that the giant branch is very sparsely populated, due to a
sudden cut-off near $K_{s}=10.5$ mag. By contrast, in the larger control
region enclosing the selected area (16 arcminute radius centred on (ra,
dec)=(215.8471, -61.1694), data plotted as small grey circles) the giant
branch is well defined and continuous. We use this larger area as a control
field to estimate the average population of red clump giants per unit
magnitude in the giant branch because smaller areas have differing levels of
extinction, leading to scatter in the results.

After scaling the area of the control field to the selected high extinction
area of the dark cloud, we adopt the red clump giant colour, absolute
magnitude and extinction law employed by Minniti et al. (2011) and we find
that the cut-off occurs between 2.3 and 3.0 kpc (i.e. apparent magnitudes
$K_{s}=10.50$ and $K_{s}=11.07$) with high confidence. At $d=3.0$ kpc, the
control field tells us that number of “missing" giant branch stars in the
selected region is $5.8\pm 2.4$ and the Poisson probability of a gap at
$10.50<K_{s}<11.07$ is $P(0)=0.3$%. We adopt a likely distance range of 2.3 to
3.0 kpc to the dark cloud, with a preferred value of 2.6 kpc. This is
consistent with kinematic distances slightly in excess of 3 kpc because
peculiar velocities of only a few km/s relative to the Galactic rotation curve
would modify the kinematic distance by several hundred pc. Inspection of the
Gaia-based distances of Bailer-Jones et al. (2018) confirms that the distance
to the dark cloud is at least 2.3 kpc. The most distant stars having parallax
measurements above 5$\sigma$ significance have a lower distance bound of c.
2.5 to 2.6 kpc in that work, at 90% confidence. If we allow for the known
negative offset of order $30$ $\mu$as in the current Gaia parallaxes
(Lindegren et al., 2018), this translates to a minimum distance of about 2.3
kpc.

##  4 Discussion

The $\sim$8 mag mid-infrared amplitude of the outburst is higher than that of
any previously recorded episodic accretion event, in any waveband. This
extreme value, together with the $\sim$1 decade duration and the mid-infrared
location of the peak in the SED provide some potentially useful constraints
regarding which disc instability mechanism is operating here. In many
previously known events, infrared amplitudes were slightly lower than optical
amplitudes but an amplitude in $K$ approaching 6 mag was seen in the embedded
V346 Nor system in 2010-11 (Kóspál et al., 2020a; Kraus et al., 2016), during
a brief dip between outbursts. Most other events in embedded YSOs have
amplitudes of 2 to 4 mag in $K$ (see Reipurth & Aspin, 2004; Persi et al.,
2007; Caratti o Garatti et al., 2011; Connelley & Reipurth, 2018), though the
eruptions of the class I YSOs OO Ser (Hodapp et al., 1996) and UKIDSS
J185318.36+012454.5 (hereafter UGPS 1853+0124, Nikoghosyan, Azatyan &
Khachatryan 2017) had amplitudes $\Delta$K=4.6 mag and $\Delta$K$\geq$4.8 mag,
respectively. With a total duration of over a decade (Kóspál et al., 2007), a
very red 1–5 $\mu$m SED and a featureless, steeply rising 2 $\mu$m spectrum,
the OO Ser event is the closest previous analogue to WISE 1422-6115, despite
some differences (see below). Previously published YSO outbursts discovered by
the VVV survey had $K_{s}$ amplitudes below $\sim$4 mag (e.g. Contreras Peña
et al., 2017a, b; Medina et al., 2018; Teixeira et al., 2018) but events with
$K_{s}$ amplitudes up to 6.7 mag have been discovered more recently (Lucas et
al., in prep.). WISE 1422-6115 appears to be an outlier but not by so much
that we need to consider it an entirely different type of event.

###  4.1 Accretion rate

The accretion rate during outburst can be estimated from the luminosity using
the equation $L$ = $GM\dot{M}/2R_{*}$ (Shakura & Sunyaev, 1973). The 1.25–22
$\mu$m luminosity was $L$ = 178–418 $(d/2.6~{}\mathrm{kpc})^{2}$ L⊙ in 2010
(section 3.4.2). The low pre-outburst luminosity indicates a low mass
progenitor but the stellar radius and mass are not known. From inspection of a
sample of 22 classical T Tauri stars with ages $<$3 Myr in Grankin (2016) we
find a mean mass of 0.65 M⊙, a mean radius of 2.04 R⊙ and mean $R/M=3.9$
R${}_{\odot}/$M⊙, with a standard deviation of 1.9 R${}_{\odot}/$M⊙. Adopting
the mean value of $R/M$, we find $\dot{M}$ = 4.4–10.3 $\times$ $10^{-5}$ M⊙
yr-1. Making some allowance for flux emitted at $\lambda>22~{}\mu$m and the
slightly brighter infrared 2.15–4.6 $\mu$m fluxes in 2011-14 would bring the
middle of this range to approximately $\dot{M}=10^{-4}$ M⊙ yr-1. A luminosity
of order $10^{2}$ L⊙ is typical of a classical FUor (Connelley & Reipurth,
2018) and the implied accretion rate is therefore also typical of the rates
inferred in the literature for such events (1–30$\times 10^{-5}$ M⊙ yr-1, Bell
et al. 1995).

###  4.2 Models and veiling

Models employing the thermal-viscous instability mechanism (Bell & Lin, 1994;
Bell et al., 1995; Lin & Papaloizou, 1985; Hartmann & Kenyon, 1985) naturally
produce a high temperature (6000 K to 10 000 K) outburst in the innermost part
of the disc (sub-au scales), leading to a lower amplitude in the infrared than
at optical wavelengths. The detailed models of Bell et al. (1995) were able to
reproduce many observed features of optically bright classical FUors,
including a lower infrared amplitude, but some thought is needed to reconcile
them with the mid-infrared outburst discussed here. The event duration can be
adjusted in these models from $\sim$100 yr to $\sim$10 yr by increasing the
adopted viscosity parameter, as noted by Kóspál et al. (2007) for the case of
OO Ser. Another class of model is the magneto-rotational instability (MRI)
models, where MRI is triggered by spiral density waves created by
gravitational instability in the outer disc, e.g. Bae et al. (2014); Kadam et
al. (2020). Again, these models lead to a temperature of several thousand K in
the disc.777In MRI models with a magnetically layered disc the temperature
might be lower (Kadam et al., 2020) but a low MRI ignition temperature was
pre-defined in these models, constraining the temperature of the outburst.

The magnetic gating model D’Angelo & Spruit (2010, 2012) involves an
instability near the co-rotation radius of the disc. For a typical YSO
rotation period of 3 days this is 0.04 au for a 1 M⊙ star, far smaller than
the radius of a few au derived in section 3.4.2. However, the scenario of
fragmentation in the outer disc, followed by inward migration to the inner
disc and infall on to the star (Vorobyov & Basu, 2006, 2010, 2015) can more
naturally explain a release of energy at a radius of a few au and a high
temperature is not necessarily required, though this is still under
investigation. In Vorobyov, Zakhozhay & Dunham (2013, see figure 7 of that
work), we see an example of a mid-infrared peak in the SED arising from
emission by a clump that has been scattered into the inner disc, though this
is not in itself a burst event.

In the thermal instability and MRI models, the innermost part of the disc
should become heated to several thousand K as the mass flux through that
region increases by orders of magnitude above the quiescent rate. Emission by
this hot matter should be observed, as is seen in classical FUors, unless it
is entirely obscured from view. A possible explanation is that the disc plane
lies close to the line of sight so that the observed mid-infrared emission
arises from reprocessed radiation further out in the disc. In this case much
of the outburst region (perhaps all of it) would be closer to the star than
the 4.6 au radius implied by the observed luminosity and the $\sim$800 K
temperature. We might expect the SED to rise from 4.6–22 $\mu$m in such a
case, as opposed to the fall that is observed, due to reprocessing further out
in the disc (Johnstone et al., 2013) but this would depend on the details of
the disc structure and any effects such as disc shadowing or disc gaps. A
scattered light peak at 4.6 $\mu$m should also be considered but it seems
unlikely, in view of the low albedo of sub-micron-sized dust grains at that
wavelength.

Another possibility, perhaps less likely, is that the optical depth of the
upper layers of the disc in the outburst region is sufficient to entirely
obscure the hotter mid-plane layers from view (despite thermal inflation of
the disc scale height) causing a shallow temperature gradient. A generally low
disc surface temperature and a shallow temperature gradient may also occur in
a new class of models where the burst is caused by an increase in turbulent
viscosity due to vertical convection and convective transfer of energy
(Pavlyuchenkov et al., 2020). A shallow temperature gradient helps to explain
the almost featureless 2 $\mu$m spectrum, though we note that it was taken 6
mag below peak 2 $\mu$m brightness so gradients are expected to be shallower,
weakening absorption features in the manner observed by Guo et al. (2020).

In comparison with most other outbursts in other embedded YSOs, WISE 1422-6115
event is redder and apparently cooler. The embedded YSO outburst events V2775
Ori, V1647 Ori and GM Cha have near infrared colours $J-K=3.7$, 3.7 and 5.2
respectively, and their infrared spectra have temperatures similar to reddened
M or L dwarf photospheres ($T$=3200 K was fitted for V2775 Ori by Caratti o
Garatti et al. 2011). The 800-1000 K temperature of WISE 1422-6115 in 2010
(section 3.4.2) is more T dwarf-like and it appears to have cooled towards the
Y dwarf range since that time (see Figure 7). With $J-K_{s}=6.9,H-K_{s}=3.5$
in outburst and a 2 $\mu$m continuum spectrum that has a positive second
derivative (see Figure 3), WISE 1422-6115 is amongst the reddest events,
alongside OO Ser, the massive YSO outburst of S255 IR NIRS3 (Caratti o Garatti
et al., 2017), UGPS 1853+0124 and V723 Car (Tapia, Roth & Persi, 2015). None
of the VVV YSO spectra in Contreras Peña et al. (2017b) have such steeply
rising 2 $\mu$m continua as WISE 1422-6115 and OO Ser. Despite the
similarities of the two, OO Ser differs from WISE 1422-6115 in that the
outburst of the former faded in a wavelength-independent manner and the
outburst luminosity was relatively low at $\sim$30 L⊙ (Kóspál et al., 2007).

We note that UGPS 1853+0124 differed from WISE 1422-6115 in that it has stayed
at peak mid-infrared brightness for at least 10 years and showed (EXor-like)
CO overtone emission bands. The class I V723 Car event (Tapia et al., 2015)
had a more complex light curve and an EXor-like spectrum, with both Br$\gamma$
and CO overtone emission features. Tapia et al. (2015) made a similar argument
for an edge-on orientation to help explain the very red colour, $H-K=4.0$ at
the peak, though very high foreground extinction also contributed. The V723
Car and UGPS 1853+0124 events may have been in relatively massive YSOs, though
this is not entirely clear.888The distance, luminosity and mass of UGPS
1853+0124 are uncertain (Nikoghosyan et al., 2017). A high mass, $M\sim$10 M⊙,
was claimed for V723 Car (Tapia et al., 2015) but the mass and luminosity
values given might be overestimated, having employed either mid-infrared data
taken during the outburst or the far IR/submm flux of a 1700 M⊙ “massive
envelope” that may be due to a self-luminous cloud core rather than
reprocessing.

Figure 7: Colour and temperature evolution. (Upper panel): the $W$1-$W$2
colour vs time. The GLIMPSE $I1$-$I2$ colour, measured pre-outburst, is
plotted as two open squares: the lower square showing the published colour and
the upper square derived after transformation to the WISE photometric system.
During outburst, the overall trend was an increasingly red colour as the
outburst first brightened slightly from 2010–2014 and then faded thereafter.
(Lower panel): best-fit temperature derived from the 2.15, 3.4 and 4.6 $\mu$m
fluxes as a function of assumed extinction, plotted at three separate epochs.
A cooling trend is apparent as the outburst faded.

###  4.3 Evolution of the event

As noted earlier, the outburst declined more slowly with increasing
wavelength, in the 1.6–4.6 $\mu$m range. In Figure 7 (upper panel) we plot the
change in $W1$-$W2$ colour vs time. The GLIMPSE $I1$-$I2$ pre-outburst colour
in 2004 (also plotted for comparison) is similar to the WISE colour measured
in 2010 if we allow for the difference between the Spitzer/IRAC and WISE
bandpasses (Antoniucci et al., 2014). However, the outburst became redder
between 2010 and the time of the brightest WISE measurements in 2014, then
reddened further as the outburst began to fade, showing a blip in 2015 but a
clear trend to the red from late 2016 to late 2019. The “redder when brighter"
change between 2010 and 2014 suggests that the outburst was propagating
outward through the disc from 2010 to 2014, cooling slightly but increasing in
luminosity as the area of the outbursting region increased. (We noted
previously that the negative gradient of the SED from 4.6–22 $\mu$m in 2010
was a sign that energy had not yet been efficiently redistributed outward in
the disc by that date). This behaviour is seen in thermal instability models
and in the MRI outburst models of Kadam et al. (2020), though not in infalling
fragment models. The data are therefore perhaps most consistent with thermal
instability or MRI models where the hot matter is veiled from view.

The behaviour as the outburst faded after 2014 can be understood as simple
cooling of the outbursting region and perhaps further outward propagation. In
the lower panel of Figure 7 we plot the results of minumum $\chi^{2}$ fits to
extinguished Planckian spectra, using the fluxes at 2.15, 3.4 and 4.6 $\mu$m.
(We interpolated the $K_{s}$ light curve using cubic spline fitting to
estimate the fluxes at the WISE epochs.) For any extinction value in the range
$0<A(K_{s})<8$ there is a temperature that gives a minimum value of
$\chi^{2}$. The best fit temperature rises slowly as a function of the adopted
extinction (shown as the curves for three separate epochs) but the $\chi^{2}$
parameter varies by only $\sim 25\%$ along each curve. Whilst the temperature
at each epoch is only loosely constrained in these three point fits, a clear
cooling trend is apparent. An isothermal fading due to increasing extinction
alone appears unlikely since it would require very large changes in
extinction.

###  4.4 The cometary nebula

Finally, the large size of the cometary reflection nebula requires some
explanation. We have interpreted it as reflection from the massive cloud core
(section 3.2) which implies that the morphology must be due to the structure
of the core. This may indicate that an outflow has cleared a low density
cavity that extends through the core and out into the more diffuse
interstellar medium. Alternatively, it may simply indicate a pre-existing
density gradient in the cloud core that causes regions of lower optical depth
to appear better illuminated. Narrow band imaging in the 2.12 $\mu$m H2 line
or radio mapping in CO could distinguish between these two options. We have
suggested a nearly edge-on disc orientation as an explanation for the the mid-
infrared flux peak, so if the structure is caused by an outflow we might have
expected a bipolar cavity. However, high resolution imaging has shown that the
inner disc of a YSO can be highly inclined with respect to the outer disc (see
e.g. Benisty et al., 2018) so the outflow cavity interpretation cannot be
ruled out.

##  5 Conclusions

We have detected 23 new highly variable mid-infrared sources ($>$2 mag
variation) in a catalogue of WISE/NEOWISE sources projected in the vicinity of
IRDCs. The majority of these relatively bright and red stars are YSOs but
almost half are periodic, hence more likely to be dusty Mira-like variables.
Since we have searched only a very small portion of the Galactic plane, this
demonstrates the potential of the ongoing NEOWISE mission for eruptive YSO
science, complementing VVV/VVVX.

The most notable discovery is the $\sim$8 mag outburst in WISE 1422-6115, a
low mass YSO at a distance $d=2.3$–$3.0$ kpc that is presumed to have
undergone an episodic accretion event. In evolutionary terms, we classify it
as a probable stage 1 protostar. The in-outburst luminosity of a few $\times
10^{2}$ L⊙ is higher than that of the surrounding massive cold cloud core so
it is possible that the core was measurably warmed by the event, though
probably not enough to cause major changes to the cloud’s chemical make-up. By
contrast, the accretion disc and any surrounding circumstellar envelope would
have been heated far more strongly.

Details of the event and the conclusions we have drawn are as follows.

  1. 1.

GLIMPSE, MIPSGAL, WISE and VVV data indicate that the outburst began between
2004 and 2006 and the duration now exceeds 13 yr. After making some allowance
for extinction and reprocessing, the luminosity is consistent with an
accretion rate approaching $10^{-4}$ M⊙yr-1, comparable to a classical FUor
event.

  2. 2.

The 4.6 $\mu$m peak in the SED corresponds to a temperature $T$ $\approx$
800–1000 K in 2010, appearing to cool further as the outburst progressed and
then faded. The 800-1000 K temperature and the measured mid-infrared
luminosity imply a disc radial location $R\approx 4.5$ au for the emitting
region. The apparent cooling as the outburst brightened slightly from
2010–2014 suggests an outward radial progression. This contrasts with the YSO
outburst of Gaia 17bpi (Hillenbrand et al., 2018), where there appears to have
been an inward progression. This disparate behaviour could be seen as evidence
that different disc instability mechanisms operated in the two outbursting
systems.

  3. 3.

The apparent lack of emission from hotter matter in the innermost disc is
unusual, given that the innermost parts of the disc are expected to be heated
to several thousand K in either the thermal-viscous instability model (Bell &
Lin, 1994; Bell et al., 1995) or in MRI models (Bae et al., 2014; Kadam et
al., 2020), with outward radial progression in both cases. An F-type disc
photosphere with supergiant gravity is observed at optical wavelengths in
classical FUor events. We suggest that the hot innermost regions of the disc
may be obscured from view by a near edge-on disc orientation. The 4.6 $\mu$m
peak might then arise in large part from reprocessing of radiation emitted
close to the star, though there could also be a contribution from a radially
extensive outburst region in the disc, with a spread of temperatures. Perhaps
less likely, if the upper layers of the disc are denser and more opaque than
in classical FUors, then the hot mid-plane layer where viscous energy is
dissipated could be hidden.

  4. 4.

In the disc fragmentation/infalling fragments model a mid-infrared peak can
arise naturally due to emission from an infalling clump in the inner disc
(Vorobyov et al., 2013). However, this is not in itself a burst event and
radially outward progression of a burst would be unexpected. In the context of
the disc fragmentation model, the low pre-outburst luminosity of the protostar
and the location inside a cold dense cloud core should lead to a low
temperature in the outer disc that would tend to facilitate fragmentation, as
noted by Vorobyov & Basu (2010).

  5. 5.

The very high amplitude, more than 13 yr duration and mid-infrared location
disfavour the magnetic gating mechanism, which was designed more to explain
EXor-like events, (see D’Angelo & Spruit, 2010, 2012).

Further observations of this system are certainly desirable to better
understand the event. Very high resolution submm continuum imaging with ALMA
might be able to detect signs of fragmentation in the outer disc. Mid-infrared
spectroscopy (see Kóspál et al. 2020b) and interferometry should be
practicable, in view of the slow rate of decline of the outburst in that
waveband. Sub-arcsecond millimetre waveband data would be able to detect the
expected circumstellar envelope within the cloud core, confirming stage I
evolutionary status. Either 13CO mapping or 2.12 $\mu$m H2 imaging could be
used to search for an outflow, investigate the arc of nebulosity and better
understand the nature of the large cometary reflection nebula.

Finally, another of the new discoveries, WISEA J185720.27+015711.8 (source 21
in Table 1) underwent a mid-infrared outburst of 4.6 mag and it has remained
in a bright state for several years. This unusually high amplitude event also
warrants further investigation.

## Acknowledgements

We thank the referee for reading the paper and encouraging us to provide more
detail in several places. This publication makes use of data products from the
WISE satellite, which is a joint project of the University of California, Los
Angeles, and the Jet Propulsion Laboratory/California Institute of Technology,
funded by the National Aeronautics and Space Administration (NASA). The work
is based in part on observations obtained at the Southern Astrophysical
Research (SOAR) telescope, which is a joint project of the Ministério da
Ciência, Tecnologia e Inovações (MCTI) do Brasil, the US National Science
Foundation s National Optical-Infrared Astronomy Research Laboratory
(NOIRLab), the University of North Carolina at Chapel Hill (UNC), and Michigan
State University (MSU). This research has made use of the NASA/IPAC Infrared
Science Archive, which is funded by the National Aeronautics and Space
Administration and operated by the California Institute of Technology. We also
acknowledge use of NASA’s Astrophysics Data System Bibliographic Services and
the SIMBAD database operated at CDS, Strasbourg, France. We also made use of
the VizieR catalogue access tool, CDS, Strasbourg, France (DOI :
10.26093/cds/vizier). The original description of the VizieR service was
published in 2000, A&AS 143, 23. PWL and ZG acknowledge support by STFC
Consolidated Grant ST/R00905/1 and CM is supported by an STFC studentship
funded by grant ST/S505419/1. We gratefully acknowledge data from the ESO
Public Survey program ID 179.B-2002 taken with the VISTA telescope, and
products from the Cambridge Astronomical Survey Unit (CASU). EV acknowledges
support from the Russian Fund for Fundamental Research, Russian-Taiwanese
project 19-52-52011. DM acknowledges support from the FONDECYT Regular grant
No. 1170121, the BASAL Center for Astrophysics and Associated Technologies
(CATA) through grant AFB170002. RKS acknowledges support from CNPq/Brazil
through project 305902/2019-9.

Data Availability
The WISE, Spitzer and 2MASS data underlying this article are publicly
available at https://irsa.ipac.caltech.edu/Missions/wise.html,
https://irsa.ipac.caltech.edu/Missions/spitzer.html and
https://irsa.ipac.caltech.edu/Missions/2mass.html respectively. The VVV and
VVVX data are publicly available at the ESO archive
http://archive.eso.org/cms.html. The relevant reduced products for the most
recent VVVX epochs have not yet been publicly released but are available on
request to the first author. Akari photometry are available at
https://vizier.u-strasbg.fr/viz-bin/VizieR-2. Herschel data are available at
the Herschel Science Archive, http://archives.esac.esa.int/hsa/whsa/. UKIDSS
data are available at the WFCAM Science Archive wsa.roe.ac.uk. The SOAR data
are available at http://star.herts.ac.uk/~pwl/Lucas/SOAR/.

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##  Appendix A Additional Photometry

The VVV $K_{s}$ light curve consists of 157 data points, after discarding
images with seeing $>$1.2″and a few highly discrepant data points. There were
four contemporaneous image stacks at each epoch (from adjoining VVV tiles d051
and d052) each showing numerous adjacent stars for visual comparison, which
made it easy to reject the outlying data.

A difficulty with the VVV/VVVX data is that the source was not only saturated
in $K_{s}$ at maximum brightness (i.e. in 2010-2013 and in a minority of
images from 2014) but also spatially extended on a scale of a few arc seconds,
though the flux profile is strongly peaked, see Figure 2. The saturation level
varies between the 16 VISTA/VIRCAM arrays. Inspection of the radial intensity
profile of the source in VISTA pawprint images from the 4 arrays with
detections showed that the profile just outside the saturated core resembles
that of a point source. From the photometric apertures provided by the
Cambridge Astronomical Survey Unit (CASU) VISTA pipeline we used apertures 3
and 4 (diameters of 2″ and 2$\sqrt{2}$″, respectively) to compute a
saturation-corrected magnitude in a similar manner to that described by Irwin
(2009).This was done for the 2010-2013 data, with an estimated uncertainty of
0.2 mag, and visual comparison with saturated stars having 2MASS photometry
was used to check that the results were sensible. Saturated data from 2014
were discarded because the image profile had changed. For unsaturated $K_{s}$
data from 2014 onward, and for the other VVV passbands, we give point source
photometry in aperture 3. This will slightly underestimate the total
brightness of the source but it provides a reasonably consistent measurement
of the brightness of central flux peak, for comparison with the earlier data.

The integrated VVV magnitudes of the saturated $K_{s}$ March 2010 photometry
used for SED fitting (see section 3.4) were calculated by computing the
difference between the fluxes measured by the CASU pipeline in apertures 7 (8″
diameter) and aperture 3 and adding this quantity to the saturation-corrected
aperture 3 flux computed earlier. After converting the resulting flux value to
a magnitude, we added the aperture correction for aperture 3 and subtracted
the aperture correction for aperture 7 to arrive at an aperture corrected
magnitude in aperture 7. The aperture corrections were provided by the CASU
pipeline for each image.

Table 3: Additional time series photometry for WISE 1422-6115 Dataset | MJD | Bandpass | Magnitude | Error
---|---|---|---|---
DENIS | 51014.69 | $K_{s}$ |  $>$13.6 |
2MASS | 51615.29 | $K_{s}$ |  $>$15.0 |
GLIMPSE | 53075.39 | [3.6] | 14.41 | 0.13
GLIMPSE | 53075.56 | [4.5] | 13.08 | 0.15
GLIMPSE | 53075.39 | [5.8] | 12.44 | 0.17
GLIMPSE | 53075.56 | [8.0] | 11.57 | 0.32
MIPSGAL | 53834.33 | [24] | 4.74 | 0.03
Akari/IRC | - | [9] | 5.90 | 0.35
Akari/IRC | - | [18] | 3.83 | 0.03
VVV/VVVX | 55274.245 | $Z$ |  $>$20.0 |
VVV/VVVX | 55281.254 | $Y$ | 19.14 | 0.10
VVV/VVVX | 55281.255 | $Y$ | 19.42 | 0.15
VVV/VVVX | 55282.162 | $Y$ | 19.44 | 0.16
VVV/VVVX | 55282.163 | $Y$ | 19.55 | 0.21
VVV/VVVX | 55274.253 | $J$ | 16.07 | 0.02
VVV/VVVX | 55274.255 | $J$ | 16.06 | 0.02
VVV/VVVX | 55288.293 | $J$ | 16.11 | 0.02
VVV/VVVX | 55288.295 | $J$ | 16.11 | 0.02
VVV/VVVX | 55274.245 | $H$ | 12.65 | 0.02
VVV/VVVX | 55288.285 | $H$ | 12.68 | 0.02
VVV/VVVX | 57176.178 | $H$ | 16.30 | 0.03
VVV/VVVX | 57176.179 | $H$ | 16.27 | 0.04
VVV/VVVX | 57181.250 | $H$ | 16.34 | 0.05
VVV/VVVX | 57181.252 | $H$ | 16.26 | 0.05
VVV/VVVX | 55260.34247 | $K_{s}$ | 9.08 | 0.20
VVV/VVVX | 55260.3431 | $K_{s}$ | 9.19 | 0.20
VVV/VVVX | 55260.34724 | $K_{s}$ | 9.27 | 0.20
VVV/VVVX | 55260.34796 | $K_{s}$ | 9.16 | 0.20
VVV/VVVX | 55262.39191 | $K_{s}$ | 9.20 | 0.20
VVV/VVVX | 55262.39336 | $K_{s}$ | 9.25 | 0.20
VVV/VVVX | 55262.39404 | $K_{s}$ | 9.16 | 0.20
VVV/VVVX | 55264.39126 | $K_{s}$ | 9.21 | 0.20
VVV/VVVX | 55264.392 | $K_{s}$ | 9.26 | 0.20
VVV/VVVX | 55264.39267 | $K_{s}$ | 9.23 | 0.20
VVV/VVVX | 55274.24962 | $K_{s}$ | 9.18 | 0.20
VVV/VVVX | 55274.36398 | $K_{s}$ | 9.05 | 0.20
VVV/VVVX | 55274.3646 | $K_{s}$ | 9.16 | 0.20
VVV/VVVX | 55274.36793 | $K_{s}$ | 9.18 | 0.20
VVV/VVVX | 55274.36857 | $K_{s}$ | 9.17 | 0.20
VVV/VVVX | 55283.24924 | $K_{s}$ | 9.08 | 0.20
VVV/VVVX | 55283.25274 | $K_{s}$ | 9.23 | 0.20
VVV/VVVX | 55283.25337 | $K_{s}$ | 9.10 | 0.20
VVV/VVVX | 55288.28806 | $K_{s}$ | 9.22 | 0.20
VVV/VVVX | 55288.28954 | $K_{s}$ | 9.07 | 0.20
VVV/VVVX | 55778.06854 | $K_{s}$ | 8.52 | 0.20
VVV/VVVX | 55778.07018 | $K_{s}$ | 8.62 | 0.20
VVV/VVVX | 55778.07102 | $K_{s}$ | 8.54 | 0.20
VVV/VVVX | 55780.07017 | $K_{s}$ | 8.68 | 0.20
VVV/VVVX | 55785.99496 | $K_{s}$ | 8.72 | 0.20
VVV/VVVX | 55785.99667 | $K_{s}$ | 8.76 | 0.20
VVV/VVVX | 55795.02149 | $K_{s}$ | 8.56 | 0.20
VVV/VVVX | 55795.02306 | $K_{s}$ | 8.72 | 0.20
VVV/VVVX | 55803.98976 | $K_{s}$ | 8.34 | 0.20
VVV/VVVX | 55803.99062 | $K_{s}$ | 8.39 | 0.20
VVV/VVVX | 55803.99142 | $K_{s}$ | 8.53 | 0.20
VVV/VVVX | 55808.99112 | $K_{s}$ | 8.27 | 0.20
VVV/VVVX | 55808.99192 | $K_{s}$ | 8.27 | 0.20
VVV/VVVX | 55808.99272 | $K_{s}$ | 8.35 | 0.20
VVV/VVVX | 55995.18952 | $K_{s}$ | 8.64 | 0.20
VVV/VVVX | 55995.19099 | $K_{s}$ | 8.73 | 0.20
VVV/VVVX | 55995.19174 | $K_{s}$ | 8.69 | 0.20
VVV/VVVX | 56014.27852 | $K_{s}$ | 8.59 | 0.20
VVV/VVVX | 56014.28026 | $K_{s}$ | 8.96 | 0.20
VVV/VVVX | 56014.28119 | $K_{s}$ | 8.60 | 0.20

Table 4: continued Dataset | MJD | Bandpass | Magnitude | Error
---|---|---|---|---
VVV/VVVX | 56058.30581 | $K_{s}$ | 8.72 | 0.20
VVV/VVVX | 56058.30765 | $K_{s}$ | 8.82 | 0.20
VVV/VVVX | 56371.21412 | $K_{s}$ | 9.02 | 0.20
VVV/VVVX | 56371.21564 | $K_{s}$ | 9.10 | 0.20
VVV/VVVX | 56371.21643 | $K_{s}$ | 8.85 | 0.20
VVV/VVVX | 56372.20321 | $K_{s}$ | 8.88 | 0.20
VVV/VVVX | 56372.20405 | $K_{s}$ | 9.01 | 0.20
VVV/VVVX | 56372.20483 | $K_{s}$ | 8.93 | 0.20
VVV/VVVX | 56372.20566 | $K_{s}$ | 8.91 | 0.20
VVV/VVVX | 56373.19458 | $K_{s}$ | 8.99 | 0.20
VVV/VVVX | 56373.19615 | $K_{s}$ | 9.04 | 0.20
VVV/VVVX | 56373.19759 | $K_{s}$ | 8.89 | 0.20
VVV/VVVX | 56374.19197 | $K_{s}$ | 9.06 | 0.20
VVV/VVVX | 56375.25579 | $K_{s}$ | 8.59 | 0.20
VVV/VVVX | 56375.2566 | $K_{s}$ | 8.61 | 0.20
VVV/VVVX | 56375.25738 | $K_{s}$ | 8.65 | 0.20
VVV/VVVX | 56375.25819 | $K_{s}$ | 8.47 | 0.20
VVV/VVVX | 56419.25141 | $K_{s}$ | 8.57 | 0.20
VVV/VVVX | 56419.25216 | $K_{s}$ | 8.70 | 0.20
VVV/VVVX | 56419.25293 | $K_{s}$ | 8.76 | 0.20
VVV/VVVX | 56428.3069 | $K_{s}$ | 9.06 | 0.20
VVV/VVVX | 56448.22414 | $K_{s}$ | 9.08 | 0.20
VVV/VVVX | 56448.22497 | $K_{s}$ | 9.14 | 0.20
VVV/VVVX | 56448.22655 | $K_{s}$ | 9.10 | 0.20
VVV/VVVX | 56453.21702 | $K_{s}$ | 9.02 | 0.20
VVV/VVVX | 56453.21779 | $K_{s}$ | 8.85 | 0.20
VVV/VVVX | 56453.21861 | $K_{s}$ | 8.92 | 0.20
VVV/VVVX | 56453.2194 | $K_{s}$ | 8.78 | 0.20
VVV/VVVX | 56460.99369 | $K_{s}$ | 8.91 | 0.20
VVV/VVVX | 56460.99446 | $K_{s}$ | 8.99 | 0.20
VVV/VVVX | 56460.99521 | $K_{s}$ | 9.12 | 0.20
VVV/VVVX | 56460.99594 | $K_{s}$ | 8.91 | 0.20
VVV/VVVX | 56463.17171 | $K_{s}$ | 9.21 | 0.20
VVV/VVVX | 56463.17253 | $K_{s}$ | 9.24 | 0.20
VVV/VVVX | 56463.17329 | $K_{s}$ | 9.15 | 0.20
VVV/VVVX | 56463.17412 | $K_{s}$ | 9.10 | 0.20
VVV/VVVX | 56466.18514 | $K_{s}$ | 9.16 | 0.20
VVV/VVVX | 56466.18668 | $K_{s}$ | 9.23 | 0.20
VVV/VVVX | 56466.18747 | $K_{s}$ | 9.14 | 0.20
VVV/VVVX | 56467.16656 | $K_{s}$ | 8.96 | 0.20
VVV/VVVX | 56467.1674 | $K_{s}$ | 9.05 | 0.20
VVV/VVVX | 56467.16816 | $K_{s}$ | 9.09 | 0.20
VVV/VVVX | 56468.16904 | $K_{s}$ | 9.07 | 0.20
VVV/VVVX | 56468.16983 | $K_{s}$ | 9.18 | 0.20
VVV/VVVX | 56468.17057 | $K_{s}$ | 9.17 | 0.20
VVV/VVVX | 56468.17137 | $K_{s}$ | 9.05 | 0.20
VVV/VVVX | 56469.12137 | $K_{s}$ | 9.10 | 0.20
VVV/VVVX | 56813.04202 | $K_{s}$ | 11.03 | 0.02
VVV/VVVX | 56813.04381 | $K_{s}$ | 10.94 | 0.02
VVV/VVVX | 56813.05533 | $K_{s}$ | 11.00 | 0.02
VVV/VVVX | 56813.0571 | $K_{s}$ | 10.92 | 0.02
VVV/VVVX | 56813.96544 | $K_{s}$ | 10.94 | 0.02
VVV/VVVX | 56813.96625 | $K_{s}$ | 10.96 | 0.02
VVV/VVVX | 56813.96822 | $K_{s}$ | 10.96 | 0.02
VVV/VVVX | 56813.97878 | $K_{s}$ | 10.94 | 0.02
VVV/VVVX | 56813.97961 | $K_{s}$ | 11.02 | 0.02
VVV/VVVX | 56813.98135 | $K_{s}$ | 10.95 | 0.02
VVV/VVVX | 56813.9912 | $K_{s}$ | 10.95 | 0.02
VVV/VVVX | 56813.99204 | $K_{s}$ | 11.01 | 0.02
VVV/VVVX | 56813.99388 | $K_{s}$ | 10.96 | 0.02
VVV/VVVX | 56814.00341 | $K_{s}$ | 10.94 | 0.02
VVV/VVVX | 56814.00422 | $K_{s}$ | 11.00 | 0.02
VVV/VVVX | 56814.00643 | $K_{s}$ | 10.98 | 0.02
VVV/VVVX | 56814.04191 | $K_{s}$ | 10.93 | 0.02

Table 5: continued VVV/VVVX | 56814.04274 | $K_{s}$ | 10.95 | 0.02
---|---|---|---|---
VVV/VVVX | 56814.04373 | $K_{s}$ | 10.95 | 0.02
VVV/VVVX | 56814.04456 | $K_{s}$ | 10.96 | 0.02
VVV/VVVX | 56825.01277 | $K_{s}$ | 11.00 | 0.02
VVV/VVVX | 56825.01364 | $K_{s}$ | 11.03 | 0.02
VVV/VVVX | 57118.15631 | $K_{s}$ | 12.03 | 0.02
VVV/VVVX | 57118.15709 | $K_{s}$ | 12.05 | 0.02
VVV/VVVX | 57134.0828 | $K_{s}$ | 12.10 | 0.02
VVV/VVVX | 57150.03191 | $K_{s}$ | 12.08 | 0.02
VVV/VVVX | 57150.03299 | $K_{s}$ | 12.08 | 0.02
VVV/VVVX | 57150.0339 | $K_{s}$ | 12.11 | 0.02
VVV/VVVX | 57150.03507 | $K_{s}$ | 12.10 | 0.02
VVV/VVVX | 57165.0114 | $K_{s}$ | 12.09 | 0.02
VVV/VVVX | 57165.01219 | $K_{s}$ | 12.16 | 0.02
VVV/VVVX | 57165.01362 | $K_{s}$ | 12.14 | 0.02
VVV/VVVX | 57165.01452 | $K_{s}$ | 12.14 | 0.02
VVV/VVVX | 57171.02674 | $K_{s}$ | 12.10 | 0.02
VVV/VVVX | 57171.02757 | $K_{s}$ | 12.16 | 0.02
VVV/VVVX | 57171.02829 | $K_{s}$ | 12.14 | 0.02
VVV/VVVX | 57171.02911 | $K_{s}$ | 12.14 | 0.02
VVV/VVVX | 57172.05397 | $K_{s}$ | 12.10 | 0.02
VVV/VVVX | 57172.05474 | $K_{s}$ | 12.12 | 0.02
VVV/VVVX | 57172.05543 | $K_{s}$ | 12.18 | 0.02
VVV/VVVX | 57172.05623 | $K_{s}$ | 12.17 | 0.02
VVV/VVVX | 57176.18236 | $K_{s}$ | 12.14 | 0.02
VVV/VVVX | 57176.18403 | $K_{s}$ | 12.17 | 0.02
VVV/VVVX | 57181.25722 | $K_{s}$ | 12.12 | 0.02
VVV/VVVX | 57585.07992 | $K_{s}$ | 12.65 | 0.02
VVV/VVVX | 57585.0807 | $K_{s}$ | 12.68 | 0.02
VVV/VVVX | 57585.08142 | $K_{s}$ | 12.69 | 0.02
VVV/VVVX | 57585.08222 | $K_{s}$ | 12.70 | 0.02
VVV/VVVX | 57928.18046 | $K_{s}$ | 13.21 | 0.02
VVV/VVVX | 57933.0333 | $K_{s}$ | 13.19 | 0.02
VVV/VVVX | 57933.03393 | $K_{s}$ | 13.18 | 0.02
VVV/VVVX | 57933.03461 | $K_{s}$ | 13.22 | 0.02
VVV/VVVX | 57933.03533 | $K_{s}$ | 13.21 | 0.02
VVV/VVVX | 58208.31044 | $K_{s}$ | 13.66 | 0.02
VVV/VVVX | 58208.31118 | $K_{s}$ | 13.67 | 0.02
VVV/VVVX | 58212.39832 | $K_{s}$ | 13.70 | 0.03
VVV/VVVX | 58262.99078 | $K_{s}$ | 13.85 | 0.02
VVV/VVVX | 58262.99156 | $K_{s}$ | 13.81 | 0.02
VVV/VVVX | 58654.20043 | $K_{s}$ | 14.28 | 0.03
VVV/VVVX | 58654.20121 | $K_{s}$ | 14.27 | 0.03
VVV/VVVX | 58654.20198 | $K_{s}$ | 14.33 | 0.03
VVV/VVVX | 58654.20271 | $K_{s}$ | 14.31 | 0.03
VVV/VVVX | 58701.08159 | $K_{s}$ | 14.34 | 0.02
VVV/VVVX | 58701.08231 | $K_{s}$ | 14.34 | 0.02
VVV/VVVX | 58701.08303 | $K_{s}$ | 14.37 | 0.02
VVV/VVVX | 58701.08378 | $K_{s}$ | 14.37 | 0.03
VVV/VVVX | 58702.06429 | $K_{s}$ | 14.32 | 0.03
VVV/VVVX | 58702.06503 | $K_{s}$ | 14.36 | 0.02
VVV/VVVX | 58702.06659 | $K_{s}$ | 14.40 | 0.02
VVV/VVVX | 58702.06742 | $K_{s}$ | 14.37 | 0.03

Table 6: Herschel/Hi-Gal and ATLASGAL scaled photometry for HIGALBM 313.6699-0.3092 Wavelength ($\mu$m) | Flux (Jy) | Error (Jy)
---|---|---
70 | 3.12 | 0.14
160 | 10.89 | 0.16
250 | 14.66 | 0.41
350 | 4.48 | 0.22
500 | 2.37 | 0.15
870 | 0.97 | 0.19