# Dippers from the TESS Full-Frame Images I: The Results of the first 1 year
data and Discovery of A Runaway dipper

Tomoyuki Tajiri11affiliation: Department of Earth and Planetary Science, The
University of Tokyo, 7-3-1, Hongo, Tokyo, Japan , Hajime
Kawahara11affiliation: Department of Earth and Planetary Science, The
University of Tokyo, 7-3-1, Hongo, Tokyo, Japan 22affiliation: Research Center
for the Early Universe, School of Science, The University of Tokyo, Tokyo
113-0033, Japan , Masataka Aizawa33affiliation: Department of Physics, The
University of Tokyo, 7-3-1, Hongo, Tokyo, Japan , Michiko S.
Fujii44affiliation: Department of Astronomy, The University of Tokyo, 7-3-1,
Hongo, Tokyo, Japan , Kohei Hattori55affiliation: Department of Physics,
Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, USA , Yui
Kasagi66affiliation: Department of Astronomy, School of Science, The Graduate
University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan
, Takayuki Kotani77affiliation: National Astronomical Observatory of Japan,
2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 88affiliation: Astrobiology
center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 66affiliation: Department
of Astronomy, School of Science, The Graduate University for Advanced Studies
(SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan , Kento Masuda99affiliation:
Institute for Advanced Study, School of Natural Sciences, 1 Einstein Drive,
Princeton, NJ 08540, USA 1010affiliation: Department of Earth and Space
Science, Osaka University, Osaka 560-0043, Japan , Munetake
Momose1111affiliation: College of Science, Ibaraki University, Bunkyo 2-1-1,
Mito 310-8512, Japan , Takayuki Muto1212affiliation: Division of Liberal Arts,
Kogakuin University, 1-24-2 Nishi-Shinjyuku, Shinjyuku-ku, Tokyo 163-8677,
Japan , Ryou Ohsawa1313affiliation: Institute of Astronomy, The University of
Tokyo, 2-21-1, Osawa, Mitaka, Tokyo 181-0015, Japan , Satoshi
Takita77affiliation: National Astronomical Observatory of Japan, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan

###### Abstract

We present a comprehensive catalog of the dippers—young stellar objects that
exhibit episodic dimming—derived from the one year’s worth of data of
Transiting Exoplanet Survey Satellite (TESS) full-frame images. In the survey,
we found 35 dippers using the convolutional neural network, most of them newly
discovered. Although these dippers are widely distributed over the first half-
hemisphere that TESS surveyed, we identified the majority’s membership with
the nearest association Scorpius–Centaurus, Velorum OB2, and nearby Orion
molecular cloud complex. However, several dippers are likely to be located in
the field. We also found three old dippers whose age exceeds ten million year,
which is considered as the disk dissipation time. The color-color diagram
indicates that these old dippers are likely to have an extreme debris disk. In
particular, we found a runaway old dipper having a large three-dimensional
velocity of $72$ $\,\mathrm{km\ s}^{-1}$. The dippers in the field, which were
probably escaping from their birth molecular clouds or were born outside the
current area of star forming regions, are more common than previously
considered.

###### Subject headings:

protoplanetary disks — stars: variables: T Tauri, Herbig Ae/Be

##  1\. Introduction

Episodic or quasi-periodic dimming in the light curve of young stellar objects
(YSOs), known as “dippers,”, have been discovered in the data obtained by
recent satellite missions, CoRoT, Spitzer (Alencar et al., 2010; Morales-
Calderón et al., 2011; Cody et al., 2014), $K2$ (Ansdell et al., 2016b; Hedges
et al., 2018; Ansdell et al., 2019), and TESS (Gaidos et al., 2019). Morales-
Calderón et al. (2011) revealed that dips are deeper in the optical wavelength
than in the infrared bands, indicating that dips are shadows of circumstellar
materials. However, the detailed mechanism has so far been poorly understood.
Various mechanisms have been proposed such as accretion inflow onto a star
(Bouvier et al., 1999; McGinnis et al., 2015), non-axisymmetric structure of
the disk induced by the Rossby wave instability (Stauffer et al., 2015), and
transiting circumstellar clumps (Ansdell et al., 2016b).

Historically, new dippers are discovered through visual inspection-based
approaches because their transit shape varies from object to object and
changes over time. The first systematic survey using machine learning was done
by (Hedges et al., 2018). They searched for the dippers from $K2$ data and
discovered about 100 dippers in famous star forming regions, Upper Scorpius
and $\rho$ Ophiuchus.

While most dippers that have been found are located in the star forming
regions and have the properties of M or K -type stars (Stauffer et al., 2015;
Ansdell et al., 2016a), a few stars near the zero-age main sequence with
episodic dimming have been also reported. RZ Psc was regarded as a variable
star with IR excess in this stage (30–50 Myr Grinin et al., 2010; de Wit et
al., 2013; Punzi et al., 2018). However, Potravnov et al. (2019) have
identified the membership of RZ Psc with Cassiopeia–Taurus OB association from
their proper motions and age as $20_{-5}^{+3}$ Myr updated by Gaia DR2 data
(Gaia Collaboration et al., 2018). Recently, Gaidos et al. (2019) discovered
episodic dimming in HD 240779, outside star forming regions in the TESS short
cadence data. HD 240779 is between UX Ori stars and M-type dipper and belongs
in the AB Doradus moving group. HD 240779 shows IR excess, but the moving
group suggests that its age is $\approx 125$Myr. At this age, primordial disks
are thought to have already dissipated, so this IR excess is thought to be due
to a secondary disk. Though the origin of the disk is not fully understood,
this discovery suggests that dimming events can be found even outside star
forming regions. In this paper, we adopt the definition of dipper as a star
with episodic or quasi periodic dimming with IR excess regardless of its age.
In this sense, HD 240779 can be called an old dipper or debris dipper.

YSO surveys have been done in star forming regions, but YSOs ($\lesssim 10$
Myr) can also be found outside of star forming regions. McBride & Kounkel
(2019) found several runaway stars with disks around the Orion Nebula Cluster
(ONC) using the proper motions obtained by Gaia. Runaway stars (Blaauw &
Morgan, 1954; Gies & Bolton, 1986) are stars with a peculiar velocity
($>30\,\mathrm{km\ s}^{-1}$) that is higher than the velocity dispersion of
young disk stars (Sharma et al., 2014). Runaway stars initially found among
OB-type stars, so called OB runaways (Gies & Bolton, 1986). There are two
mechanisms to form runaways: supernova explosion of a companion star in a
tight binary (Blaauw, 1961) or binary-single stellar encounters in star
clusters (Gies & Bolton, 1986). Recent observations (Evans et al., 2010;
Roman-Lopes et al., 2011; Gvaramadze et al., 2013; Lennon et al., 2018;
McBride & Kounkel, 2019) and simulations (Fujii & Portegies Zwart, 2011;
Banerjee et al., 2012; Fujii et al., 2012) have suggested that young star
clusters can actually produce runaway stars. Most recent study done by McBride
& Kounkel (2019) reported that $\approx 30\%$ of runaway young stars from the
ONC have a disk. This suggests that protoplanetary disks can survive close
stellar encounters, which are necessary for the formation of runaway stars.
Therefore, we do not need to restrict the dipper survey to star forming
regions. In this paper, we searched for dippers both inside and outside the
star forming regions by systematically analyzing the full-frame images of TESS
(Ricker et al., 2015).

The rest of the paper is organized as follows.In Section 2 we describe the
pipeline to identify dippers from TESS full frame images. In Section 3, we
show the statistical properties of 35 dippers we find. We also identify the
memberships of these dippers. In section 4, We focus on the runaway dipper TIC
43488669. We discuss the origins of dippers and the occurrence of debris
dippers in Section 5.

##  2\. Identification of Dippers

###  2.1. Aperture photometry of the TESS full-frame images

In the survey, we performed the simple aperture photometry of the stars in the
Candidate Target List version 8 (CTLv8; Stassun et al., 2019) using the TESS
full-frame images (FFIs; Sullivan et al., 2015) of Sectors 1–13, corresponding
to the first 1 year of the TESS mission. We extracted data cubes of a time-
series of $13\times 13$ pixels around the position of the stars in the CTLv8
from the FFIs. The aperture is defined by the pixels whose flux is 3 $\sigma$
times larger than the median flux of the 13 $\times$ 13 pixels and is
connected to the central pixel by other pixels in the aperture. We also
defined the 30 or more background pixels distinguished from its aperture whose
flux does not exceed 0.5 quartile deviation. We assume the median value of the
background pixels as the background flux, which is constant for all of the
target pixels. We subtracted the background flux from the light curve. We
automatically removed poor quality data points caused by “momentum dump” and
so on using time-series data of pointing of spacecraft provided in the FFIs.
In this way, we created about $4\times 10^{6}$ light curves in 30 minute
cadence from FFIs.

###  2.2. Convolutional Neural Network

Figure 1.— The convolutional neural network architecture used in this work.
One dimensional time-series data of light curves are fed into four
convolutional layers and one fully connected layer and ends in a sigmoid
function. Following Ansdell et al. (2018), the convolutional layers are
denoted as
CONV-$\langle\mathrm{kernel\,size}\rangle$-$\langle\mathrm{number\,of\,feature\,maps}\rangle$,
the max pooling layers are denoted as
MAXPOOL-$\langle\mathrm{window\,length}\rangle$-$\langle\mathrm{stride\,length}\rangle$,
and the fully connected layer is denoted as
FC-$\langle\mathrm{number\,of\,units}\rangle$, where
$\langle\mathrm{kernel\,size}\rangle$ indicates the length of the 1D
convolution window, $\langle\mathrm{number\,of\,feature\,maps}\rangle$ is the
dimensionality of the output space, and
$\langle\mathrm{stride\,length}\rangle$ is the length of each window and
$\langle\mathrm{number\,of\,units}\rangle$ means the dimensionality of the
output space. The value of null in each light curve was removed and data were
cut down to 950 points.

To find episodic events of the dippers from more than $4\times 10^{6}$ light
curves, we first identified objects that exhibited a large photometric
variation such as eclipsing binaries by using a convolutional neural network
(CNN). The CNN model is shown in Figure 1, which is based on the Exonet-XS
model (Ansdell et al., 2018). We slightly modified the Exonet-XS model for
TESS light curves; we do not use the local view (a light curve localized
around a transit signal) used in the original Exonet-XS , because we do not
have a common transit shape and we also do not use stellar parameters. We add
one more convolutional layer to the Exonet-XS model.

So far, we do not have sufficient training sets of dippers because HD 240779
is the only dipper found in TESS. Instead, we decided to use eclipsing
binaries found in the TESS data as the training data set. This is because a
large and sharp dip in the light curve of an eclipsing binary resembles those
of dippers in a local view around a dip. First, we searched for eclipsing
binaries from TESS light curves by finding outliers defined by the four
quartile deviation, whose parameters such as threshold values was adjusted by
Kepler Eclipsing Binary Catalog (Abdul-Masih et al., 2016). We detect light
curves which have data points far from the baseline. A catalog of hundreds of
TESS eclipsing binaries was created by visual inspection from the sets of
light curves obtained by this process.

Then using these data, we trained a support vector machine, visually examined
the data judged to be true, and found 4000 eclipsing binaries. We used
thundersvm (Wen et al., 2018) for the support vector analysis. Using these
training data set, we trained our neural network model. For each phase,
training sets for a negative detection were randomly chosen from the light
curves. The final CNN identified $36,674$ eclipsing binary candidates.

###  2.3. Identifining and Screening of the Dipper Candidates

####  2.3.1 Visual Inspection

We visually inspected all of the $36,674$ eclipsing binary candidates to
classify the types of variation. We classified them into six types, stellar
eclipse, stellar pulsation, variation by star spots, asteroid passing,
systematic noise, and dippers. The eclipse by a companion or planet is
completely cyclic and has the same shape in every transits. Although there are
many types of the pulsating stars whose period is about from a day to a week
such as Cepheid, RR Lyrae, Delta Scuti, and Gamma Doradus, all of them are
almost periodic and well modeled as in the case of eclipse and transit
signals. The variation by the star spots is also periodic, but it exhibits
various shapes in the light curves. While the variation of the star spots,
whose periodic shape can be roughly modeled by a smooth sine-like curves, is
indistinguishable from the variation of the eclipsing binary, it can be easily
distinguished from the dippers, whose shape exhibits complex structure. Also,
a large single dip is observed when an asteroid crosses over the target star.
Although the episodic dimming event owing to asteroid crossing is similar to
the dipper event, it can be distinguished by examining pixels outside the
aperture; if an asteroid crosses, pixels outside the aperture will also
exhibits large variation. A systematic noise also induces large variations
outside the aperture. We can distinguish the systematic noise from the dipper
because the common variation pattern is observed in the other targets in the
CCD chip. As a result, we discovered 41 dipper candidates in the TESS first
one year data.

####  2.3.2 Pixel-level Difference Imaging

Figure 2.— The pixel-level difference image of TIC 243324939. Black, blue and
orange points in the upper panel are normalized flux of TIC 243324939. The red
points in bottom images indicate stars, TIC 243324946, 243324939 and 243324936
from the top to the bottom. White range is an aperture for TIC 243324939 and
blue regions are used to determine background flux. The bottom right image
indicates that the dip occurred in the central four pixels around TIC
243324939.

The contamination from nearby stars was investigated using the pixel-level
difference imaging (Bryson et al., 2013; Kawahara & Masuda, 2019). Figure 2
shows an example of the pixel-level difference image for TIC 243324939. We
take the averages of pixels in and out of the dip (orange and blue) and
compute the difference image (bottom right panel). The red points indicate
stars, TIC 243324946, 243324939 and 243324936 from the top to the bottom,
whose magnitudes in TESS band are 15.9, 13.7 and 13.6. We do not plot stars
whose magnitudes are more than 16 in the figure. For this example, though both
TIC 243324939 and 243324936 are located in the center pixel and have similar
magnitudes, the difference image of the dip reveals that pixels around TIC
243324939 show large values; this implies that the source star that shows
dimming event is TIC 243324939.

We also analyze the relative positions of the moment-weighted centroid of the
dipper candidates using photutils. All of their centroid shifts are consistent
with our identification of the dimming stars.

Through the above process, we determined 37 dipping stars from contaminated
images. However, we were not able to associate the dipping signal with any
specific star for four systems by the above process.

####  2.3.3 SED Analysis

Young stellar objects generally exhibit the infrared excess owing to the
circumstellar structure. We checked spectral energy distributions (SEDs) of
the dipper candidates using the data of three bands ($G_{BP}$, G, $G_{RP}$)
provided by Gaia DR2, three bands (J, H, $K_{s}$) from 2MASS, and four bands
($W1$, $W2$, $W3$, $W4$) from WISE. Each SED was compared with the best-fit
black body model obtained by MCMC using $G_{BP}$, G, $G_{RP}$, J, H, $K_{s}$
bands. The distance from Gaia is fixed, and the effective temperature and
radius are regarded as free parameters. We compared observed WISE fluxes with
the expected energy distributions derived by black body model to check the
infrared excess. Comparing with the result of Virtual Observatory SED Analyzer
(VOSA Bayo et al., 2008), we excluded one target with no clear infrared excess
from our list of dippers because we focus on the clear examples of the dippers
with infrared excess in this paper.

####  2.3.4 Other Variables

There are variable stars which shows infrared excess other than T-Tauri stars.
For instance, asymptotic giant branch (AGB) stars and R Coronae Borealis (RCB)
stars exhibit such excess in their SEDs. Both have infrared excess due to
circumstellar envelopes and sometimes exhibit quasi-periodic or aperiodic
variability. Because both AGBs and RCBs are giant stars, we used the stellar
radius provided by TESS Input Catalog version 8 (TICv8; Stassun et al., 2019)
to exclude such stars from dipper candidates. We dropped one object with a
stellar radius greater than $3R_{\odot}$. We finally identified 35 dippers as
listed in Table 1. We additionally analyzed Reduced Unit Weighted Error to
distinguish multiple stars from each other, as summarized in Appendix.

##  3\. Disk and stellar properties of dippers

In this survey, we discovered 35 dippers. All light curves are shown in Figure
20. All of them exhibit infrared excess in their SED diagrams. There are a
wide variety of the shape, duration, and frequency of the dips. The depths of
dips are typically from 10 percent to several ten percent. Stellar properties
of the dippers from Gaia DR2 and TICv8 are listed in Table 1.

Table 1Properties of dippers  TIC ID111TESS Input Catalog ID | Gaia222Gaia Source ID associated with TIC ID | Membership333The possible membership determined in Section 3. The blank means that we cannot identify the membership. | Disk Type444Disk Type determined from Figure.3: F=full, E=evolved, T=transitional. Dippers with two classes are those whose classes we cannot identify with Figure.3 |  $l$555Galactic longitude, latitude and parallax listed in Gaia DR2 |  $b$555Galactic longitude, latitude and parallax listed in Gaia DR2 | parallax555Galactic longitude, latitude and parallax listed in Gaia DR2 | Tmag666TESS magnitude, effective temperature, and stellar radius listed in TICv8 | Teff666TESS magnitude, effective temperature, and stellar radius listed in TICv8 | Radius666TESS magnitude, effective temperature, and stellar radius listed in TICv8
---|---|---|---|---|---|---|---|---|---
|  |  |  | deg | deg | mas |  | K | $R_{\odot}$
24775707 | 3208869061545298048 | Orion | T? | 209.191 | -21.826 | $3.02\pm 0.03$ | 12.9 | $4223\pm 129$ | $1.23\pm 0.12$
34397579 | 3208985468042433792 | Orion | F | 208.992 | -21.071 | $2.89\pm 0.05$ | 9.88 | $6330\pm 140$ | $2.3\pm 0.12$
37947642 | 3179285734830216192 |  | E | 206.123 | -37.112 | $3.99\pm 0.18$ | 15.52 | $3082\pm 157$ | $0.48\pm 0.02$
38044097 | 3179048313334809472 |  | E | 206.93 | -36.098 | $4.69\pm 0.07$ | 14.62 | $3210\pm 157$ | $0.55\pm 0.02$
43488669 | 3183786280737192704 |  | E | 207.954 | -28.688 | $3.87\pm 0.02$ | 12.73 | $5193\pm 117$ | $0.64\pm 0.04$
47390297 | 6607586870253814144 |  | F | 19.839 | -62.749 | $1.48\pm 0.05$ | 11.45 | $5997\pm 133$ | $2.29\pm 0.14$
50745680 | 3216722873103020288 |  | F | 206.566 | -19.14 | $3.5\pm 0.21$ | 15.4 | $3190\pm 161$ | $0.63\pm 0.03$
57830249 | 3478940625208241920 | TW Hydrae | E | 285.143 | 29.71 | $20.52\pm 0.1$ | 11.96 | $3085\pm 157$ | $0.49\pm 0.01$
67822046 | 6041816673812516864 | ScoOB2 | F | 347.056 | 16.905 | $6.57\pm 0.04$ | 12.49 | $3662\pm 122$ | $0.98\pm 0.12$
81198421 | 5519376920648594304 | Vel OB2 | F | 262.533 | -7.188 | $2.84\pm 0.02$ | 12.66 | $4655\pm 127$ | $1.3\pm 0.09$
124072515 | 5530952441620701440 | Vel OB2 | F | 259.267 | -10.03 | $2.56\pm 0.02$ | 12.99 | $4381\pm 126$ | $1.33\pm 0.11$
144697607 | 3336447113703238784 | Orion | F | 195.72 | -11.048 | $2.54\pm 0.1$ | 15.3 | $3290\pm 159$ | $0.71\pm 0.03$
167303776 | 5280226578890455168 | Carina | E | 278.41 | -26.817 | $10.67\pm 0.06$ | 14.15 | $3149\pm 157$ | $0.35\pm 0.01$
226241509 | 5944710143324257792 | ScoOB2 | F | 340.049 | 4.136 | $6.67\pm 0.05$ | 11.57 | $3954\pm 127$ | $1.32\pm 0.14$
243324939 | 6083123916915057664 | ScoOB2 | E | 310.949 | 14.543 | $7.93\pm 0.09$ | 13.61 | $3285\pm 158$ | $0.54\pm 0.02$
248473576 | 3213713956452945024 | Orion | E/T | 204.159 | -23.562 | $2.66\pm 0.12$ | 15.6 | $3214\pm 157$ | $0.61\pm 0.03$
248992945 | 3215349651798096768 | Orion | T? | 202.128 | -22.341 | $2.92\pm 0.03$ | 12.31 | $4564\pm 126$ | $1.39\pm 0.11$
266079454 | 5336376503118529536 | ScoOB2 | E | 293.79 | 2.527 | $9.75\pm 0.08$ | 14.22 | $3030\pm 157$ | $0.4\pm 0.01$
276070646 | 6107552866260657408 | ScoOB2 | E | 314.213 | 15.951 | $6.65\pm 0.16$ | 14.26 | $3196\pm 157$ | $0.5\pm 0.02$
276664304 | 3017275483925567872 | Orion | F | 209.697 | -19.194 | $2.55\pm 0.03$ | 12.97 | $6158\pm 180$ | $1.21\pm 0.07$
284730577 | 3291818722708659200 | AB Doradus | E | 190.329 | -17.322 | $10.53\pm 0.06$ | 9.25 | $5677\pm 131$ | $0.98\pm 0.05$
317873721 | 2995720039486652672 |  | F | 219.838 | -18.11 | $1.51\pm 0.04$ | 11.94 | $8211\pm 148$ | $1.67\pm 0.06$
323354855 | 5209023133585929984 | ScoOB2 | F/E | 292.586 | -21.437 | $10.14\pm 0.03$ | 10.1 | $4308\pm 150$ | $1.22\pm 0.13$
334999132 | 5843724192198636416 | ScoOB2 | E | 303.399 | -8.46 | $9.72\pm 0.05$ | 14.02 | $3018\pm 157$ | $0.46\pm 0.01$
361957000 | 5789045856890265216 | ScoOB2 | F | 303.692 | -13.981 | $5\pm 0.03$ | 11.49 | $4064\pm 129$ |
412308868 | 6061152101197951104 | ScoOB2 | E | 301.848 | 5.8 | $8.75\pm 0.05$ | 12.71 | $3496\pm 118$ | $0.74\pm 0.1$
419954392 | 6069015327310399104 | ScoOB2 | E | 309.034 | 10.088 | $7.29\pm 0.13$ | 14.8 | $3230\pm 159$ | $0.45\pm 0.01$
427334205 | 3209524233037903360 | Orion | F | 208.721 | -19.567 | $2.43\pm 0.03$ | 12.71 | $4777\pm 69$ |
427395967 | 3017187110667027200 | Orion | F | 209.595 | -19.513 | $2.63\pm 0.06$ | 15.37 | $5321\pm 319$ |
427399794 | 3217577258060423808 |  | F | 205.343 | -17.396 | $4.66\pm 0.53$ | 14.3 | $3473\pm 158$ | $0.6\pm 0.05$
434229695 | 3304172564776041728 |  | F | 181.268 | -27.171 | $3.4\pm 0.04$ | 11.34 | $5312\pm 147$ | $1.92\pm 0.11$
442549866 | 6011594294619304064 | ScoOB2 | E | 338.978 | 15.028 | $9.55\pm 0.09$ | 14.79 | $3114\pm 160$ | $0.36\pm 0.01$
452694529 | 5218060397614069120 | ScoOB2 | E | 290.321 | -15.185 | $7.39\pm 0.03$ | 12.73 | $3572\pm 115$ | $0.83\pm 0.11$
454365494 | 5201143758382913792 | Chamaeleon? | E | 297.654 | -15.686 | $5.29\pm 0.08$ | 14.39 | $3101\pm 124$ |
457231768 | 3221803265361341056 | Orion | F | 202.573 | -18.473 | $2.19\pm 0.08$ | 9.88 | $8785\pm 158$ | $1.96\pm 0.09$

Although all of the dippers we found showed IR excess possibly resulting from
disks, the extent of the excess is different, which means that each dipper has
different disk structure. We classified the dippers according to their IR
colors. Luhman & Mamajek (2012) classified the disk structure in the members
of Upper Scorpius (USco) with the color-color diagram. They used $K_{s}$ band,
which is dominated by emission from stellar photosphere, and $W3$ and $W4$
bands as emission from a disk. They corrected the extinction of $K_{s}$, $W3$
and $W4$ bands. Then, they subtracted the component of IR colors from stellar
photospheres assuming the same spectral type stars. The IR colors calculated
by this process are denoted by $E(K_{s}-W3),E(K_{s}-W4)$.

Using 2MASS and WISE data, we re-computed their $E(K_{s}-W3),E(K_{s}-W4)$
values in their catalog and compare them with those of our dippers. The
extinction was corrected as follows: we first obtained $A_{Ks}$ values from
the dust map mwdust (Bovy et al., 2016); then we calculated $A_{W3}$ and
$A_{W4}$ according to Xue et al. (2016). Figure 3 shows the classification of
the disk stages. The colored markers represent USco members with different
disk types and the black circles indicate our dippers. We estimated a typical
error of $E(K_{s}-W3)$ and $E(K_{s}-W4)$ by propagating typical errors of the
temperature given in the TIC and magnitude of 2MASS and WISE. Most of the
dippers are classified as a full or evolved system. As exceptions, TIC
248992945 and 24775707 appear not to belong any disk classes drawn in the
diagram. Because these two dippers exhibit a weak emission in $W3$ band and a
strong emission in $W4$ band, which implies that they have a large inner
cavity, we classified them as a transitional system. No dipper is classified
as a debris system. We summarize the classes in Table 1.

Figure 3.— IR color excess for classification of the disk type. The $x$ and
$y$ axes are the extinction-corrected IR excess $(K_{s}-W4)$ and $(K_{s}-W3)$
relative to the stellar photosphere. The colored markers are the members of
Upper Sco with different disk types listed in Luhman & Mamajek (2012). The
black points with labels indicate our dippers. We do not put labels for TIC
167303776, 276070646, 419954392, 37947642 and 454365494 in the crowded region,
but all of them are classified as an evolved system. The typical error
embedded in the panels is estimated with the typical errors of temperature in
TIC and magnitudes of 2MASS and WISE.

In Figure 4, we construct the Hertzsprung–Russell diagram of our dippers using
the Gaia DR2 photometry and parallax, where the absolute magnitude in the Gaia
$G$-band was calculated from the distance $r_{\rm est}$ estimated by Bailer-
Jones et al. (2018). The open circles show the values corrected for
interstellar extinction, and they are connected with lines to smaller dots
that show the values without the correction.

We attempted to correct for the extinction as follows: we first retrieved
$E(B-V)$ from the dust map mwdust (Bovy et al., 2016) and converted it to
$A_{V}=3.1E(B-V)$; we then used the effective temperature of each star and the
empirical relation in Danielski et al. (2018) to convert $A_{V}$ into the
total extinction in the $G$-band $A_{G}$; finally $A_{G}$ was converted to
$E(G_{\rm BP}-G_{\rm RP})$ interpolating the results by Jordi et al. (2010)
and assuming $G_{\rm BP}-G_{\rm RP}\approx(V-I_{C})_{0}$. In the left panel,
our dippers are compared to nearby stars within $50\,\mathrm{pc}$ from Gaia
DR2 (small gray dots), as well as the main sequence of the Pleiades cluster
(solid line) constructed by Hamer & Schlaufman (2019). In the right panel, the
dippers are compared to the stellar isochrones (Dotter, 2016; Choi et al.,
2016) computed with Modules for Experiments in Stellar Astrophysics (MESA;
Paxton et al., 2011, 2013, 2015) for ages of $1$, $10$, and
$100\,\mathrm{Myr}$ and for $\mathrm{[Fe/H]}=0$. Both of these comparisons
suggest that our dippers, as a whole, have luminosities and colors consistent
with low-mass stars younger than $\sim 100\,\mathrm{Myr}$. Here we do not
attempt to draw any further quantitative conclusion because the accuracy of
the dust map may be limited for those stars in star forming regions and
because some of our dippers might exhibit strong circumstellar extinction that
we did not take into account. That said, the above qualitative conclusion
would remain unchanged even if we underestimate $A_{\rm G}$ by $\sim 1$ due,
for example, to ignoring circumstellar extinction (Ansdell et al., 2016b).

Figure 4.— The TESS dippers on the HR diagram. The open circles show the
absolute magnitudes in the Gaia $G$-band against Gaia colors both corrected
for extinction, and they are connected to the values without correction
(smaller dots) with solid lines; see Section 3 for details. Left — Comparison
with stars within $50\,\mathrm{pc}$ from the Gaia DR2 (gray dots), as well as
the main sequence of the Pleiades cluster (black solid line). Right —
Comparison with MESA isochrones for the ages of 1 Myr (dotted line), 10 Myr
(dashed line), and 100 Myr (solid line).

Figure 5 shows the spatial distribution of the dippers in galactic
coordinates. The survey area is shown by yellow rectangles. The markers with
arrows indicate our dippers with proper motion, colored based on the
classification of the disk type. While most of the dippers are located in the
star forming regions as indicated by red (nearby molecular clouds; Zucker et
al., 2019) and pink (nearby OB association; de Zeeuw et al., 1999) rectangles,
some of them are far from any star forming regions. Moreover, some dippers
have high proper motion as shown by gray arrows.

Figure 5.— The distribution of the TESS dippers on the galactic coordinate.
Each color of dipper represents disk type classified by Figure 3. The
Evolved/Transitional dipper is colored by blue as well as well as Full/Evolved
disk types. The red and pink rectangles show the approximate locations of
nearby star forming regions and OB associations. The names of OB associations
are labeled in this figure. The yellow area indicates the survey region in the
first-year data of TESS. The gray arrows indicate tangential velocities of the
dippers. The length of gray arrows is proportional to the tangential
velocities ($\,\mathrm{km\ s}^{-1}$) calculated from Gaia DR2 data. The arrow
corresponding to 10$\,\mathrm{km\ s}^{-1}$velocity is shown in at the bottom.

##  4\. Characterization of individual stars

Next, we analyzed the individual dippers to determine their memberships and
ages. We mainly used kinematic data of their positions, parallaxes, and proper
motions obtained from Gaia DR2 data to determine their memberships. Dividing
the dippers into several areas according to their positions (Sco OB2 and
Chamaeleon, Vel OB2, Orion, and Field), we compare their kinematics with that
of the stars that associates with known clusters. we additionally performed
follow-up observations of four out of the six dippers near the Orion (TIC
434229695, 284730577, 317873721, and 43488669), and also TIC 34397579 and TIC
457231768 with high-dispersion spectra obtained by High-Dispersion
Spectrograph (HDS Noguchi et al., 2002) installed on the Subaru telescope. The
observation was performed on UT 16 September 2019 with the non-standard setup
and a 2 $\times$ 1 binning without the image rotator. We use the image slicer
(IS) #2 with a spectral resolution of about 80,000. The non-standard setup of
the wavelength range includes H$\alpha$ and lithium 6707 Å line as an
indicator of YSOs and also Mg triplet lines (5165–5185 Å) for estimates of
stellar temperature, log $g$, and radial velocity. The data were reduced by
the standard procedure (the bias subtraction, flat fielding, order
tracing/extraction, and wavelength calibration), which yields one-dimensional
spectra with a signal-to-noise ratio of 80–100 per pixel at 6563 Å.

###  4.1. Sco OB2 and Chamaeleon

We found 13 dippers near the Scorpius-Centaurus OB Association (Sco OB2) and
Chamaeleon complex (Chamaeleon). Sco OB2 is the nearest OB association to the
Sun. Sco OB2 consists of three large sub-associations, Upper Scorpius (USco),
Upper Cen-Lup (UCL), and Lower Cen-Crux (LCC). Their mean ages were estimated
by Pecaut et al. (2012), $\sim 11$Myr old for US, which is the youngest part
of Sco OB2, $\sim 16$Myr and $\sim 17$Myr for UCL and LCC. Thanks to Gaia
mission, today, more than 10,000 pre-main sequence members for them have been
discovered (Damiani et al., 2019). The dippers, TIC 67822046, 226241509,
266079454, 276070646, 334999132, 412308868, 419954392, 442549866, and
243324939 are located close to Sco OB2 (shown in Figure6 as red circles).

Chamaeleon is one of the nearest star forming regions and consists of three
sub regions, Chamaeleon I (Cha I), Chamaeleon II (Cha II), and Chamaeleon III
(Cha III) 111Because of the lack of multi-band photometric data, Zucker et al.
(2019) do not determine a precise area of Cha III. Therefore, we did not
include Cha III in Figure 5.  The catalog of Cha I members is provided by
Luhman (2007). They calculated the isochrone ages of southern and northern
clusters of Cha I as 3–5 and 4–6 Myr. TIC 323354855, 361957000, 452694529 and
454365494 are located close to Chamaeleon (shown in Figure 6 as orange
circles). We note that we do not plot the members of Cha II and Cha III
because no sufficient information on the population has been reported yet.

Checking the population catalogs of Sco OB2 and Cha I by Damiani et al. (2019)
and Luhman (2007), we find that TIC 67822046, 226241509, 266079454, 276070646,
334999132, 412308868 and 243324939 are known as the members of Sco OB2. The
others are not listed in these catalogs.

Figure 6.— The galactic positions of Sco OB2 members, Cha I members, and the
13 dippers located near these systems. The blue and green dots indicate the
members of Sco OB2 and Cha I listed in Damiani et al. (2019) and Luhman
(2007). The red and orange points are dippers located close to Sco OB2 and Cha
I in the celestial plane. Figure 7.— The distance distribution for members of
Sco OB2 and Cha I. The red and orange lines represent the distance of dippers
whose colors inherits Figure 6. We only show TIC IDs for dippers close to Cha
I. The black vertical lines represent the average distances of ChaII and
ChaIII.

To identify the memberships of the other six dippers not listed in these
catalogs, we analyzed their distances and kinematic properties using Gaia DR2
data. The fundamental astrometric parameters (positions, parallaxes, and
proper motions) provided Gaia DR2 enabled us to well determine the memberships
from the dippers’ positions and kinematic natures. Figure 7 shows the
distributions of the distances in the Sco OB2 (blue) and Cha I (green)
catalogs. The dippers close to Sco OB2 and Chamaeleon in the celestial plane
are indicated by the red and orange lines. Because Damiani et al. (2019)
surveyed only Sco OB2 members whose parallaxes are larger than 5, the outer
steep edge of Sco OB2 shown in Figure 7 can be explained by the selection
bias. Note that they have also searched for the members of Sco OB2 whose
parallaxes are $3.3<\pi<5$ mas and found that no significant presence of Sco
OB2 members beyond 200 pc. We also indicate the average distances of Cha II
and Cha III as the black lines. Their distances ($d_{ChaII}$ and $d_{ChaIII}$)
are estimated by Voirin et al. (2018) as $181^{+11}_{-10}$ pc and
$199^{+12}_{-11}$ pc. From the distances, all the dippers close to Sco OB2,
TIC 323354855 and 452694529 are consistent with the memberships of Sco OB2.
TIC 361957000 and 454365494 can belong either to Sco OB2 or to Chamaeleon.

Figure 8.— The distribution of proper motion and tangential velocities of Sco
OB2 (blue), Cha I (green), and the 13 dippers (red and orange) located near
these systems. The right and left panels have the different unit of the
velocities corresponding to milliarcsecond $\mathrm{(mas)\,year^{-1}}$ and
$\,\mathrm{km\ s}^{-1}$. In the right panel, we only show TIC ID of dippers
located close to Cha I in the celestial plane. The color of each star inherits
Figure 6.

Figure 8 shows the distributions of the proper motions and tangential
velocities in the Sco OB2 (blue) and Cha I (green) catalogs. The kinematics
also shows that all the dippers close to Sco OB2 are consistent with the
memberships of Sco OB2. TIC 452694529 and 361957000 are slightly separated
from Cha I but well agrees with Sco OB2. Because TIC 323354855 and 454365494
are consistent with both clusters, we cannot determine their memberships only
by the kinematics. Considering their positions, distances and kinematic
nature, we conclude that TIC 454365494 is possibly a member of Chamaeleon and
other dippers near the Sco OB2 and Chamaeleon are members of Sco OB2.

###  4.2. Vel OB2

Vel OB2 is the OB association and contains $\gamma^{2}$ Velorum, which is a
spectroscopic binary composed of a massive O star and a Wolf-Rayet star(North
et al., 2007). Using the Gaia DR2 data, Beccari et al. (2018) reported that
Vel OB2 consisted of six kinematic clusters with the different ages.

Figure 9.— Similar as the left panel of Figure 8, but for Vel OB2. The two
dippers are shown by red and purple points. The green ellipses with numbers
correspond to approximate cluster regions defined by Beccari et al. (2018).
Cluster 5 is excluded because of the large variation of the proper motion.

We identified the star clusters proposed by Beccari et al. (2018) for each
dipper in the distribution of the proper motion. Because there are no member
catalogs of each cluster, we initially made the distribution of proper motions
near Vel OB2 using Gaia DR2 catalog. The range we used is the right-ascension
of $116<\alpha<126$ degree and declination of $-50<\delta<-44$ degree and the
range of parallax $\pi>2$ mas because the average distance of Vel OB2 is much
closer than 500 pc. Furthermore, We exclude the sources whose proper motion
values were not measured.

Figure 9 shows the proper motions of 23,240 stars identified by the procedure
described above (blue dots). The green ellipses are approximate areas of the
clusters Beccari et al. (2018) reported. Cluster 5 is excluded because of the
large variation of the proper motion. The proper motions of TIC 81198421 and
TIC 124072515 are consistent with the memberships of Cluster 1 and Cluster 4.
The sky positions and distances are also consistent with these
classifications. Using the isochrone analysis, Beccari et al. (2018) derived
the mean ages of Cluster 1 and 4 as about 10 Myr old. This age is also
consistent with the presence of the primordial disk in TIC 81198421 and TIC
124072515.

###  4.3. Around the Orion

Figure 10.— The dippers near the Orion. Each color of dipper represents the
disk type classified by the color-color diagram (Fig 3). The background
picture is synthesized by the images of Digitized Sky Survey and AKARI.

We found 17 dippers around the Orion as shown in Figure 10. Some of them are
close to the Orion molecular cloud complex, while some are relatively far from
it. First, we focus on the following eleven systems, TIC 24775707, 34397579,
50745680, 248992945, 276664304, 457231768, 248473576, 427399794, 427334205,
427395967, and 144697607. These are located near the Orion molecular cloud
complex. The remaining six systems, TIC 284730577, 317873721, 37947642,
38044097, 434229695, and 43488669, are relatively far from the Orion molecular
cloud complex, which means that these six systems do not belong to the Orion
molecular cloud complex.

We note that comprehensive analyses of spectral features will be presented in
forthcoming papers while we present several parameters from the spectral
analysis for individual objects in this paper.

####  4.3.1 Orion complex

Figure 11.— Distributions of positions, distances and proper motions of
members of Orion A(green), Orion B(orange), Orion C(cyan), Orion D(red), and
$\lambda$ Ori(blue) listed in Kounkel et al. (2019). TIC 427399794 and
50745680 (bold) do not belong to any clusters.

Orion complex is one of the largest star forming regions near the Sun.
Kinematic structure of Orion complex was studied by Kounkel et al. (2018)
using Gaia DR2 data. They revealed that Orion Complex had five main kinematic
groups, Orion A and B, which are younger parts harboring molecular gases,
Orion C and D, which are older parts with little molecular gases, and
$\lambda$ Orion, whose gas has mostly been dissipated by a supernova. Based on
the population catalog of the clusters (Kounkel et al., 2019) (hereafter K19
catalog), we found that TIC 276664304 and 427334205 belongs to Orion A, TIC
457231768 to Orion C, TIC 24775707, 34397579 and 248992945 to Orion D, and TIC
144697607 to $\lambda$ Orion. To identify the memberships of the rest of the
dippers, we checked the spatial distributions of the stars listed in the
catalogs and the rest of the dippers. Figure 11 shows the distribution of the
positions, parallaxes and proper motion in the direction of right ascension
and declination. The figure shows that TIC 427395967 and 248473576 are likely
the members of Orion A and D. However, TIC 427399794 and 50745680 are isolated
from the clusters. These two dippers whose membership we could not identify
are possibly slowly escaping from their birth place, were born in an isolated
small cloud, or have already dissipated birth places. Radial velocity
measurements of these dippers will enable further discussion.

We estimated the ages of the nine dippers whose membership we were able to
identify. TIC 427395967 and 248473576 are not listed in K19 catalog. We find
that the following seven dippers are listed in K19 catalog as the member of
sub-groups: TIC 276664304 and 427334205 as ONC-2 ($\sim 2.9$ Myr), TIC
457231768 classified as Ori C north-3 ($\sim 4.8$ Myr), TIC 34397579 as Ori D
south-1 ($\sim 3.0$ Myr), TIC 24775707 as Ori D south-5 ($\sim 5.0$ Myr),
248992945 as L1616-4 ($\sim 7.9$ Myr), and TIC 144697607 as B35-1 ($\sim 3.0$
Myr). These results are consistent in that TIC 24775707 and 248992945,
classified as relatively older sub-groups, have a transitional disk, whereas
the others have a full disk.

####  4.3.2 TIC 284730577

TIC 284730577 (HD 240779) is located at a distance of about 0.5 degree from
the dippers in the Orion complex (see Fig 10). This system is a quasi-periodic
variable star with infrared excess reported by Gaidos et al. (2019). This
system is also the first field dipper found in the TESS data. We recover this
known dipper by our pipeline. Gaidos et al. (2019) reported that TIC 284730577
was a member of AB Doradus moving group with an age of about 125 Myr, which is
consistent with the observation of lithium and rotational period. The
equivalent width of the lithium from our observation is consistent with the
value (0.12 Å) reported by Gaidos et al. (2019).

####  4.3.3 TIC 37947642 and 38044097

TIC 37947642 and 38044097 are located far from any star forming regions near
the Orion (Fig.10) except for very wide area of Cassiopeia–Taurus OB
association (Cas–Tau; see Fig. 5). Cas–Tau is a young association covering a
very large area on the celestial plane and the range of distance is also wide
(125–300 pc de Zeeuw et al., 1999). Cas–Tau is regarded as a physical group
and has the same origin as $\alpha$ Persei cluster. Using the membership
catalog of Cas–Tau (de Zeeuw et al., 1999) and Gaia DR2 data, we estimated the
tangential velocity distribution of Cas–Tau as $V=25\pm 6$ $\,\mathrm{km\
s}^{-1}$.222We note that we exclude HIP 2377 from Cas-Tau member because its
parallax is much smaller than other members. Although the sky positions of TIC
37947642 and 38044097 are in the Cas–Tau region, these are likely not to
belong to Cas–Tau because of much smaller tangential velocities ($\sim 5$
$\,\mathrm{km\ s}^{-1}$) compared with those of Cas–Tau.

The distances of TIC 37947642 and 38044097 are much larger than 100 pc.
Because all the well-known young moving groups are located at less than 100
pc, we cannot identify the memberships of them. However, the dimming events
with the IR excess supports that TIC 37947642 and 38044097 are young stars
with circumstellar disks. The fact that their proper motions go toward almost
the same direction and their sky positions are very close supports that TIC
37947642 and 38044097 could belong to a unknown common young moving group.

####  4.3.4 TIC 43488669

TIC 43488669 is far from any star forming regions except for Cas–Tau as shown
in Figure 10. Because its tangential velocity is very high ($\sim 55$
$\,\mathrm{km\ s}^{-1}$), this system surely does not have Cas–Tau membership.
While we detected an absorption feature of a H$\alpha$ line, we also detected
the lithium absorption line. The spectrum also exhibits the high radial
velocity of $\sim 46$$\,\mathrm{km\ s}^{-1}$. Its high velocity supports the
fact that TIC 43488669 is a runaway star. We will revisit this system in
Section 5.

####  4.3.5 TIC 434229695 and 317873721

TIC 434229695 is also far from Orion molecular cloud complex and Taurus
molecular cloud as shown in Figure 10. Gaia measured the radial velocity of
TIC 43422969, so we can calculate its galactic UVW space motion. The galactic
UVW space motion, $(UVW)=(-8.93,-11.51,-3.64)$$\,\mathrm{km\ s}^{-1}$does not
matche to any young moving groups and associations even to Cas–Tau
($(UVW)=(-13.24,-19.69,-6.38)$$\,\mathrm{km\ s}^{-1}$). In addition to the IR
excess, the high-resolution spectrum shows a clear H$\alpha$ emission and also
a lithium absorption, supporting the fact that TIC 43422969 is a young stellar
object with an accreting disk. TIC 43422969 has also been identified as the
T-Tauri star candidate based on the infrared satellite survey, AKARI (Takita
et al., 2010).

TIC 317873721 is far from any star forming regions. We detected an inverse
P-Cyg type of H$\alpha$ emission line. However the temperature is so high
($\sim 8,000$ K as shown in Table.1) that we do not detect the lithium
absorption line, which is one of the clues to determine its age. We will
present the detailed analysis of the spectral features in a forthcoming paper.

###  4.4. Dippers in the Field

Three dippers (TIC 57830249, 47390297 and 167303776) are located far from any
nearby molecular clouds and OB associations. We tried to identify their
memberships in the following sections.

####  4.4.1 TIC 57830249

TIC 57830249 was previously reported as TWA 33 by Schneider et al. (2012).
They reported that TWA 33 was a member of TW Hydrae Association (TWA) based on
its position, proper motion, spectral signature, distance estimate, and a mid-
IR excess. TWA is the nearest association of very young low-mass stars
(usually classified as T association), whose distance is within 100pc. TWA 33
exhibits the infrared excess (Schneider et al., 2012), but optical variability
has not been reported yet. The age of TWA 33 was estimated as $10\pm 3$ Myr
(Bell et al., 2015), being consistent with a primordial disk structure.
Schneider et al. (2012) measured equivalent width of H$\alpha$ emission line
(EWH$\alpha$ = $5.8$Å) and spectral type (M$4.7\pm 0.5$) with the
spectroscopic observation. According to White & Basri (2003), who proposed
empirical criteria for distinguishing classical T-Tauri stars (CTTSs) and
weak-line T-Tauri stars (WTTSs) using equivalent width of H$\alpha$ emmision
line, TWA 33 is classified as WTTS. Our classification of TIC 57830249 as
evolved system supports this results. This indicates that TIC 57830249 has no
accretion signature.

####  4.4.2 TIC 47390297

TIC 47390297 is quite far from any star forming regions. Its large distance
(676 pc) prevents us from identifying membership of any young moving groups.
From Gaia DR2 data, TIC 47390297 has very high proper motion ($V=\sim
40$$\,\mathrm{km\ s}^{-1}$). This velocity is above galactic deviation near
the Sun ($\sigma V_{gal}=\sim 30$$\,\mathrm{km\ s}^{-1}$), TIC 47390297 is
possibly a runaway star.

####  4.4.3 TIC 167303776

TIC 167303776 is located near the south ecliptic pole and also far from any
star forming regions. The direction of proper motion is different from the
other dippers close to Sco OB2, which implies TIC 167303776 is not a member of
Sco OB2. Using its position, parallax and proper motion data provided Gaia
DR2, we examined possibility of any young moving group by BANYAN $\Sigma$
(Gagné & Faherty, 2018). BANYAN $\Sigma$ is an online Bayesian classifier to
statically determine the memberships of stars to a nearby young moving group.
Based on the stellar position, parallax and proper motion, BANYAN $\Sigma$
computes the membership probability of each young moving group even without
radial velocity data.

From the results of BANYAN $\Sigma$, TIC 167303776 has a membership of Carina
association with $80.4\%$ probability, as well as field probability is
$19.3\%$ and a probability of Carina-near association moving group is $0.4\%$.
Because its probability are less than $90\%$, Gagné & Faherty (2018) have not
reported TIC 167303776 as a Carina member. Because radial velocity data is not
yet available, we do not fully identify its membership although the infrared
excess indicates TIC 167303776 is young, and it is consistent that TIC
167303776 belongs to the young moving group. However, TIC 167303776 is
potentially a member of Carina association with a mildly high probability.

Bell et al. (2015) calculated the isochronal ages for young moving groups in
the solar neighbourhood. They determined the age of Carina association as
$45^{+11}_{-7}$ Myr. According to the discussion so far, the age of TIC
167303776 is at least 38 Myr. Comparing the classical dissipation time scale
of primordial disk ($\sim 10$Myr), TIC 167303776 is much older and it is
unlikely to still have a primordial disk structure. Like HD 240779, the
infrared excess and dimming event of TIC 167303776 is potentially explained by
collisions of planetesimals in the debris disk.

##  5\. Runaway old dipper: TIC 43488669

Among 35 dippers that we found, TIC 43488669 is of particular interest; it was
subjected to follow-up observations because of its large proper motion and
radial velocity. Moreover, the infrared excess indicates that TIC 43488669 is
between class II and class III. The age of TIC 43488669 is much older than
that of most dippers found near the star forming regions. In this section, we
discuss the runaway old dipper, TIC 43488669.

###  5.1. Light curve of TIC 43488669

Figure 12.— Light curves of TIC 43488669 from TESS (solid line) and ASAS-SN
(error bars).

The light curve of TIC 43488669 shown in Figure 12 exhibits several asymmetric
dips with tails. The tail feature is similar as ones found in the evaporating
planet candidates KIC 12557548b (e.g. Rappaport et al., 2012; Kawahara et al.,
2013) and exocomets (e.g. Rappaport et al., 2018; Zieba et al., 2019) but the
depth of dips TIC 43488669 shows is much deeper an order of magnitude than
both. The tail in TIC 43488669 is also likely due to a dusty tail by radiation
pressure. However, further study is required for unraveling what produces the
tail. We also confirm that the dips are in the ASAS-SN light curve (Shappee et
al., 2014; Kochanek et al., 2017). The ASAS-SN light curves well agree with
the TESS light curve and we found that TIC 43488669 continuously exhibits such
dips at least in several year scale. As will be explained in the following
subsections, from the lithium line and IR excess, this system is likely to
have a debris disk and the age is about 100–400 Myr. Considering the tail
feature and no gas disk, the dips are likely to be produced by asteroids or
debris in a disk.

###  5.2. Stellar Properties

We obtained stellar parameters by fitting the stellar model to the high
dispersion spectrum around the Mg triplet by Subaru/HDS (Noguchi et al., 2002)
as shown in Figure 13. The parameters were derived using the public code,
specmatch-emp (Yee et al., 2017), the stellar temperature
$T_{\mathrm{eff}}=5080\pm 100$ K, The stellar radius = 0.71, and the
metallicity of [Fe/H]= $-0.53\pm 0.09$. Also, the radial velocity is $46\pm 1$
$\,\mathrm{km\ s}^{-1}$. We also measure the equivalent width (EW) of the
lithium 6,708.6 Å line with the Gaussian model and obtain
$\mathrm{EW_{Li}}=56\pm 4$ mÅ from the spectrum of TIC 43488669. Following
Gaidos et al. (2019), we estimated the age of the system using the EW of
lithium based on known young clusters near the Sun. Measurement of Hyades, UMa
and Pleades are from Thorburn et al. (1993), and Ammler-von Eiff & Guenther
(2009) Barrado et al. (2016). Measurement of other clusters are from Mentuch
et al. (2008). Compared with the stars associated with known clusters, the age
of TIC 43488669 is estimated as 100–400 Myr. We note that the depletion rate
of lithium in pre-main sequence stars is strongly dependent on metallicity
while the depletion rate in the main sequence star is insensitive to
composition: Metal-poor pre-main sequence stars deplete more slowly than
metal-rich ones (Somers & Pinsonneault, 2014). In this sense, TIC 43488669 is
potentially older than 100–400 Myr because of its poor-metalicity. However, it
is difficult to estimate the age with consideration of this effect. Therefore,
we assume 100–400 Myr as a minimal value in the following discussion.

Figure 13.— The high-dispersion spectrum of TIC 43488669 around the Mg-triplet
(black). The red curve is the best-fit model by specmatch-emp (Yee et al.,
2017).  Figure 14.— The equivalent width of lithium 6707 Å line, following
Gaidos et al. (2019). The black dot with the error bar indicates TIC 43488669.

###  5.3. Orbit from 3 dimensional velocity

We noted that TIC 43488669 shows a large 3D velocity ($\sim 78\,\mathrm{km\
s}^{-1}$) relative to the Sun. To understand the origin of this star, we
computed its orbit in the past. First, we evaluated the heliocentric position
and velocity of TIC 43488669 by using the astrometric data from Gaia and the
measured radial velocity of $v_{\mathrm{los}}=56\,\mathrm{km\ s}^{-1}$. Given
the high-quality data, we did not take the observational error into account.
Then, we estimated the Galactocentric positon and velocity of this star to be
$(x,y,z)=(-8.32,-0.11,-0.12)\,\mathrm{kpc}$ and
$(v_{x},v_{y},v_{z})=(-14.87,214.96,-74.56)\,\mathrm{km\ s}^{-1}$,
respectively, by assuming that the Galactocentric position and velocity of the
Sun is given by $(x_{\odot},y_{\odot},z_{\odot})=(-8.112,0,0)\,\mathrm{kpc}$
(Gravity Collaboration et al., 2018) and
$(v_{x,\odot},v_{y,\odot},v_{z,\odot})=(11.1,245.31,7.25)\,\mathrm{km\
s}^{-1}$ (Reid & Brunthaler, 2004; Schönrich et al., 2010), respectively.
Based on the current position and velocity of this star, we computed the orbit
of this star in the last $300$ Myr in a realistic model potential of the Milky
Way given by McMillan (2017).

We note that, if we assume that the Local Standard of Rest (LSR) velocity is
$233.09\,\mathrm{km\ s}^{-1}$ (which corresponds to the circular velocity of
our model potential at the Sun), the 3D velocity of TIC 43488669 relative to
the LSR is $(U,V,W)=(-14.87,-18.13,-74.56)\,\mathrm{km\ s}^{-1}$.

The orbit in the Galactocentric $(x,y)$\- and $(x,z)$-planes is shown in
Figure 15. We note that this star shows a nearly circular orbit in
$(x,y)$-plane with an eccentricity of $e=0.06$, but shows a large excursion
above and below the Galactic plane with the maximum excursion of
$z_{\mathrm{max}}=2\,\mathrm{kpc}$. This large excursion is unusual given the
young age of this star ($\sim 100$–$400$ Myr from the Lithium line), which
hints that this star is a runaway star ejected from its birthplace. The orbit
passes through the Galactic plane every $\sim 75$ Myr. Because of the near-
circular nature of the in-plane motion, the vertical motion of this star is
well decoupled from its in-plane motion. Consequently, this star passes
through the disk plane with a velocity of $\sim 80\,\mathrm{km\ s}^{-1}$
relative to the local circular velocity in the last $300$ Myr. This indicates
that, if this star is a runway star ejected from the Galactic plane, its
ejection velocity can be estimated to be $\sim 80\,\mathrm{km\ s}^{-1}$. This
estimate of the ejection velocity is not sensitive to the age of this star or
when it was ejected as long as the ejection happened at the disk plane.

Figure 15.— Orbit of TIC 43488669 in the last 300 Myr. The left and right
panels indicates the trajectory on the Galactic $(x,y)$-plane and
$(x,z)$-plane. The current location of the Sun corresponds to
$(x_{\odot},y_{\odot},z_{\odot})=(-8.122,0,0)\,\mathrm{kpc}$. To guide the
eye, the disk-crossing locations are marked with blue thick region.

If we assume that the system was ejected from a star cluster near the Galactic
plane (indicated by blue), the ejection velocity should be $\sim
80\,\mathrm{km\ s}^{-1}$ toward almost $z$-direction. Such a high velocity is
expected by dynamical ejection of stars from star clusters (Banerjee et al.,
2012). Thus, the large ejection velocity supports the scenario that TIC
43488669 is a runaway star ejected from its birth cluster a few hundred Myr
ago.

##  6\. Discussion

###  6.1. The dippers whose memberships are unidentified

Table 2The dippers whose memberships are not identified in the paper.  TIC ID | PM$>30$$\,\mathrm{km\ s}^{-1}$ | Disk | H$\alpha$ | Origins
---|---|---|---|---
43488669 | ✓ | E | absorption | runaway
47390297 | ✓ | F | N/A | runaway?
38044097 |  | E | N/A | moving group?
37947642 |  | F/E | N/A | moving group?
50745680 |  | F | N/A | close to SFR
427399794 |  | F | N/A | close to SFR
317873721 |  | F | emission |
434229695 |  | F | emission |

In the previous section, we showed a possible scenario of the origin of the
runaway dipper, TIC 43488669: The system was ejected from a birthplace, likely
located at a star cluster on the galactic plane in several Myr ago. From the
IR excess and the age estimated from the lithium line, we determined that it
was the debris disk part of its evolution stage. Considering the evolution
stage and the tail-like features in the light curve, the dips were likely
produced by dust, asteroids or material formed in the debris disk.

Besides TIC 43488669, the membership for seven dippers was not associated with
known nearby star forming regions, and moving groups in this paper. Table 2
summarizes their properties. Some of them (TIC 38044097 and 37947642) will
potentially remain unidentifiable due to lack of the catalogs of moving
groups. However, the rests are likely stars that exists outside of any star
forming regions, or moving groups.

The ejection mechanism discussed in §4 can explain the escape of such systems
from their birthplace. TIC 47390297, which exhibits high proper motion ($\sim
40$ $\,\mathrm{km\ s}^{-1}$), might be a runaway dipper. Also, TIC 50745680
and 427399794 are located near the Orion clusters and have small proper
motions. The sky positions and distances of these two dippers can be
attributed to the slow escape from their birthplaces. However, TIC 434229695
and 317873721, which also have small proper motion but clearly show H$\alpha$
emission lines, cannot be attributed to a runaway process from their
birthplace because their location are far from any nearby star forming
regions. A birth in a small clump like “Bok globule” or “ quick” dissipation
of clouds can explain their origins. These results are also indicative of the
YSO distribution because only parts of the T-Tauri stars exhibit dips in their
light curve. Our results imply that it is likely that there are numerous YSOs
outside the star forming regions, which have been overlooked thus far. The IR
excess with astrometric information by Gaia DR2 can be used to find such
isolated YSOs. Indeed, TIC 43422969 was already been identified as a T-Tauri
star candidate, from the analysis of the infrared catalog by AKARI (Takita et
al., 2010), located outside the star forming regions.

###  6.2. Occurrence of the debris dipper in field

In our survey, we discovered three old dippers (TIC 43488669, 167303776 and
284730577) whose age are $>10$ Myr. Since the typical dissipation timescale of
disk is 10 Myr, these systems are unlikely to have a primordial disk and the
IR excess originates from a secondary disk, namely a debris disk. We call such
stars “debris dipper” here although the term of “dipper” is usually used for a
pre-main sequence star whose light curve exhibits dips.

Figure 16.— IR color-color diagrams for classification of the disk type
similar as the Figure 3. The newly plotted red points are the extreme debris
disks reported by Meng et al. (2015) and RZ Psc. The left panel shows the IR
color at the $W3$ band relative to the $W4$ band and the right panel indicates
the IR color at the $W2$ band relative to the $W4$ band. All of the systems
exhbit a strong IR excess relative to the other debris disks (brown diamonds).

Figure 16 shows the debris dippers on the color-color diagram, same as Figure
3. These debreis dippers have a similar excess as that observed in the evolved
disks. However, the debris dippers cannot be classified as the evolved or full
disk systems owing to their old ages. It is known that about $1\%$ of the
debris disk has a high infrared excess due to a warm circumstellar dust (Balog
et al., 2009; Kennedy & Wyatt, 2013). Such debris disks are dubbed “extreme
debris disks”, defined by the star whose disk luminosity is roughly 1 % larger
than the stellar luminosity. Then, the IR excess of these three debris dippers
should be classified as the extreme debris disk (see section 3). We also plot
six known extreme debris disk hosts, P1121 (80 Myr), HD 15407A (80 Myr), HD
23514 (120 Myr), HD 145263 (10 Myr), BD+20 307($\gtrsim 1$ Gyr), and ID8 (35
Myr) listed in Meng et al. (2015), and RZ Psc which is one of the extreme
debris disk candidates (de Wit et al., 2013). We also compute a $W2$ color
excess ($E(K_{s}-W2)$) by the same procedure described in Section 3, and plot
them against $W4$ color excess. All the extreme debris disks and the debris
dippers exhibit a strong IR excess at $W3$ and $W4$ bands, compared to the
regular debris disk hosts (brown diamonds) regardless of their ages. At the
$W2$ band, most of the extreme debris disks and debris dippers have large
dispersion, but, exhibits a larger excess than those of the regular debris
disks. These results suggest that the debris dippers we found should be
classified as the extreme debris disk. TIC 167303776 exhibits a relatively
weak IR excess at $W3$ respect to $W4$ compared to the other extreme debris
disks, which is possibly owing to the weak dust heating; the effective
temperature of TIC 167303776 listed in TICv8 (3149 K) is lower than any other
systems ($>5000$ K).

Su et al. (2019) reported the variability in infrared starlight of the two
extreme debris disks, ID8 and P1121. However photometric variability of the
extreme debris disk in the optical band is rare. RZ Psc was believed to be the
one of extreme case but its revised age ($20_{-5}^{+3}$ Myr; Potravnov et al.
(2019)) challenges the claim that the system has a debris disk. The revised
age rather supports that the IR excess originates from a protoplanetary disk.
Another example is HD 240779 (TIC 284730577) (Gaidos et al., 2019), which is
also identified as the dipper by our pipeline. The high infrared excess with
the age of $125$Myr old supports that HD 240779 has an extreme debris disk.
They suggested that an occultation of a dust cloud as an origin of dips, which
is probably produced by a rapid disintegration or evaporation of
planetesimals. However the scenario of dimming event is poorly understood. To
constrain the dimming scenario, the occurrence of such a debris dipper is
required.

Here, we derive the occurrence of the debris dipper to test the consistency
with the geometric occultation probability of the dust clouds. First, we
estimated the number of the systems in CTLv8 whose age is less than 500 Myr.
CTLv8 consists of a combination of main-sequence stars whose magnitude in the
TESS band is lower than 13 and the stars selected from the Cool Dwarf List
(Stassun et al., 2019). Therefore, CTLv8 exhibits a bimodal mass distribution.
We divided the stars in CTLv8 into high- and low-mass groups by the threshold
of $M=0.68M_{\odot}$. The median masses of the two groups are
1.18$\mathrm{}{M_{\odot}}$ and 0.53 $\mathrm{}{M_{\odot}}$. Then, we estimated
a mean lifetime $\tau_{ms}$, 5.2 Gyr for the high-mass group and 121 Gyr for
the low-mass group, using the following relationship

$\tau_{ms}=\frac{M}{L}\approx 10^{10}\left(\frac{M}{M_{\odot}}\right)^{4}.$
(1)

For the high-mass group, we estimated the number of the stars whose age is
less than 500 Myr assuming a uniform distribution from 0 to 5.2 Gyr. However,
the mean lifetime of low-mass group is much larger than the age of the
universe. Instead, we assumed a uniform distribution of the stellar age from 0
to 10 Gyr based on the age distribution near the sun (Rendle et al., 2019) to
derive the number of the stars with $<500$ Myr. From this estimate, the
approximate number of the stars whose age is less than 500 Myr is $4\times
10^{5}$.

The occurrence ratio of extreme debris disks to young post-main sequence stars
($<120$ Myr) is estimated as about $1\%$ (Kennedy & Wyatt, 2013). Assuming
that this ratio is constant up to 500 Myr, we derived the approximate number
of the extreme debris disk in CTLv8 as $4\times 10^{3}$.

We assumed that we did not overlook most of the dimming events by field stars
in our pipeline. Note that we are likely to overlook some dippers in star
forming regions because stars in dense clusters are highly contaminated by
nearby stars, which obscures their dimming events. The contamination by nearby
stars is less severe for the stars in the filed than those in star forming
regions. The three debris dippers in $4\times 10^{3}$ extreme debris disks
means that the probability of variability in extreme debris disks is about
$p\sim 0.1\%$.

In addition, we assumed that all the occultation in extreme debris disks are
caused by dust clouds and each dip is caused by a single dust cloud in a
circular orbit. The semi-major axis of a dust cloud with a half of transit
duration $d_{\mathrm{half}}$ is written as

$a=\frac{GMd_{\mathrm{half}}^{2}}{R^{2}},$ (2)

where $G$ is the gravitational constant, $M$ and $R$ is the stellar mass and
radius. Using equation (2), the transit geometric probability
$p_{\mathrm{geo}}\approx R/a$ is estimated as

$p_{\mathrm{geo}}\sim
0.01\left(\frac{\rho}{\rho_{\odot}}\right)^{-1}\left(\frac{d_{\mathrm{half}}}{4\,\mathrm{hour}}\right)^{-2},$
(3)

where $\rho$ is the stellar density and $\rho_{\odot}$ is the solar value of
$\rho$ and we adopt the typical half duration time of the dip of $4\pm 1$ hour
from the light curve. Denoting the occurrence of clouds which are much dense
and clustered for obscuring the stellar photosphere in extreme debris disk by
$p_{c}$, we obtain $p_{c}=p/p_{\mathrm{geo}}\sim 0.1$. At least, because
$p_{c}$ does not exceed unity, we cannot reject the cloud scenario as the
origin of the debris dipper. We note that several dippers locating in Sco OB2,
whose age is estimated as $\sim 11-17$ Myr (Pecaut et al., 2012), unlikely
host a protoplanetary disk, but an (extreme) debris disk, which might increase
$p_{c}$. Note that the equation.3 is assumed that a dust clump is much smaller
than the radius of the star. Assuming a clump with a scale similar to or
larger than the star radius, $p_{c}$ can be larger than $0.1$.

###  6.3. Possible scenario of the debris dipper

All of the debris dippers we found exhibit a strong IR excess at the $W3$ and
$W4$ bands and the probability of variability in the extreme debris disks is
about $p\sim 0.1\%$, which is consistent the geometrical probability of the
transiting dust cloud. This fact indicates that it is feasible that the
dimming event of the debris dipper is caused by the occultation of the dust
clouds or similar structures locating at the inner disk. The question is the
origin of the dust clouds orbiting around the star.

The key feature to unravel the origin of dust clouds is periodicity. Gaidos et
al. (2019) revealed that HD 240779 has two peaks in the Lomb-Scargle
periodogram of the light curve: one is located at about 4.2 day of the
rotational period and another one is found at 1.51 day of the dimming period.
They focused on the dimming periodicity and suggested that the occultation was
caused by a transiting clustered dust cloud orbiting at a distance of 6
stellar radii. We examined if their scenario could be applied to the other two
debris dippers.

Figure 17.— Upper — The Lomb Scargle periodogram of the entire sector of TIC
167303776 except for sector 7 (black solid line). This periodogram exhibits a
single clear peak at 1.86 day (indicate by the red dashed line). Bottom — The
small dots show the light curve of TIC 167303776 folded by 1.86 day. The large
dots show the binned flux. The red curve is the best-fit sine function of the
binned flux. The folded flux is well expressed as the sine curve, indicating
that the 1.86 day signal is likely to be owing to the rotational period.

TIC 167303776, which is located near the south ecliptic pole, have been
monitored for the entire sectors of TESS one year’s mission except for sector
7. The Lomb-Scargle periodogram (Scargle, 1982) of the entire light curve
exhibits the only single clear peak at 1.86 day (as shown in the upper panel
of Figure 17). Because the light curve folded by 1.86 d is well approximated
by a sine curve (the lower panel of Figure 17), this signal is likely to
originates from the stellar rotation. The absence of the other peaks in the
periodogram indicates that the dimming event is not (quasi) periodic but
aperiodic in contrast to HD 240779.

Figure 18.— Upper — The ASAS-SN light curves of TIC 43488669 in six years.
Both $V$ and $B$ magnituded are converted into the normalized flux and are
plotted together in the same panel, ignoring the wavelength dependence of the
dimming. We devided the ASAS-SN light curve into six terms with the different
colors. Bottom — The Lomb Scargle periodogram of the TIC 43488669. The solid
colored lines show the periodograms in terms corresponding the color of the
upper panel. The solid black line corresponds to the entire light curve of
ASAS-SN and the gray dotted line corresponds to the TESS light curve. The
dashed line shows the false alert probability levels corresponding to each
color.

TIC 43488669 was monitored only in one sector of the TESS first one year
observation. Instead, ASAS-SN has monitored this system in about six years and
provides plenty of photometric data although 167303776 is a bit faint
($\mathrm{Tmag}=14.5$) for the ASAS-SN data. The upper panel of Figure 18
shows the ASAS-SN light curves in the six years. We converted both $V$ and $B$
magnitudes into the normalized flux and plotted together, ignoring the
wavelength dependence of the dimming. The bottom panel shows the Lomb Scargle
periodogram. Although the periodogram of the entire ASAS-SN light curve
exhibits several peaks for each term, no common peak is found in the entire
light curve. The periodogram indicates that the dimming period changes during
the ASAS-SN observation or the dimming events are episodic.

Figure 19.— The light curves of TIC 167303776 monitored in the entire sector
except for sector 7 (red and black lines). The red lines indicate the dips
whose depth are larger than 10 % against the flat level. Seventy three large
dips are contained in the entire light curve.

We did not find clear periodicity both in TIC 43488669 and 167303776. An
alternative scenario to the one (Gaidos et al., 2019) proposed is required to
account for their dips. The simple explanation is that those are transiting
events by dust clumps randomly distributed in the orbits. To make a dust
clump, one needs to assume a planetesimal or embryo which traps dust in their
Hill radius. From here, we discuss the case of TIC 167303776 whose radius and
mass is, according to TICv8, 0.35 times of the Sun. A dust clump with a radius
of 8,000 km is required to obscure the area more than 10% of the stellar disk.
Assuming that a planetesimal or embryo core envelopes dust all over its Hill
radius, a planetesimal whose mass is larger than $3\times 10^{20}\mathrm{kg}$
can form such a dust clump at $1\mathrm{AU}$, which is the preferred distance
derived by mean transit duration of TIC 167303776 (see Equation 2). We note
that the mass of the planetesimal core required to be the 8,000km dust clump
is inversely proportional to the cube of the semi-major axis. The next
question is how many dust clumps are required to account for the dimming
events, which are observed in the entire sector of TESS observation. One the
author (T.T.) visually inspected the number of the dips in the entire light
curve of TIC 167303776 and found 72 dips whose depth are larger than 10%
against the flat level (Figure 19). A period of a dust clump located at 1 AU
is about 1.7 year in the case of TIC 167303776. Therefore, to explain the
entire light curve by transiting dust clumps, at least a hundred of $\gtrsim
3\times 10^{20}\mathrm{kg}$ planetesimals are required, which corresponds to a
$300\mathrm{km}$ radius assuming a bulk density of $3\mathrm{gcm^{-3}}$. The
total mass of planetesimals is roughly consistent with the minimum mass solar
nebula about 1–2 AU (Hayashi, 1981).

We thank David Latham, Joey Rodriguez, Masahiro Ikoma, Shogo Tachibana, and
Hirokazu Kataza for fruitful discussion. This work is supported by a Grant-in-
Aid from Japan Society for the Promotion of Science (JSPS), Nos. 17K14246,
18H01247, 18H04577, JP20H00170 (H.K.), 17H01103, 18H05441(T.M. & M.M.),
18K13599 (R.O.), 19K03932 (T.M.), 19H01933 (M.F.), and 14J07182 (M.A.). M.F.
is supported by University of Tokyo Excellent Young Researcher Program. M.A.
is supported by the Advanced Leading Graduate Course for Photon Science
(ALPS). This work was also supported by the JSPS Core-to-Core Program
(Planet2) and SATELLITE Research from Astrobiology center (AB022006).

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Figure 20.— All light curves of dippers we found. Figure 20.— Continued

##  Appendix A Reduced Unit Weighted Error

Our analysis depends on the solution of Gaia data release 2 and TICv8. TICv8
also depends on the resluts of Gaia. The Reduced Unit Weighted Error (RUWE) is
often used to distinguish single stars from binary or multiple unresolved
stars or the targets of Gaia. RUWE is defined as

$RUWE=\frac{\sqrt{\chi\/\left(N-5\right)}}{u_{0}\left(G,C\right)},$ (A1)

where $\chi$ is astrometric_chi2_al, $N$ is astrometric_n_good_obs_al, which
are listed in the Gaia database, and $u_{0}\left(G,C\right)$ is a reference
value to the normalized Unit Weighted Error, which depends on stellar
magnitudes and colors. The $u_{0}\left(G,C\right)$ is provided by
https://github.com/agabrown/gaiadr2-ruwe-tools. If RUWE is close to $1.0$, the
source is well-behaved as a single star. We calculated RUWE for all of the
dippers and found that all of RUWE values range between 0.8 and 2 except for
TIC 50745680 and 427399794, whose RUWEs are $2.3$ and $10.7$ respectively.
This suggests that these stars are not well resolved from nearby stars.