# Chemistry in externally FUV irradiated disks in the outskirts of the Orion
Nebula Cluster

Javiera K. Díaz-Berríos Institute of Astrophysics, Pontificia Universidad
Católica de Chile, Vicuña Mackenna 4860, Macul, Región Metropolitana, Chile
School of Physics and Astronomy, University of Leeds, Leeds, United Kingdom,
LS2 9JT  Viviana V. Guzmán Institute of Astrophysics, Pontificia Universidad
Católica de Chile, Vicuña Mackenna 4860, Macul, Región Metropolitana, Chile
Núcleo Milenio de Formación Planetaria (NPF), Chile  Catherine Walsh School of
Physics and Astronomy, University of Leeds, Leeds, United Kingdom, LS2 9JT
Karin I. Öberg Center for Astrophysics $\mid$ Harvard & Smithsonian, 60 Garden
St., Cambridge, MA 02138, USA  L. Ilsedore Cleeves Department of Astronomy,
University of Virginia, 530 McCormick Rd, Charlottesville, VA 22904  Elizabeth
Artur de la Villarmois Institute of Astrophysics, Pontificia Universidad
Católica de Chile, Vicuña Mackenna 4860, Macul, Región Metropolitana, Chile
Núcleo Milenio de Formación Planetaria (NPF), Chile  European Southern
Observatory, Av. Alonso de Córdova 3107, Vitacura, Santiago, Chile  John
Carpenter Joint ALMA Observatory, Av. Alonso de Córdova 3107, Vitacura,
Santiago, Chile

###### Abstract

Most stars are born in stellar clusters and their protoplanetary disks, which
are the birthplaces of planets, can therefore be affected by the radiation of
nearby massive stars. However, little is known about the chemistry of
externally irradiated disks, including whether or not their properties are
similar to the so–far better–studied isolated disks. Motivated by this
question, we present ALMA Band 6 observations of two irradiated Class II
protoplanetary disks in the outskirts of the Orion Nebula Cluster (ONC) to
explore the chemical composition of disks exposed to (external) FUV radiation
fields: the 216–0939 disk and the binary system 253–1536A/B, which are exposed
to radiation fields of $10^{2}-10^{3}$ times the average interstellar
radiation field. We detect lines from CO isotopologues, HCN, H2CO, and C2H
toward both protoplanetary disks. Based on the observed disk–integrated line
fluxes and flux ratios, we do not find significant differences between
isolated and irradiated disks. The observed differences seem to be more
closely related to the different stellar masses than to the external radiation
field. This suggests that these disks are far enough away from the massive
Trapezium stars, that their chemistry is no longer affected by external FUV
radiation. Additional observations towards lower–mass disks and disks closer
to the massive Trapezium stars are required to elucidate the level of external
radiation required to make an impact on the chemistry of planet formation in
different kinds of disks.

Astrochemistry — Circumstellar matter — Interstellar molecules —
Protoplanetary disks — Submillimeter astronomy

##  1 Introduction

Protoplanetary disks of gas and dust around young stars are the birthplace of
planets; hence, understanding the physical and chemical structures of disks is
essential to determine the composition of the material that can be
incorporated into planets and planetesimals.

During the last decades, the chemical structure and distribution of numerous
molecules have been widely studied in protoplanetary disks (e.g. Dutrey et
al., 2007; Pegues et al., 2021; Öberg et al., 2021). A variety of molecules
have been observed and detected using the Atacama Large
Millimeter/submillimeter Array (ALMA), the Submillimeter Array (SMA), the
Northern Extended Millimeter Array (NOEMA), and the Institute for Radio
Astronomy in the Millimeter Range (IRAM) 30m telescope; examples are CO (and
isotopologues; e.g., Koerner et al., 1993; Dutrey et al., 1997; Thi et al.,
2001; Booth et al., 2019), small organics (C2H, CS, CN, HCN, HNC, H2CO, HCO+,
DCO+; e.g., Dutrey et al., 1997; van Dishoeck et al., 2003; Qi et al., 2008;
Öberg et al., 2011; Guzmán et al., 2015; Hily-Blant et al., 2017; Furuya et
al., 2022), and even complex species (CH3OH, CH3CN, HC3N, $c-$C3H2, HCOOH, and
CH3OCH3; e.g., Chapillon et al., 2012; Qi et al., 2013; Öberg et al., 2015;
Walsh et al., 2016; Favre et al., 2018; Brunken et al., 2022). These studies
have advanced our understanding of the chemistry of planet formation; however,
they have focused on disks around isolated stars in low-mass star-formation
regions, whereas many stars form in clusters, with low-mass and high-mass
stars forming in conjunction (Lada & Lada, 2003; Krumholz et al., 2019). Thus,
the chemistry of externally irradiated protoplanetary disks remains poorly
constrained observationally. Following the nomenclature also used in Walsh et
al. (2013), “isolated” refers to a disk irradiated by its central star only,
whereas “irradiated” refers to a disk illuminated by the central star and the
interstellar radiation field (ISRF), which includes at least one nearby
massive star.

Most studies of extremely irradiated protoplanetary disks have been focused on
the disk mass (e.g., Eisner et al., 2018; Mann et al., 2014; Boyden & Eisner,
2020, 2023), radius (e.g., Vicente & Alves, 2005; Clarke, 2007; Mann et al.,
2014; Boyden & Eisner, 2020, 2023), evolution (e.g., Champion et al., 2017;
Haworth et al., 2017), and lifetime (e.g., Adams et al., 2004; Haworth et al.,
2021; Winter et al., 2019). Theoretical models and observational studies have
predicted and discovered, respectively, that disks exposed to external
radiation decrease in their mass and size when they are close to the massive
stars, due to photoevaporation (e.g., Johnstone et al., 1998; Störzer &
Hollenbach, 1999; Matsuyama et al., 2003; Adams et al., 2004; Mann et al.,
2014; Boyden & Eisner, 2020; Concha-Ramírez et al., 2023; Boyden & Eisner,
2023). This is expected to affect the ability of these disks to form planets.
However, photoevaporation is expected to mainly remove gas from the outer
disk. This is because dust can grow in size and the subsequent settling/drift
lead to a depletion of dust in the surface and outer regions of the disk where
external photoevaporation is mainly operating (Facchini et al., 2016).
Therefore, rocky planets may still be able to form in the inner disk midplane
(Adams et al., 2004; Concha-Ramírez et al., 2023).

We currently do not know the impact of an external radiation field on the
chemistry of a disk. This is important to investigate because isolated disks
are not the norm in the Galaxy. Instead, the vast majority of stars form
within rich stellar clusters where disks are constantly affected by intense
radiation fields from nearby massive stars, and exposed to extreme-ultraviolet
(EUV) radiation and far-ultraviolet (FUV) radiation (Adams et al., 2004), that
affects their evolution through time. Our Sun is thought to have been born in
such a cluster, so the protosolar nebula would have been irradiated by its
massive neighbors (Lada & Lada, 2003; Adams, 2010; Parker, 2020). Hence,
understanding the initial chemical conditions of the protosolar nebula
requires an understanding of the chemistry in disks located in massive star-
forming regions.

Observing molecular lines in disks that are close to massive stars is
challenging, because of their physical size ($R_{\mathrm{disk}}\sim
10^{2}-10^{3}~{}\mathrm{au}$; Mann et al., 2014; Boyden & Eisner, 2023) and
the large distances to massive star-formation regions, meaning that they
typically span only $\sim
0\hbox{$.\\!\\!^{\prime\prime}$}2-1\hbox{$.\\!\\!^{\prime\prime}$}2$ on the
sky (at the distances of the nearest massive star forming regions).
Additionally, many disks in clustered regions are still embedded in the parent
molecular cloud, and it can be challenging to disentangle the disk molecular
line emission from the cloud molecular line emission.

A few astrochemical disk models have investigated the effect of strong
external FUV fields, characteristic of the environments close to O/B stars, on
disk chemistry. These models predict that external FUV sources will
significantly impact the thermal and chemical structure of the disk (Nguyen et
al., 2002; Walsh et al., 2013, 2014). Therefore, the conclusions reached for
isolated disks may not be directly transferred to disks born near massive
stars. For example, the gas temperature is expected to be significantly higher
in the outer disk midplane in externally irradiated disks, which will result
in the release of molecules that would usually be frozen out onto dust grains
(Walsh et al., 2013). However, these models remain speculative and their
predictions are yet to be tested through comparisons with observations.

In this work, we present observations of two protoplanetary disks around pre-
main sequence stars located in the outskirts of the Orion Nebula Cluster
(ONC), which is the closest ($\sim 414$ pc) massive star-forming region
(Menten et al., 2007; Rzaev et al., 2021). Our aim is to investigate the
possible differences between the chemistry of isolated disks and of externally
irradiated disks. By studying disks with similar initial conditions to the
protosolar nebula, such as externally irradiated disks, we can learn about the
formation conditions of planets in the Solar System. The ONC is an excellent
laboratory to study these kinds of sources. This rich cluster has thousands of
stars, but the radiation field is dominated by a single star: $\theta^{1}$ Ori
C (O’dell & Wen, 1994; Smith et al., 2005; Ricci et al., 2008), a young ($\sim
1$ Myr; Hillenbrand, 1997), massive ($45~{}\mathrm{M}_{\odot}$; Kraus et al.,
2009; Rzaev et al., 2021), O6-type star (O’Dell et al., 2017). Hubble Space
Telescope (HST) observations revealed hundreds of disks in Orion that are
externally irradiated, some of them surrounded by a cometary ionization front
– the so-called proplyds (O’dell et al., 1993; O’dell & Wen, 1994; Ricci et
al., 2008; Eisner et al., 2018). More recent ALMA surveys have shown that
disks close to the stellar cluster ($\lesssim 0.5$ pc) can have a significant
amount of their disk mass removed due to external photoevaporation, as was
predicted by Johnstone et al. (1998) and Störzer & Hollenbach (1999), while
disks that are farther away have disk masses that are relatively intact (Mann
et al., 2014; Eisner et al., 2018; van Terwisga et al., 2019). However, the
chemistry of disks in these less exposed regions could be affected because the
FUV radiation, even if it is not as extreme as in the EUV regime, is still
higher than the FUV field to which the isolated disks are exposed.

Section 2 describes the ALMA observations, and provides information on the
sources, including the disk and stellar properties, and the spectroscopic
parameters of the molecular lines. Section 3 presents the results of the
continuum emission and molecular lines. In Section 4 we compare the results of
the externally irradiated disks with those for isolated disks. Finally,
Section 5 presents our conclusions.

Table 1: Stellar and disk properties.

| 216–0939 | 253–1536A/B
---|---|---
RA | $5^{h}35^{m}21.57^{s}$ |  $5^{h}35^{m}25.30^{s}$/$5^{h}35^{m}25.23^{s}$
DEC | $-5^{\circ}9^{\prime}38.9^{\prime\prime}$ |  $-5^{\circ}15^{\prime}35.4^{\prime\prime}$/$-5^{\circ}15^{\prime}35.69^{\prime\prime}$
$\mathrm{M_{\star}}$ | 2.17 $\mathrm{M_{\odot}}$aaAverage rms estimated from the line free channels which correspond to the first $5$ and last $5$ channels in the image cube. | 3.5 $\mathrm{M_{\odot}}$/$>0.2~{}\mathrm{M}_{\odot}/\sin^{2}i_{\mathrm{B}}$
Spectral type | K5 | F/M2
DistancebbThe same process was performed on the ALMA Band 7 data obtained from Cycle 0 project #2011.0.00028.s (Mann et al., 2014) | 1.59 pc | 0.92 pc
Inclination | $32^{\circ}$ |  $65^{\circ}$/-
PA | $173^{\circ}$ |  $69.7\pm 1.4^{\circ}$/ $136\pm 15^{\circ}$
Disk mass | 46 MJup | 79/30 MJup
$v_{\mathrm{LSR}}$ | 10.75 km/s | 10.55/10.85 km/s
FUV fieldccfootnotemark:  | $G_{0}\sim 180$ | $G_{0}\sim 500$

aaAverage rms estimated from the line free channels which correspond to the
first $5$ and last $5$ channels in the image cube.

216–0939 could also be a tight equal-mass binary of two $\sim
1~{}\mathrm{M_{\star}}$ stars (see Section 2). bbThe same process was
performed on the ALMA Band 7 data obtained from Cycle 0 project
#2011.0.00028.s (Mann et al., 2014) Projected distance to $\theta^{1}$ Ori.
ccfootnotemark:  Based on _Herschel_ FIR observations (Pabst et al., 2021).

Note. — The stellar and disk properties were taken from Factor et al. (2017)
and Williams et al. (2014) for the 216–0939 and 253–1536A/B system,
respectively.

##  2 Observations

We have targeted two protoplanetary disk systems located in the Orion Nebula
Cluster at a distance of $414$ pc (e.g., Rzaev et al., 2021). The two systems,
216–0939 and 253–1536A/B, are located in the outskirts of the ONC (projected
distance of $>0.9$ pc from $\theta^{1}$ Ori C), where the radiation field is
estimated to be $<10^{3}~{}\mathrm{G}_{0}$ (Mookerjea et al., 2003), where
$\mathrm{G}_{0}$ is the average strength of the integrated ISRF;
$\mathrm{G}_{0}\approx 1.6\times 10^{-3}$ erg cm-2 s-1 (Draine, 1978). The
sources have been previously identified and classified with HST (e.g., Smith
et al., 2005; Ricci et al., 2008, 2011a), and have been observed with ALMA as
part of a survey that observed the continuum emission in 22 disks near the ONC
(e.g., Mann et al., 2014). The three disks have estimated gas masses of the
same order of magnitude, ranging between $\sim
30-80~{}\mathrm{M}_{\mathrm{Jup}}$. The two systems are exposed to different
external radiation fields (by almost an order of magnitude), which allows us
to investigate for the first time the effect of different external radiation
fields on the chemistry. The stellar and disk propeties are summarized in
Table 1. This section presents the sources, observational details, and data
reduction process.

Source 216–0939 is a K5 star, determined spectroscopically by Hillenbrand
(1997). However, the dynamical mass of $2.17\pm 0.07$ M⊙ estimated by Factor
et al. (2017) is inconsistent with this spectral type. They propose that the
star could instead be a tight equal-mass binary of two 1.1 M⊙ stars. For the
purposes of this study we consider both cases. The disk is located at a
projected distance of $1.59$ pc from $\theta^{1}$ Ori C (O6 star; Hillenbrand,
1997; Factor et al., 2017), and at $0.8$ pc from $\nu$ Ori (B3V star; Terada &
Tokunaga, 2012). The disk around 216–0939 (J2000 R.A.
$\mathrm{=05h35m21.57s}$; J2000 DEC. $\mathrm{=-05d~{}09m~{}38.9s}$) has an
inclination of $32^{\circ}$, a position angle ($\mathrm{PA}$) of
$173^{\circ}$, a systemic velocity ($v_{\mathrm{LSR}}$) of $10.75$ km/s
(Factor et al., 2017), and a dust disk size of $\sim 290~{}\mathrm{au}$ (Mann
& Williams, 2009). This source is one of the most massive protoplanetary disks
in the ONC. The gas mass of the disk is estimated to be $\sim
46~{}\mathrm{M}_{\mathrm{Jup}}$ ($\sim 0.04~{}\mathrm{M}_{\odot}$; Ricci et
al., 2011b; Mann et al., 2014; Mann & Williams, 2009; Factor et al., 2017),
computed from the 856$\mu$m dust continuum emission. Additionally, because of
its distance to the massive stars, the external FUV radiation field is
estimated to be $\mathrm{G}_{0}\approx 180$, based on Herschel FIR
observations (Pabst et al., 2021). Previous studies by Mann & Williams (2009,
2010) and Factor et al. (2017) reported the detection of HCO+ ($4-3$), CO
($3-2$), and HCN ($4-3$) lines and a weak detection of CS ($7-6$) with an rms
noise of $0.41~{}\mathrm{mJy~{}beam^{-1}}$ and a synthesized beam of
$0\hbox{$.\\!\\!^{\prime\prime}$}57\times 0\hbox{$.\\!\\!^{\prime\prime}$}52$,
and provided the first view of the molecular content and structure of the
disk.

Additionally, HST observations of the 216–0939 star revealed the presence of
water ice absorption, concluding that it most likely originated from the
surrounding disk (Terada & Tokunaga, 2012). The water ice detection is
associated with a large silhouette disk, about $\sim 1000~{}\mathrm{au}$ in
diameter and showed that there are regions of the disk that are sufficiently
cold to host substantial ice reservoir (Terada & Tokunaga, 2012; Terada et
al., 2012).

Source 253–1536A/B is a wide binary system with dynamically estimated stellar
masses of $\sim 3.5~{}\mathrm{M}_{\odot}$ and $\gtrsim
0.2~{}\mathrm{M}_{\odot}/\sin^{2}i$, for the A and B members, respectively
(Williams et al., 2014). The lower-mass star, 253–1536B, has a spectral type
M2, while a spectral type of F/G has been reported for 253–1536A by Ricci et
al. (2011b). The disk of 253–1536A has an inclination of $65^{\circ}$, and a
position angle ($\mathrm{PA}$) of $69.7^{\circ}$ (Williams et al., 2014). The
inclination is unknown for 253–1536B, but the position angle has been
estimated to be $136^{\circ}$ (Williams et al., 2014). The systemic velocity
of the system ($v_{\mathrm{LSR}}$) is $10.55$ and 10.85 km/s for the A/B
members (Williams et al., 2014). This system, an ONC proplyd (Smith et al.,
2005), is located inside the M43 HII region and is associated with a bright
proplyd ionization front (Williams et al., 2014) and a bipolar jet (Smith et
al., 2005). This binary system is at a projected distance of $0.92$ pc from
$\theta^{1}$ Ori (J2000 R.A. $\mathrm{=05h35m25.30s/05h35m25.23s}$; J2000 DEC.
$\mathrm{=-05d15m35.40s/-05d15m35.69s}$), and its external FUV radiation field
is estimated to be $\mathrm{G}_{0}\sim 500$, based on Herschel FIR
observations (Pabst et al., 2021). The gas masses of the disks are estimated
to be $\sim 79$ and $30~{}\mathrm{M}_{\mathrm{Jup}}$ for A and B ($\sim 0.08$
and $0.03~{}\mathrm{M}_{\odot}$), respectively, computed from the 856$\mu$m
dust continuum emission (Williams et al., 2014). 253–1536A has an estimated
disk radius of 0$.\\!\\!^{\prime\prime}$75 equivalent to $\sim 300$ au, and is
1$.\\!\\!^{\prime\prime}$1 (440 au) away from 253–1536B (Williams et al.,
2014). No evidence of a larger circumbinary disk around the system has been
found (Smith et al., 2005; Mann & Williams, 2009).

The three protoplanetary disks of this study are massive enough ($\geq 30$
MJup) to potentially form planets (Mann & Williams, 2009), providing test
cases for studying planet formation under extreme irradiation.

Table 2: Spectroscopic parameters of targeted molecular lines.

Molecule | Line | Rest. Freq. (GHz) | $\log(A_{ij}(\mathrm{s}^{-1}))$ |  $E_{u}$ (K) | $g_{u}$
---|---|---|---|---|---
DCN | J = $3-2$ | $217.2385$ | $-3.3396$ | $20.85$ | $21$
$c-$C3H2 | (J${}_{\mathrm{K_{a},K_{c}}}$) = $6_{06}-5_{15}$ | $217.8221$ | $-3.2679$ | $38.61$ | $13$
H2CO | (J${}_{\mathrm{K_{a},K_{c}}}$) = $3_{03}-2_{02}$ | $218.2222$ | $-3.5504$ | $20.96$ | $7$
H2CO | (J${}_{\mathrm{K_{a},K_{c}}}$) = $3_{22}-2_{21}$ | $218.4756$ | $-3.8037$ | $68.09$ | $7$
H2CO | (J${}_{\mathrm{K_{a},K_{c}}}$) = $3_{21}-2_{20}$ | $218.7601$ | $-3.8024$ | $68.11$ | $7$
C18O | J = $2-1$ | $219.5604$ | $-6.2211$ | $15.81$ | $5$
13CO | J = $2-1$ | $220.3987$ | $-6.2191$ | $15.87$ | $5$
12CO | J = $2-1$ | $230.5380$ | $-6.1605$ | $16.60$ | $5$
N2D+ | J = $3-2$ | $231.3218$ | $-3.1465$ | $22.20$ | $63$
$c-$C3H2 | (J${}_{\mathrm{K_{a},K_{c}}}$) = $7_{07}-6_{16}$ | $251.3144$ | $-3.0704$ | $50.67$ | $45$
$c-$C3H2 | (J${}_{\mathrm{K_{a},K_{c}}}$) = $6_{25}-5_{14}$ | $251.5273$ | $-3.1706$ | $47.49$ | $39$
C2H | N = $3-2$, J = $\frac{5}{2}-\frac{3}{2}$, F = $3-2$ | $262.0650$ | $-4.1521$ | $25.16$ | $7$
HCN | J = $3-2$ | $265.8864$ | $-3.0766$ | $25.52$ | $21$
| J = $4-3$ | $354.5055$ | $-3.1614$ | $42.53$ | $27$

Note. — Molecular data extracted from the CDMS (Müller et al., 2001, 2005;
Endres et al., 2016), JPL (Pickett et al., 1998), and LAMDA molecular (Schöier
et al., 2005) catalogues, obtained through www.splatalogue.net (Remijan et
al., 2007).

###  2.1 Observations details

The observations of the 216–0939 and 253–1536A/B protoplanetary disks were
obtained with ALMA as part of the Cycle 5 project #2018.1.01190.S (PI: V.
Guzmán). The ALMA Band 6 observations included two spectral settings, at 1.2
and 1.3 mm. The correlator setup was configured with narrow spectral windows
targeting different molecular lines. The main targets of the observations were
lines from species commonly observed in isolated disks, such as the CO
isotopologues, HCN, small carbon chains, H2CO, and deuterated species. Table 2
summarizes the molecular line targets and their spectral properties.

The Band 6 observations were carried out in August 2018 with baseline lengths
spanning between 41 and 3640 m approximately. The total on-source time was 82
and 84 minutes per spectral setting, for the 216–0939 and 253–1536 disks,
respectively. The quasar J0423–0120 was observed to calibrate the frequency
bandpass and amplitude, and the quasars J0529–051 and J0607–0834 were observed
to calibrate phase temporal variations (see Table A1).

Additionally, we used archival ALMA Band 7 observations of the two
protoplanetary disks obtained as part of the Cycle 0 project #2011.0.00028.S
(PI: R. Mann), which include the HCO+ ($4-3$) and HCN ($4-3$) lines. More
details about these observations can be found in Mann et al. (2014).

Table 3: Molecular line and continuum image parameters/properties.

Species | Transition | Beam size | RMSaaAverage rms estimated from the line free channels which correspond to the first $5$ and last $5$ channels in the image cube. | Beam size | RMSaaAverage rms estimated from the line free channels which correspond to the first $5$ and last $5$ channels in the image cube.
---|---|---|---|---|---
|  | (arcsec) | (deg) | (mJy/beam) | (arcsec) | (deg) | (mJy/beam)
|  | 216–0939 | 253–1536A/B
Continuum (high-res.) |  | $0.12\times 0.13$ | $-57.18$ | $0.04$ | $0.09\times 0.12$ | $82.47$ | $0.08$
Continuum (smooth) |  | $0.48\times 0.51$ | $62.96$ | $0.25$ | $0.47\times 0.50$ | $64.07$ | $0.78$
DCN | $3-2$ | $0.48\times 0.52$ | $67.34$ | $2.37$ | $0.47\times 0.50$ | $71.95$ | $3.04$
$c-$C3H2 | $6_{06}-5_{15}$ | $0.48\times 0.52$ | $67.59$ | $2.24$ | $0.47\times 0.50$ | $70.31$ | $2.88$
H2CO | $3_{22}-2_{21}$ | $0.48\times 0.52$ | $67.63$ | $2.06$ | $0.46\times 0.50$ | $69.54$ | $2.66$
C18O | $2-1$ | $0.48\times 0.52$ | $65.97$ | $2.18$ | $0.47\times 0.50$ | $66.92$ | $2.83$
13CO | $2-1$ | $0.48\times 0.51$ | $63.96$ | $3.18$ | $0.47\times 0.50$ | $64.07$ | $4.14$
12CO | $2-1$ | $0.48\times 0.51$ | $62.68$ | $2.89$ | $0.46\times 0.49$ | $65.95$ | $3.76$
N2D+ | $3-2$ | $0.47\times 0.51$ | $65.55$ | $2.91$ | $0.46\times 0.49$ | $68.78$ | $3.74$
C2H | $3-2$ | $0.45\times 0.49$ | $-69.88$ | $3.09$ | $0.45\times 0.49$ | $-70.97$ | $3.61$
$c-$C3H2 | $7_{07}-6_{16}$ | $0.46\times 0.50$ | $-72.74$ | $3.44$ | $0.45\times 0.49$ | $-72.94$ | $3.94$
$c-$C3H2 | $6_{25}-5_{14}$ | $0.46\times 0.50$ | $-71.89$ | $3.56$ | $0.45\times 0.49$ | $-73.99$ | $4.11$
HCN | $3-2$ | $0.45\times 0.49$ | $-69.94$ | $3.39$ | $0.45\times 0.49$ | $-68.58$ | $4.03$

Note. — Lines were imaged with a spectral resolution of $0.4~{}\mathrm{km/s}$.

###  2.2 Data reduction

The initial data calibration was performed by ALMA staff using standard
procedures in CASA (Common Astronomy Software Applications) version 6.4
(McMullin et al., 2007). Additionally, to increase the signal-to-noise (SNR)
of the observations, we further self-calibrated the data using the continuum.
The first step was to create pseudo-continuum visibilities by flagging all
channels that contained line emission. Then, we ran a total of seven phase-
only calibration iterations and one amplitude calibration iteration. This
procedure improved the SNR of the continuum emission only by a factor of $\sim
1.2$ for 216–0939 and $\sim 1.8$ for 253–1536A/B on average. Then, the self-
calibration solutions were applied to each spectral window including the
channels with line emission. The continuum was then subtracted from the
visibilities using the uvcontsub routine to produce the self-calibrated
visibilities of the different lines.

The continuum images were produced from the self-calibrated visibilities by
first extracting the continuum channels using the split routine and imaging
the visibilities using the tclean routine with a Briggs robust weighting
parameter of $+0.5$, which balances the SNR and spatial resolution. We used an
elliptical cleaning mask created using CASA regions for each source (216–0939,
253–1536A, and 253–1536B). The continuum images are shown in Fig. 1, and the
final rms and beam sizes of the continuum images are listed in Table 3.

Figure 1: Left: Dust continuum emission images at high-angular resolution from
the protoplanetary disks 216–0939 (top) and 253–1536 (bottom). Right:
Deprojected radial profiles for the continuum emission of each disk. For
216–0939, the radial profile is divided in north (blue) and south (orange)
side emission. In particular, we estimated each radial profile including the
emission from an aperture of $30^{\circ}$ in each side. For 253–1536, the
radial profile of 253–1536A and 253–1536B are shown in blue and orange,
respectively. Vertical lines represent the estimated disk size for each case,
containing $95\%$ of the total flux of each disk. For 216–0939, the black
vertical line represents the disk size estimated from the full azimuthally
averaged radial profile.

For the molecular lines, we used a Briggs robust parameter value of $+0.5$
because the line emission was not bright enough to be imaged at higher angular
resolution. We also used the uvtaper parameter in tclean to reduce the weight
of the longest baselines in the $uv-$plane, in particular we used uvtaper=0.5
arcsec. In this way, higher spatial frequencies are weighted down relative to
lower spatial frequencies, increasing the sensitivity to larger-scale
emission. This technique is used when there are poorly sampled areas in the
$uv-$plane or to increase the SNR. We also explored the effects of $uv$ cuts
to minimize the cloud contamination, which affected some of the lines. The
same was investigated by Factor et al. (2017) for the HCO+ and HCN lines
(4-3). They excluded baselines shorter than $70~{}k\lambda$ and found some
improvement in the quality of the observations. However, we did not find a
significant improvement in the quality of our observations after removing the
shortest baselines. This can be explained by the different baseline coverage,
which spans from 41 to 3640 m in our dataset, while it spans from 21.2 to
384.2 m in the data analyzed by Factor et al. (2017).

To help the cleaning process, we created a Keplerian mask adapted to each
source in the tclean process task, that selects regions with line emission in
each channel. The Keplerian masks were generated with the publicly available
Python code from Teague (2020), that computes the Keplerian motion of the disk
given the mass of the central star, the disk geometry and the cube parameters,
such as spectral resolution, line frequency, source position, and systemic
velocity. For the binary system, we created a Keplerian mask for 253–1536A and
253–1536B separately111For this we used a modified version of the code from
https://github.com/kevin-flaherty/ALMA-Disk-Code and then added them to
produce a total mask that was used in the cleaning process. The inclination
and PA used to create the masks are listed in Table 1. However, the reported
PA for 253-1536B disk did not capture well the Keplerian rotation of the disk,
so after some visual inspection we instead used a more conservative mask with
an inclination of $45^{\circ}$ and PA of $-90^{\circ}$. Appendix B shows the
resulting Keplerian masks overlaid on the channel maps. The rms and beam sizes
of the line cubes can be found in Table 3.

In order to better compare the dust continuum emission with the molecular line
emission, we created a second version of the dust continuum images, using the
imsmooth routine to degrade the angular resolution of the continuum
observations. This task performs a Fourier-based convolution to smooth the
image and increase the SNR. In particular, we used it to obtain dust continuum
images with the same angular resolution as the images of the different
molecular lines.

##  3 Results

In this section, we present the continuum and molecular line detections and
non-detections in the two externally irradiated systems. First, we describe
the high-resolution continuum emission of the disks. Then we present the
results for the molecular line emission. In particular, we extract disk-
integrated flux densities for the molecular lines, and disk-averaged column
densities. Finally, we estimate the HCN excitation temperature for both disks,
using archival Band 7 observations.

###  3.1 Dust continuum emission

The beam size of the high-angular resolution continuum images of our sources
is $\sim 0\hbox{$.\\!\\!^{\prime\prime}$}13$, resulting in a spatial
resolution of $\sim 54$ au (for a distance of 414 pc). These images are shown
in the left panels of Fig. 1. The smoothed continuum images have an angular
resolution of $\sim 0\hbox{$.\\!\\!^{\prime\prime}$}$5 and are shown in the
left panels in Figure 2.

The new high-angular resolution observations allow us to spatially resolve the
dust emission in these disks. Using the Python package GoFish (Teague, 2019a)
to generate deprojected radial profiles (see right panels in Fig. 1), we
estimate a disk size of $311.5\pm 14.5~{}\mathrm{au}$ for the 216–0939 disk,
defined as the radius containing $95\%$ of the total flux. The observations
also reveal a central cavity with an outer edge at $120-135$ au. The cavity is
also clearly seen in the cleaned image (see Figure 1). We note that this inner
cavity is consistent with the scenario of the central star being a tight
binary system, that would clear the inner disk from material due to tidal
interactions with the disk, which was proposed by Factor et al. (2017).
Additionally, the 216–0939 disk seems to be asymmetric or eccentric, with the
southern side being more elongated and $23\%$ brighter than the northern side.
The origin of this eccentricity is unknown and should be investigated in the
future.

We are able to spatially resolve the region between the two members of the
binary system 253–1536A/B. We estimate an angular separation of
0$.\\!\\!^{\prime\prime}$3 between the two disk edges (equivalent to $\sim
124$ au), and a disk size of $239.1\pm 14.5$ au and $108.7\pm 14.5$ au for
253–1536A and 253–1536B, respectively. At the current angular resolution, the
continuum emission looks very symmetrical for both disks, contrary to other
binary systems where spirals have been observed due to the interaction between
the disks (e.g., Kurtovic et al., 2018). However, substructure could appear
with even higher angular resolution observations.

Figure 2:  Left: Smoothed dust continuum emission for the 216–0939 (top) and
253–1536A/B (bottom) disks. Right: Moment zero maps for CO $(2-1)$, 13CO
$(2-1)$, C18O $(2-1)$, HCN $(3-2)$, H2CO $(3-2)$, and C2H $(3-2)$, integrated
over the full line width (colormap) and over the red- and blue-shifted parts
of the line (contours). Labels and ticks are the same as those in the dust
continuum images For 216–0939, CO $(2-1)$ contour levels correspond to 3, 18,
and 32$\sigma$, and for 13CO $(2-1)$ and C18O $(2-1)$ contours correspond to
3, 12, and 24$\sigma$. On the other hand, HCN $(3-2)$ contour levels
correspond to 3, 12, and 24$\sigma$, for H2CO $(3-2)$ they correspond to 3, 9,
and 15$\sigma$, and for C2H $(3-2)$ contours correspond to 3 and 6$\sigma$.
For 253–1536, CO $(2-1)$ contours correspond to 3, 48, and 120$\sigma$, and
for 13CO $(2-1)$ and C18O $(2-1)$ contours correspond to 3, 28, and
48$\sigma$. Finally, HCN $(3-2)$ contour levels correspond to 3, 18, and
40$\sigma$, and for H2CO $(3-2)$ and C2H $(3-2)$ contours correspond to 3 and
6$\sigma$.

###  3.2 Molecular lines

In this section, we present the results for the observed molecular lines in
both disks. First, we explain our criteria to determine whether a line is
detected or not. We then describe the spatial distribution of the detected
lines.

####  3.2.1 Line detections

A line was considered to be detected if emission within the Keplerian mask was
$\geq 3\sigma$ (rms) in at least $3$ velocity channels. Following this
criteria, 12CO $(2-1)$, 13CO $(2-1)$, C18O $(2-1)$, and HCN $(3-2)$ are
robustly detected in both sources. Indeed, $>3\sigma$ emission is detected in
almost every channel in both sources, with $10\sigma$ emission detected in at
least 3 channels. H2CO $(3-2)$ and C2H $(3-2)$ are also detected according to
this criteria but at lower SNR. In particular, $5\sigma$ emission is detected
in a couple of channels for H2CO $(3-2)$, and C2H $(3-2)$ is only detected at
the $3\sigma$ level in five or six channels in both disks. The other observed
lines did not show emission $\geq 3\sigma$ in any channel, and are therefore
considered as non-detections. For examples of channel maps of detected and
non-detected lines, see Appendix B.

To confirm the detection of H2CO and C2H, we used the matched-filter method
described in Loomis et al. (2018). The advantage of this method is that it
looks for a signal directly in the visibilities, and therefore removes the
uncertainties of the cleaning process which is present in the images. This
method requires a model image as a matched filter; we used the HCN $(3-2)$ and
C18O $(2-1)$ cubes as filters, as these molecular lines were robustly detected
and they are less affected by the cloud contamination. A molecular line is
considered detected if a $\geq 3\sigma$ peak is found near the source velocity
in the filter-response spectrum, using at least one of the filters222We note
that the filter response spectrum depends on the used filter because the
method assumes that the two lines (weak data and filter) have the same spatial
distribution. If this is not the case, it is possible to obtain a false
negative (see Fig. C.2 for an example).. Following this criteria, we confirm
the detection of both H2CO and C2H $(3-2)$ lines in both disks. We applied the
same method to the lines that were not detected in the image plane, and
confirm that DCN $(3-2)$, $c-$C3H2 $(6_{06}-5_{15})$, $c-$C3H2
$(6_{25}-5_{14})$, $c-$C3H2 $(7_{07}-6_{16})$ and N2D+ $(3-2)$ are not
detected. Fig. C.1 shows an example of the filter response for H2CO $(3-2)$
(detected) and DCN $(3-2)$ (not detected) using HCN $(3-2)$ line as the
filter.

Figure 3:  Deprojected radial and azimuthally averaged profiles for 216–0939
(left) and 253–1536A/B (right). Upper panels: $\mathrm{{}^{12}CO~{}(2-1)}$,
$\mathrm{{}^{13}CO~{}(2-1)}$ and $\mathrm{C^{18}O~{}(2-1)}$,
$\mathrm{HCN~{}(3-2)}$. Lower panels: $\mathrm{H_{2}CO~{}(3-2)}$ and
$\mathrm{C_{2}H~{}(3-2)}$. The profile of the smoothed dust continuum emission
is shown in black in all panels, in units of mJy/beam. Color-shaded regions
show the 1$\sigma$ scatter of the averaged emission at each radial bin. The
black vertical line marks the edge of the high-resolution dust continuum
emission on each system, defined as the radius within which the $95\%$ of the
total flux arises. In the case of the binary system, the radial profiles are
obtained with 253–1536A as the center of the emission, and the dust edge shown
corresponds to 253–1536A.

####  3.2.2 Spatial distribution

We created velocity-integrated maps for each detected molecular line. Figure 2
shows the resulting maps for 216–0939 (top) and 253–1536A/B (bottom). In this
figure, the low-angular resolution images of the continuum emission are shown
in the left panels for comparison. The top panels show the 12CO $(2-1)$, 13CO
$(2-1)$, and C18O $(2-1)$ line emission, and the bottom panels show the
emission from HCN $(3-2)$, H2CO $(3-2)$ and C2H $(3-2)$. The red and blue
contours show the Keplerian rotation of the disks, created by integrating the
line emission over the red- and blue-shifted parts of the line. To do this, we
integrated the emission from the first channel that presented $>3\sigma$ line
emission to the channel that was closest to the systemic velocity of the
source (blue-shifted); the same was done to the red-shifted part, integrating
the channels from the systemic velocity to the last channel that presented
$>3\sigma$ line emission. We find that the 12CO line emission is more extended
than the 13CO and C18O line emission in both 216–0939 and 253–1536A/B systems.
Moreover, the CO isotopologue emission is more extended compared to the dust
continuum emission. The line emission from H2CO and C2H is weaker than the HCN
and CO isotopologue line emission for both systems. In addition, we note that
the 12CO line emission is heavily contaminated by extended cloud emission and
foreground absorption near the systemic velocity. To a lesser degree, the
other CO isotopologues and HCN also suffer from cloud contamination (see the
channel maps in Appendix B and the disk-integrated spectra in Appendix D).

In the velocity-integrated maps it is also possible to disentangle the
Keplerian rotation of the smaller disk from the larger disk in 253–1536A/B, in
particular for the CO isotopologues and HCN, where the emission of the minor
companion can be observed in the red- and blue-shifted contours (see also the
channel maps in Appendix B). The emission can also be tentatively disentangled
for C2H. Figure E.1 shows the first moment map for the CO $(2-1)$ line in
253–1536A/B, where a tentative deviation from Keplerian rotation can be seen
towards the northern side of 251–1536A (black arrow). This deviation could be
associated with the interaction between the two companions or be due to the
cloud contamination. Unfortunately, the angular resolution of
0$.\\!\\!^{\prime\prime}$5 is not high enough to resolve the separation
between the two disks and to disentangle if smaller spatial signals of
dynamical interaction are present.

To further investigate the spatial distribution of the different molecular
lines, we generated radial profiles using GoFish to deproject and azimuthally
average the line emission in the zero moment maps, using the disk parameters.
Figure 3 shows the radial profiles of the brighter (top panels) and weaker
(bottom panels) lines. For both 216–0939 and 253–1536A/B disks, all molecular
line emission is more extended than the dust continuum emission. Most of the
lines show little sub-structure at the current angular resolution (the
apparent gaps/rings in C2H $(3-2)$ are probably related to the noise in the
images). However, an interesting feature is that the central emission of the
C2H line is flat (and not centrally peaked), suggesting the emission could be
arise from a ring. C2H ringed emission has been observed in other isolated
disks (e.g., Bergin et al., 2016; Cleeves et al., 2021; Pegues et al., 2021;
Guzmán et al., 2021).

Table 4: Disk integrated fluxes.

|  | 216–0939 | 253–1536A/B
---|---|---|---
Molecule | Line | Integrated Intensity
|  | [mJy km s-1]
Detected
12CO | $2-1$ | $1261\pm 218$ | $3381\pm 1163$
13CO | $2-1$ | $1020\pm 485$ | $610\pm 567$
C18O | $2-1$ | $341\pm 144$ | $543\pm 97$
C2H | $N=3-2,J=\frac{5}{2}-\frac{3}{2},F=3-2$ | $159\pm 76$ | $211\pm 87$
HCN | $3-2$ | $989\pm 145$ | $2193\pm 255$
|  $4-3$bbThe same process was performed on the ALMA Band 7 data obtained from Cycle 0 project #2011.0.00028.s (Mann et al., 2014) | $1276\pm 212$ | $3099\pm 338$
H2CO | $3_{22}-2_{21}$ | $283\pm 76$ | $106\pm 54$
Non–detectedaaReported fluxes for non-detections correspond to $3\sigma$ upper
limits where $\sigma$ is the uncertainty estimated via bootstrapping.
$c-$C3H2 | $6_{06}-5_{15}$ | $<97$ | $<107$
| $6_{25}-5_{14}$ | $<138$ | $<183$
| $7_{07}-6_{16}$ | $<126$ | $<132$
DCN | $3-2$ | $<112$ | $<117$
N2D+ | $3-2$ | $<79$ | $<169$

Note. — The integrated fluxes are measured within the Keplerian masks. In the
case of 253–1536, they include both A and B members.

####  3.2.3 Disk integrated fluxes

Disk-integrated fluxes were estimated for the detected lines, and upper limits
are reported for non-detections. To calculate the integrated flux of each
line, we first multiplied the image cubes by the same Keplerian masks used to
clean the data, using the immath routine in CASA, and then summed over the
line emission. This additional step removes some of the noise in the image and
results in a better-integrated disk spectrum where the Keplerian rotation is
shown. To estimate the uncertainty in the integrated flux for each line, we
used the same method described above to extract the fluxes but now changing
the disk center in $1000$ random samples in regions without emission outside
of the original Keplerian mask via bootstrapping. The uncertainty in the
integrated flux was then computed as the standard deviation of the resulting
distribution added in quadrature with a $10\%$ systematic flux calibration
uncertainty. The number of samples is chosen to be sufficiently large so that
the estimated uncertainty does not vary significantly. The resulting
integrated intensities of the detected lines are listed in Table 4. We also
list upper limits for the non-detected lines. The CO and CO isotopologue
fluxes have larger errors because of the cloud contamination. Indeed, the
cloud is more pronounced in CO than in the other species.

###  3.3 Column density retrieval

To estimate the disk-averaged column densities ($N_{\mathrm{T}}$) we consider
that the gas obeys local thermal equilibrium (LTE), and assume a range of
possible excitation temperatures ($T_{\mathrm{ex}}$) based on typical gas
temperatures observed in the outer regions of disks (Guzmán et al., 2021). The
LTE assumption is reasonable because typical gas densities in disks are high
compared to the critical densities of the observed HCN, H2CO, and C2H
lines333For gas temperatures between $10$ and $50$ K, the HCN $(3-2)$, H2CO
$(3-2)$, and C2H $(3-2)$ lines, have a critical density of $\sim 7\times
10^{7}$, $(2-3)\times 10^{6}$, and $(5-6)\times 10^{6}~{}\mathrm{cm^{-3}}$,
respectively., assuming their emission arises mainly from layers that are
close to the midplane (Law et al., 2021a).

Under LTE conditions, the energy levels are populated following Boltzmann’s
law,

$N_{u}=\frac{N_{\mathrm{tot}}}{Q(T_{\mathrm{ex}})}g_{u}\exp{\left(\frac{-E_{u}}{k_{\mathrm{B}}T_{\mathrm{ex}}}\right)},$
(1)

where $N_{u}$ corresponds to the upper-level column density,
$N_{\mathrm{tot}}$ is the total column density of the molecule,
$Q(T_{\mathrm{ex}})$ is the partition function, $g_{u}$ is the upper-level
degeneracy, $E_{u}$ is the upper-level energy, and $k_{\mathrm{B}}$ is the
Boltzmann constant. If the line is optically thin, the upper-level column
density can be written as

$N_{u}^{\mathrm{thin}}=\frac{8\pi k_{\mathrm{B}}\nu^{2}W}{hc^{3}A_{ul}},$ (2)

Figure 4: Disk-averaged column densities for HCN, H2CO and C2H, assuming
optically thin emission. The colors represent different excitation
temperatures ($T_{\mathrm{ex}}$) assumed in the calculation.

where $\nu$ is the line frequency, $W$ is the integrated line intensity, $h$
is the Planck constant, $c$ is the speed of light, and $A_{ul}$ is the
Einstein coefficient for spontaneous emission. All frequencies, Einstein
coefficients, and partition functions values were taken from the CDMS catalog
(Müller et al., 2001, 2005; Endres et al., 2016), and can be found in Table 2.

We assume the optically thin approximation and a disk-averaged excitation
temperature that ranges from $20-50$ K, which are typical temperatures found
in the disk molecular layer (e.g., Walsh et al., 2010). The results are shown
in Fig. 4. We note that for the 253–1536A/B system, the reported column
densities include the contribution from both members A and B. We can see that
the HCN, H2CO, and C2H column densities are not too sensitive to the
excitation temperature assumptions in this range. Overall, we find
$N_{\mathrm{tot}}(\mathrm{HCN})\sim(0.5-1.8)\times 10^{13}$ cm-2,
$N_{\mathrm{tot}}(\mathrm{H}_{2}\mathrm{CO})\sim(0.4-6.2)\times 10^{13}$ cm-2,
and $N_{\mathrm{tot}}(\mathrm{C}_{2}\mathrm{H})\sim(0.3-1.0)\times 10^{14}$
cm-2 for both disks. These column densities have to be considered as lower
limits because we are assuming that the lines are optically thin, which might
not be the case, in particular for HCN.

We find that the HCN column density is quite similar between the two disks,
with differences of only a factor of $\sim$2\. Moreover, the C2H column
density is almost the same for the two systems. In contrast, the formaldehyde
column density varies between the disks. In particular, the H2CO/HCN ratio is
a factor $\sim 5$ larger in 216–0939 compared to 253–1536A/B. These column
densities are within the range of what has been found in previous studies of
isolated disks (e.g., Pegues et al., 2020; Guzmán et al., 2021), which
reported disk integrated column densities for HCN and C2H in the range of
$10^{12}-10^{15}$ cm-2 in disks around low-mass stars, and $>10^{16}$ cm-2 in
Herbig Ae/Be disks.

###  3.4 HCN excitation temperature

Figure 5: Zeroth–moment maps of HCN $(3-2)$ (left) and HCN $(4-3)$ (right)
line emission of the two disks. Top: 216–0939, Bottom: 253–1536A/B. Figure 6:
Molecular line fluxes for FUV irradiated disks in the Orion Nebula Cluster
(this work) as a function of stellar mass, compared with isolated disks
compiled from the literature, including M4-M5 stars (Pegues et al., 2021), T
Tauri and Herbig Ae/Be (Huang et al., 2017; Bergner et al., 2019b, a, 2020;
Pegues et al., 2020; Öberg et al., 2021; Law et al., 2021b; Guzmán et al.,
2021; Pegues et al., 2023). The fluxes have been normalized to a common
distance of $414$ pc. The left red diamond corresponds to the 216–0939 disk,
assuming the source is a binary system of two $\sim 1~{}\mathrm{M_{\odot}}$
stars. A horizontal error bar is added to consider the possibility the system
is a single massive star of $2.17~{}\mathrm{M_{\odot}}$. The right red diamond
corresponds to the 253–1536A disk. For CO $(2-1)$, lower limits are shown
since these lines are highly affected by cloud contamination. a C2H $(3-2)$
includes $J=5/2-3/2$ and $J=7/2-5/2$ lines.

We estimated the excitation temperature of HCN using the ratio between the HCN
$(3-2)$ observations presented in this work and the HCN $(4-3)$ line initially
published in Mann et al. (2014) and later presented in more detail in Williams
et al. (2014) and Factor et al. (2017). Figure 5 shows the HCN $(3-2)$ and
$(4-3)$ moment zero maps for both sources. For consistency, we computed the
disk-integrated flux of the HCN $(4-3)$ line in the same manner as was done
for the $(3-2)$ line. Additionally, we smoothed the HCN $(3-2)$ emission to
match the angular resolution of the HCN $(4-3)$ line
$(0\hbox{$.\\!\\!^{\prime\prime}$}59\times
0\hbox{$.\\!\\!^{\prime\prime}$}53)$. We used the spectroscopic parameters of
HCN listed in Table 2, and assumed $\tau=0$ (the optically thin case). We
infer an excitation temperature of $20.0^{+4.1}_{-2.9}$ K and
$22.5^{+5.4}_{-3.6}$ K for 216–0939 and 253–1536A/B, respectively. The derived
excitation temperature of $\sim 20$ K is similar for both disks and also
similar to what has been found for other disks (e.g., Bergner et al., 2019a;
Guzmán et al., 2021), suggests that the HCN emission arises from a relatively
cold layer close to the midplane.

In order to investigate whether the excitation temperature increases in the
outer disk, which could be expected for externally irradiated disks, and
taking advantage of the angular resolution of the observations, we also
derived the excitation temperature as a function of radius. We found that, at
the current angular resolution, the excitation temperature is constant across
both disks, with no visible increase in the outer disk.

##  4 Discussion

In this section, we first compare the measured line fluxes of 216-0939 and
253-1536A/B with those found in disks around stars of different masses that
are located in low-mass star forming regions, where the external radiation
field is significantly lower compared to the disks in our sample. Then, we
compare our findings with predictions from chemical models.

###  4.1 Comparison between irradiated and isolated disks

Figures 6 and 7 show the distance-normalized integrated fluxes and upper
limits as a function of stellar mass, for the detected and non-detected lines,
respectively. For 216–0939, we consider two possible stellar scenarios: a
tight equal mass binary of two 1.1 M⊙ stars and a single star of $2.17$ M⊙, as
discussed in Section 2. For 253–1536A/B, we only consider the mass of the
primary A star ($3.5$ M⊙), since most of the emission we detect comes from
this source. For comparison, we also include fluxes reported for isolated
disks around M4–M5 stars, T Tauri stars, and Herbig Ae/Be stars. The
literature disk sample was compiled from Huang et al. (2017), Bergner et al.
(2019b), Bergner et al. (2019a), Bergner et al. (2020), Pegues et al. (2020),
Pegues et al. (2021), Law et al. (2021b), Guzmán et al. (2021), Öberg et al.
(2021), and Pegues et al. (2023). We note that the different molecular line
fluxes were compiled from different studies, and in some cases the same line
flux is reported in two or more studies. In those cases, we selected the most
recent study. The total disk sample includes $5$ M4–M5 stars, $6$ Herbig Ae
disks, $13$ T Tauri disks, and the $2$ externally irradiated systems from our
sample. The stellar masses range from $0.12$ to $3.5~{}\mathrm{M}_{\odot}$,
and the stellar ages range between 0.4 to $>10$ Myr. The two disks in Orion
have ages within this range ($\sim 1-3$ Myr; Williams et al., 2014; Factor et
al., 2017).

Considering the combined sample of isolated and externally irradiated disks,
we find a positive correlation between the fluxes and stellar mass, for all
the lines presented here (see Fig. 6). This trend was previously reported for
isolated disks only by Pegues et al. (2021) and Pegues et al. (2023). The
estimated Spearman correlation coefficient of the combined disk sample is
shown in the bottom right corner of each panel, which corresponds to the
dispersion of the data and measures the correlation between the two variables.
The closer to one the value of the Spearman correlation coefficient, the
stronger the correlation. The strongest correlations are found for C18O, HCN
and C2H. In principle, one could have expected to find brighter CO emission in
the two irradiated disks compared to the isolated disks because they are
expected to be warmer. However, we find that the 12CO and 13CO lines are
weaker than expected according to the observed trend, with an emission flux
similar to the Herbig disks of lower masses. This is probably related to the
cloud contamination in the irradiated disks, which results in a drop of the CO
emission in the central channels (see the channels maps in Appendix B). This
is also consistent with the fact that the less contaminated C18O emission
shows a stronger correlation than the more abundant CO isotopologues. This
suggests that there is no difference in the chemistry between the isolated
sources and the two irradiated disks presented here, and that the differences
between the line fluxes in the disks are mainly due to the stellar masses.

Figure 7:  Same as Figure 6, but displaying the upper limits for a subset of
the non-detected molecular lines.

Regarding the non-detected molecular lines, the derived upper limits are
consistent with the fluxes measured in other disks, where these lines have
been detected (see Figure 7).

Figure 8: The molecular line ratios for FUV irradiated disks in the Orion
Nebula Cluster (this work) compared with isolated disks from the literature.
The two disk systems from this work are shown as red diamonds. The dashed line
shows a one-to-one correlation. 12CO is not included in this comparison
because the derived fluxes are affected by cloud contamination.

In order to remove the dependence of the fluxes on the stellar mass, in Fig. 8
we show the normalized line fluxes of each line against one another, in a
similar manner to Bergner et al. (2019b), Bergner et al. (2019a), Pegues et
al. (2020), Pegues et al. (2021), and Pegues et al. (2023). Overall, we find
that for the combined disk sample, every molecular line combination in Fig. 8
has a strong and positive correlation, as was previously found by Bergner et
al. (2019a); Pegues et al. (2021, 2023) for the isolated disks only. The C2H
vs H2CO pair shows one of the worst correlation, with a Spearman coefficient
of $0.66$. Indeed, the observed C2H flux in the irradiated disks is lower than
expected based on the observed trend in isolated disks. The strongest
correlation corresponds to HCN vs ${}^{13}\mathrm{CO}$ with a Spearman
coefficient ($\rho=0.91$); followed by C18O vs 13CO ($\rho=0.89$), and C2H vs
HCN, with a correlation coefficient of $0.82$.

Finally, Figure 10 shows four different flux ratios (C2H/C18O, H2CO/C18O,
HCN/C18O, and C2H/HCN) as a function of stellar mass. Considering the isolated
disks only, Pegues et al. (2021) found no clear correlation between these flux
ratios and stellar mass. However, with the addition of the externally
irradiated disks 216–0939 and 253–1536A/B, we find that the C2H $(3-2)$/HCN
$(3-2)$ flux ratio shows a tentative trend, with the ratio decreasing with
stellar mass (Spearman coefficient of $-0.24$). This result is unexpected
since the C2H emission should be brighter in irradiated disks because the C2H
formation is expected to be enhanced in the presence of FUV radiation.
However, other factors, like carbon depletion (Bergin et al., 2016) and dust
growth/settling, also play a role in the formation of carbon chains, and could
explain the faint C2H emission in the two Orion disks. In addition, HCN is
also known to be sensitive to photochemistry (Guzmán et al., 2015; Visser et
al., 2018; Bergner et al., 2021; Pegues et al., 2021). Future observations
towards more irradiated disks are needed to confirm the observed tentative
trend.

A similar comparison of flux ratios is presented in Figure 9 but for a subset
of the non-detected molecular lines (DCN and $c-$C3H2). We find that the
observed upper limits in the two irradiated disks are, in general, consistent
with the values found in isolated disks. However, the ratios involving DCN
(e.g., DCN/HCN and DCN/H2CO) seem to be slightly lower for the irradiated
disks compared to the isolated ones. If this is confirmed, it would suggest
that irradiated sources have less cold material than isolated disks, which is
consistent with the expectation of irradiated disks being warmer. However, in
that case we would also expect to see an enhancement of H2CO in the outer disk
due to ice desorption in the warm gas, assumming that H2CO was inherited, and
this is not observed with the current observations (see Fig. 3). Observations
with better sensitivity are needed to confirm this result.

Figure 9: Same as Figure 10 for a subset of the non–detected molecular lines.

These results suggest that the chemistry of the two irradiated disk systems
presented here is not too different from the chemistry of isolated sources, at
least for the molecular lines considered here. Indeed, the two irradiated
disks follow the trends observed for nearby isolated disks. However, we note
that the results presented here may not be representative of the chemistry of
all irradiated disks, because our sample consists of only two particularly
massive irradiated disk systems; 216–0939 and 253–1536A/B are massive disks,
located at $1.59$ and $0.92$ pc away from the radiation field source,
respectively. Indeed, it is possible that some material in the outer disk has
been stripped away by the external radiation field, and our observations are
just tracing the part of the disk that has survived, and is no longer affected
by the external radiation field. Another possibility is that the external
radiation fields are lower than the ones assumed here, due to projection
effects or to the extinction of UV photons by surrounding cluster material.

Molecular line surveys of smaller disks and disks that are closer to the
massive stars are needed to draw more general conclusions about the chemistry
of externally irradiated disks. Unfortunately, this is very challenging
because lines are usually heavily contaminated by the emission from the
molecular cloud. However, recent surveys have been able to detect CO and HCO+
lines in disks close to the ONC (ranging between $0.03-0.15~{}\mathrm{pc}$),
thanks to very sensitive and high angular resolution ALMA observations (Boyden
& Eisner, 2020).

Figure 10: The molecular line ratios for FUV irradiated disks in the Orion
Nebula Cluster (this work) as a function of stellar mass compared with
isolated disks from the literature. Note that this figure shows only a subset
of the non detected lines.

###  4.2 Comparison to model predictions

Our sample consists of two systems around intermediate-mass stars ($1-3.5$
$\mathrm{M}_{\odot}$); unfortunately, chemical models that include an external
radiation field have not been developed for this type of star/disk systems
yet. However, previous theoretical studies have found that the chemistry of T
Tauri and Herbig disks are not too different (Agúndez et al., 2018).
Therefore, we compare our results with models of disks around lower-mass T
Tauri stars. We use the chemical models presented in Walsh et al. (2013) and
Walsh et al. (2014), that were developed for a disk around a T Tauri star with
a mass of $0.5~{}\mathrm{M}_{\odot}$, a radius of $2~{}\mathrm{R}_{\odot}$,
and an effective temperature of $4000~{}\mathrm{K}$. In the models, the disk
is externally irradiated by UV photons from the interstellar radiation field
and a nearby massive O-type star at a distance of $0.1$ pc. We note that the
radiation field in the model goes up to $4\times 10^{4}~{}\mathrm{G}_{0}$,
which is higher than the radiation field illuminating the two disks presented
here ($<10^{3}~{}\mathrm{G}_{0}$). The main results of these models are the
following:

Brighter molecular line emission. Some molecular lines may be brighter for the
irradiated disks because of the higher disk temperatures. In particular, this
should occur for CO (and their isotopologues), CN, and HCN. Our detections of
12CO, 13CO, and C18O are not consistent with this prediction, but this could
be due to the cloud contamination discussed in Section 3.2. In the case of
HCN, the brighter line emission in irradiated disks compared to isolated disks
seems to be related to the stellar mass and not to the external radiation
field. In addition, the measured HCN excitation temperature of $\sim 20$ K is
similar to what has been found in isolated disks, suggesting that the HCN
emission arises from relatively cold gas with the caveat that the lines could
be optically thick. A possible explanation is that we are observing the inner
regions of the disk that are currently shielded and no longer affected by the
external radiation. But more importantly, it is possible that the external
radiation field is just not high enough to produce a significant difference in
the line emission.

COMs enhancement. Chemical models predict that complex organic molecules that
are typically frozen out onto dust grains could be observed in the gas-phase
in warmer and externally irradiated disks because of their higher temperatures
that sublimate these molecules into the gas-phase, in particular in the outer
disk. For example, formaldehyde, as a precursor and tracer of COMs, is
expected to be enhanced in the outer disk, where the temperature should be
higher. However, the observed H2CO radial profiles in the two irradiated disks
are centrally peaked, with no additional emission component in the outer disks
that would be indicative of ice desorption (see Figure 3). However, we note
that the SNR of the detected H2CO lines is low, so it is possible that the
current observations are not sensitive enough to detect such a component in
the outer disk.

Radiation field tracers. The HCN/HCO+ and CN/HCN line ratios are expected to
be larger in irradiated disks compared to isolated disks. HCN/HCO+ ratios
larger than one can be indicative of enhanced external radiation, because HCO+
traces the cold, dense regions of the disk, which are smaller for irradiated
disks (Walsh et al., 2013). Factor et al. (2017) measured a HCN/HCO+ ratio of
$0.58\pm 0.04$ for the 216–0939 disk, which is consistent with the isolated
models from Walsh et al. (2013). $\mathrm{CN/HCN}>1$ can also be indicative of
enhanced radiation fields, similar to what is observed in photodissociation
regions (PDRs), where the CN/HCN ratio is found to correlate with the FUV
field (Fuente et al., 1993). This is because FUV photons enhance the abundance
of CN in the outer disk, which is more exposed to external radiation (Guzmán
et al., 2015; Bergner et al., 2021). Unfortunately, CN has not yet been
observed in these irradiated disks. Future observations of CN lines are needed
to verify this prediction.

##  5 Summary

We have presented ALMA observations of the continuum and line emission of
several molecules toward two externally FUV-irradiated protoplanetary disk
systems around pre-main sequence stars in the outskirts of the Orion Nebula
Cluster. In particular, we presented observations of the CO isotopologues, the
small organic molecules HCN and H2CO, the carbon chains C2H and $c-$C3H2, and
the deuterated species DCN and N2D+, all of which have been previously
detected in isolated disks. The main conclusions are the following:

  * •

The high-angular resolution observations of the dust continuum emission in
216–0939 shows the presence of a large gap in the inner disk, that is well
resolved for the first time in our data. Additionally, the dust emission is
asymmetric or eccentric, with the southern side of the disk being $23\%$
brighter than the northern side. We estimate the outer edge of this gap to be
around $120-135~{}\mathrm{au}$, and a disk size of $311.5\pm
14.5~{}\mathrm{au}$. The high-angular resolution observations of the dust
continuum emission allow us to separate the two members in the binary system
253–1536A/B, measure their disk sizes ($239.1\pm 14.5~{}\mathrm{au}$ for A,
and $108.7\pm 14.5~{}\mathrm{au}$ for B) and the separation between their
edges ($\sim 124~{}\mathrm{au}$). In addition, we do not observe substantial
substructure with the current observations, in neither 216–0939 nor
253–1536A/B.

  * •

We detected the 12CO $(2-1)$, 13CO $(2-1)$ and C18O $(2-1)$ lines, as well as
the HCN $(3-2)$, H2CO $(3-2)$, and C2H $(3-2)$ lines toward the 216–0939 and
253–1536A/B disks. The CO and CO isotopologue emission is affected by cloud
contamination. We estimated the disk-integrated column densities of HCN, H2CO
and C2H, assuming optically thin emission, and a range of excitation
temperatures, and found values in the range of $10^{13}-10^{14}$ cm-2, similar
to what is observed in isolated disks.

  * •

Molecular lines such as $c-$C3H2 $(6_{06}-5_{15})$, $c-$C3H2
$(6_{25}-5_{14})$, $c-$C3H2 $(7_{07}-6_{16})$, DCN $(3-2)$ and N2D+ $(3-2)$
were not detected in neither the 216–0939 nor 253–1536A/B disks. The cold
tracers N2D+ and DCN are expected to be less abundant in the warmer irradiated
disks compared to isolated disks, since these molecules are formed more
efficiently at low temperatures. However, the estimated upper limits for the
disk-integrated fluxes are consistent with detections of these lines in
isolated disks.

  * •

In general, we do not observe significant differences between the chemistry of
isolated and the two irradiated disks presented in this work, based on the
observed disk-integrated fluxes and flux ratios for the molecular lines
presented here. The differences between the 216–0939 or 253–1536A/B disks and
typical T Tauri and Herbig Ae/Be disks found in low-mass star forming regions,
seem to be more closely related to the different stellar masses than to the
presence of an enhanced external radiation field.

  * •

The observed disk integrated fluxes and line ratios in the two irradiated
disks presented here are not consistent with chemical model predictions of
externally irradiated disks presented by Walsh et al. (2013). However, the
216–0939 and 253–1536A/B disk are irradiated by a weaker FUV field
($<10^{3}\mathrm{G}_{0}$) than the one included in the models ($\sim
10^{4}~{}\mathrm{G}_{0}$). Our results, therefore, suggest that these disks
are far enough away from the ONC so that their chemistry is no longer
substantially affected, but disks located closer to the stellar cluster may
experience stronger chemical effects.

The results presented in this work show that the chemical composition in these
moderately irradiated systems is similar to that in isolated disks, which
suggests that the assembly of planetary systems and their atmospheres will
proceed in a similar manner to that expected in the better studied isolated
systems.

Future observations of disks exposed to higher radiation fields are needed to
better determine the differences between isolated and externally irradiated
disks. In particular, observations of disks closer to the ONC are needed to
investigate how the chemistry changes with distance from the ionizing source.
In addition, observations with better sensitivity are needed to detect lines
from cold molecular tracers, such as DCN, and determine whether they are
indeed less abundant in the warmer irradiated disks compared to isolated
disks. Finally, chemical models including more moderate radiation fields are
needed to further investigate how the chemistry is affected by an external
radiation field.

## Acknowledgments

We thank the referee for a careful read of the manuscript, and helpful
comments that improved the presentation and discussion of the paper.

J. K. D.-B. acknowledges support in the form of a studentship from the Science
and Technology Facilities Council (grant number ST/X508536/1).

V.V.G gratefully acknowledges support from FONDECYT Regular 1221352, and ANID
CATA-BASAL project FB210003.

C.W. acknowledges financial support from the Science and Technology Facilities
Council and UK Research and Innovation (grant numbers ST/X001016/1 and
MR/T040726/1).

L.I.C. acknowledges support from NASA ATP 80NSSC20K0529. L.I.C. also
acknowledges support from NSF grant no. AST-2205698, the David and Lucille
Packard Foundation, and the Research Corporation for Scientific Advancement
Cottrell Scholar Award.

E.A.dlV. acknowledges financial support provided by FONDECYT grant 3200797.

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##  Appendix A Spectral windows

The spectral settings for Band 6 observations are shown in Table A1.

Table A1: Spectral settings.

Molecule | Line | Resolution [km s-1] | Bandwidth [MHz]
---|---|---|---
220 GHz Setting
DCN | $3-2$ | $0.195$ | $58.59$
$c-$C3H2 | $6_{06}-5_{15}$ | $0.194$ | $58.59$
H2CO | $3_{22}-2_{21}$ | $0.194$ | $58.59$
C18O | $2-1$ | $0.193$ | $58.59$
13CO | $2-1$ | $0.192$ | $58.59$
12CO | $2-1$ | $0.184$ | $117.19$
N2D+ | $3-2$ | $0.183$ | $117.19$
265 GHz Setting
C2H | $3-2$ | $0.161$ | $234.38$
$c-$C3H2 | $6_{25}-5_{14}$ | $0.168$ | $58.59$
| $7_{07}-6_{16}$ | $0.168$ | $58.59$
HCN | $3-2$ | $0.159$ | $234.38$

Note. — Spectral settings of the molecular line transitions. It includes the
bandwidth for each spectral window.

##  Appendix B Channel Maps

The channel maps of the observed molecular lines are shown in Figures B.1,
B.2, B.3, and B.4 for the 216–0939 disk, and in Figures B.5, B.6, B.7, and B.8
for the 253–1536A/B system.

Figure B.1:  Channel maps for the CO ($2-1$), 13CO ($2-1$), and C18O ($2-1$)
lines in 216–0939 (systemic velocity of $10.75$ km/s). The Keplerian mask is
shown in black. Red contours correspond to the $3\sigma$ level of the
emission. The synthesized beam is shown in the bottom left panel.  Figure B.2:
Same as B.1 but for HCN ($3-2$), H2CO ($3-2$), and C2H ($3-2$) lines in
216–0939. Figure B.3:  Same as B.1 but for $c-$C3H2 ($6_{06}-5_{15}$),
$c-$C3H2 ($6_{25}-5_{14}$), and $c-$C3H2 ($7_{07}-6_{16}$) lines in 216–0939.
Figure B.4:  Same as B.1 but for DCN ($3-2$), and N2D+ ($3-2$) lines in
216–0939. Figure B.5:  Same as B.1 but for CO ($2-1$), 13CO ($2-1$), and C18O
($2-1$) lines in 253–1536 (systemic velocity of $10.55$ km/s). Figure B.6:
Same as B.1 but for HCN ($3-2$), H2CO ($3-2$), and C2H ($3-2$) lines in
253–1536. Figure B.7:  Same as B.1 but for $c-$C3H2 ($6_{06}-5_{15}$),
$c-$C3H2 ($6_{25}-5_{14}$), and $c-$C3H2 ($7_{07}-6_{16}$) lines in 253–1536.
Figure B.8:  Same as B.1 but for DCN ($3-2$), and N2D+ ($3-2$) lines in
253–1536.

##  Appendix C Matched filter results

Figs. C.1 and C.2 show examples of the filter response spectra from the
matched–filter technique (VISIBLE; Loomis et al., 2018). The first figure
shows the difference in the response for a bright detected line (H2CO)
compared to a weak non-detected line (DCN), using the HCN line as a filter in
both cases. The second figure shows the difference in the response for C2H
when using different filters. A clear peak is seen in the response when the
C18O line is used as a filter, suggesting the C2H line is detected, and a less
clear detection is seen when instead HCN is used as a filter. This suggests
that the distribution of C2H is likely more similar to that of C18O than that
of HCN.

Figure C.1: Filter response spectra for H2CO $(3-2)$ (left) and DCN $(3-2)$
(right) in the 216–0939 disk. Both impulse responses are obtained using HCN
$(3-2)$ as the filter.  Figure C.2:  Filter response spectra for C2H $(3-2)$
toward the 216–0939 disk, obtained using C18O $(2-1)$ (left) and HCN $(3-2)$
(right) as filters.

##  Appendix D Disk integrated spectra

Figure D.1 shows the disk-integrated spectra for the detected lines in
216–0939 and 253–1536A/B. These spectra are obtained by adding all the
emission inside the Keplerian mask in each channel. We note that the disk-
integrated spectra of the 253–1536A/B system includes emission from both
binary members. The CO isotopologues and HCN emission show a double peaked
profile, typical of Keplerian rotation of an inclined disk, in both sources.
However, the CO isotopologue lines are heavily affected by absorption from the
molecular cloud, in particular in the central channels. For molecular lines
such as H2CO and C2H, it is more difficult to distinguish the Keplerian
rotation associated with the disk, because these lines are faint.

Figure D.1:  Disk integrated spectra for 216–0939 (left panels) and for
253–1536A/B (right panels). Upper panels: CO and 13CO. Central panels: C18O)
and HCN. Lower panels: H2CO and C2H.

##  Appendix E Deviations from Keplerian rotation in 253–1536A/B

Figure E.1 shows the first-moment map of the CO $(2-1)$ line emission for the
253–1536A/B system. The moment was obtained using the bettermoments Python
package (Teague et al., 2018; Teague, 2019b), which collapses the emission
cube and estimates the intensity weighted average velocity in each pixel. The
figure shows the Keplerian rotation of the disk, mainly for the 253–1536A
member. In addition, a tentative deviation from Keplerian rotation is seen
towards the northern side of the 253–1536A star, which is indicated by the
black arrow.

Figure E.1:  First–moment of the CO $(2-1)$ line emission for 253–1536A/B.
Colors represent red- and blue-shifted parts of the cube relative to the
source velocity. The beam size is shown in the bottom left corner of the
figure. White contours correspond to $2$, $15$, and $30\sigma$ of the dust
continuum emission of the sources. The black arrow represents a tentative
deviation from Keplerian rotation.