11institutetext: Niels Bohr Institute $\&$ Centre for Star and Planet
Formation, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K.,
Denmark
11email<EMAIL_ADDRESS>
# Revealing the chemical structure of the Class I disc Oph-IRS 67

E. Artur de la Villarmois 11 L. E. Kristensen 11 J. K. Jørgensen 11

###### Abstract

Context. Recent results suggest that the first steps towards planet formation
may be already taking place in protoplanetary discs during the first 100,000
years after stars form. It is therefore crucial to unravel the physical and
chemical structures of such discs in their earliest stages while they are
still embedded in their natal envelopes and compare them with more evolved
systems.

Aims. The purpose of this paper is to explore the structure of a line-rich
Class I protobinary source, Oph-IRS 67, and analyse the differences and
similarities with Class 0 and Class II sources.

Methods. We present a systematic molecular line study of IRS 67 with the
Submillimeter Array (SMA) on 1–2$\arcsec$ (150–300 AU) scales. The wide
instantaneous band-width of the SMA observations ($\sim$30 GHz) provide
detections of a range of molecular transitions that trace different physics,
such as CO isotopologues, sulphur-bearing species, deuterated species, and
carbon-chain molecules.

Results. We see significant differences between different groups of species.
For example, the CO isotopologues and sulphur-bearing species show a
rotational profile and are tracing the larger-scale circumbinary disc
structure, while CN, DCN, and carbon-chain molecules peak at the southern edge
of the disc at blue-shifted velocities. In addition, the cold gas tracer DCO+
is seen beyond the extent of the circumbinary disc.

Conclusions. The detected molecular transitions can be grouped into three main
components: cold regions far from the system, the circumbinary disc, and a UV-
irradiated region likely associated with the surface layers of the disc that
are reached by the UV radiation from the sources. The different components are
consistent with the temperature structure derived from the ratio of two H2CO
transitions, that is, warm temperatures are seen towards the outflow
direction, lukewarm temperatures are associated with the UV-radiated region,
and cold temperatures are related with the circumbinary disc structure. The
chemistry towards IRS 67 shares similarities with both Class 0 and Class II
sources, possibly due to the high gas column density and the strong UV
radiation arising from the binary system. IRS 67 is, therefore, highlighting
the intermediate chemistry between deeply embedded sources and T-Tauri discs.

###### Key Words.:

ISM: molecules – stars: formation – protoplanetary discs – astrochemistry –
ISM: individual objects: Oph-IRS 67

##  1 Introduction

Protoplanetary discs are a crucial stage between warm cores and the formation
of planetesimals and planets, and the final composition of planets likely
depends on the chemical processing within the disc. The chemical complexity of
the disc is established by the material that is incorporated from the inner
envelope and by physical processes that promotes a chemical reset (e.g.
outflows, accretion shocks, UV radiation field; Bergin et al. 2003; Ceccarelli
2004; Herbst & van Dishoeck 2009; Sakai & Yamamoto 2013; Miura et al. 2017).
The detail of how and when the disc forms are still not well understood. In
particular, the physics and chemistry of the innermost regions are challenging
to observe and study, as they are embedded within a large amount of gas and
dust. It is, therefore, essential to study and understand the first stages of
discs formation. This paper presents an extensive Submillimeter Array (SMA)
survey of the molecular line emission on a few hundred AU scales towards the
Class I protobinary system Oph-IRS 67, and investigates the physical and
chemical signatures associated with the system.

The complex structure of embedded discs is associated with a huge range of
temperatures ($\sim$10$-$1000 K) and densities (105$-$1013 cm-3; Herbst & van
Dishoeck 2009; van Dishoeck 2018), making molecules excellent diagnostics of
the physical conditions and processes. Chemical surveys towards deeply
embedded sources show the potential of the chemistry to shed light on the
physical structure of these regions (e.g. Jørgensen et al. 2016; Lefloch et
al. 2018; Yoshida et al. 2019). Also, the chemical inventories and initial
conditions of young discs can then be compared with discs around more evolved
(Class II or T Tauri) young stars (e.g. Öberg et al. 2011; Kastner et al.
2018).

Class I discs represent the bridge between deeply embedded Class 0 sources and
the emergence of planet-forming discs, that is, Class II sources. Recent high-
sensitivity and spatial resolution observations suggest that planet formation
starts at early stages (Class I), based on the evidence of considerable grain
growth (e.g. ALMA Partnership et al. 2015; Harsono et al. 2018). The study of
Class I discs is, therefore, essential in order to constrain the first steps
of planet formation and to analyse the chemical evolution of discs at
different stages. The observations of Class I discs are challenging since they
are small in size ($\sim$100 AU; Harsono et al. 2014; Yen et al. 2015) and
other components, such as inner envelope and outflow material, can contribute
to the emission on small scales.

The Class I protobinary system IRS 67 is located in the Ophiuchus star forming
region and belongs to the L1689 cloud. It has been associated with a large-
scale outflow structure ($\gtrsim$1000 AU; Bontemps et al. 1996), bolometric
temperature (Tbol) of 130 K, and bolometric luminosity (Lbol) of 4.0 L⊙
(Dunham et al. 2015). The binarity of the system was proven by McClure et al.
(2010) at infrared wavelengths, where a separation of
0$\aas@@fstack{\prime\prime}$6 ($\sim$90 AU for a distance of 151.2 pc; Ortiz-
León et al. 2017) was found between the sources. More recently, a Keplerian
circumbinary disc has been associated to IRS 67 with an extent of $\sim$620 AU
(Artur de la Villarmois et al. 2018). In comparison with other Class I
sources, IRS 67 shows a particular rich chemistry and bright emission of
c-C3H2 (Artur de la Villarmois et al. 2018, 2019), a molecule commonly
associated with photon-dominated regions (PDRs; Guzmán et al. 2015; Murillo et
al. 2018).

This paper presents an SMA chemical survey towards the Class I proto-binary
system IRS 67 and is laid out as follows. Section 2 presents the observational
details, data calibration, and spectral coverage. The continuum emission,
detected molecular transitions, contour maps, moment 0 maps, and spectral
features are described in Sect. 3. We discuss the chemical and physical
structure of IRS 67 in Sect. 4, together with an analysis of the non-detected
molecular lines, and a comparison with other stages of low-mass star
formation. Finally, the main results are summarised in Sect. 5.

##  2 Observations

We observed IRS 67 using the Submillimeter Array (SMA), in the extended and
compact configurations on 2017 February 24 and 2017 April 21, respectively
(program code: 2016B-S0004; PI: Elizabeth Artur de la Villarmois). The antenna
configuration provided projected baselines between 10 and 188 metres. A
pointing centre of $\alpha$ = 16h32m00$\aas@@fstack{s}$98, $\delta$ =
$-$24$\degr$56$\arcmin$43$\aas@@fstack{\prime\prime}$4 (J2000) was used,
corresponding to the peak of the continuum emission at 0.87 mm (Artur de la
Villarmois et al. 2018). The observations covered a frequency range of
$\sim$30 GHz, between 214.3 and 245.6 GHz (Table 1) with a spectral resolution
of 139.64 kHz (0.18 km s-1).

The calibration and imaging were done in CASA111http://casa.nrao.edu/
(McMullin et al. 2007). For the extended track, the complex gains were
calibrated through observations of the quasar 1626-298, passband calibration
on 3c273, and flux calibration on Ganymede. For the compact track, the gain,
passband, and flux calibrations were implemented by observing the quasar
1626-298, 3c454.3, and Callisto, respectively. The extended and compact
observations were combined and a Briggs weighting with robustness parameter of
1 was applied to the visibilities. The resulting dataset has a typical beam
size of 1$\aas@@fstack{\prime\prime}$7 $\times$1$\aas@@fstack{\prime\prime}$4
($\sim$260 $\times$ 210 AU) and a maximum angular scale of $\sim$25$\arcsec$.
The continuum and line rms are given in Table 1.

Table 1: Summary of the observations.

Receiver | Sideband | Frequency range | Continuum rms | Spectral rms
---|---|---|---|---
|  | (GHz) | (Jy beam-1) | (Jy beam-1 channel) (a)
230 | lower | 214.3 - 222.3 | 0.001 | 0.08
240 | lower | 221.6 - 229.6 | 0.003 | 0.15
230 | upper | 230.3 - 238.3 | 0.001 | 0.10
240 | upper | 237.6 - 245.6 | 0.004 | 0.17
222(a) The channel width is 0.18 km s-1.

##  3 Results

###  3.1 Continuum

Table 2: Results of 2D Gaussian fits towards the continuum peaks.

Receiver | Sideband | Size (a) | PA | Integrated flux | Intensity peak
---|---|---|---|---|---
|  | ($\arcsec$) | ($\degr$) | (Jy) | (Jy beam-1)
230 | lower | 1.4 $\pm$ 0.1 $\times$ 0.7 $\pm$ 0.2 | 38 $\pm$ 9 | 0.087 $\pm$ 0.004 | 0.056 $\pm$ 0.002
240 | lower | 1.4 $\pm$ 0.2 $\times$ 1.0 $\pm$ 0.4 |  3 $\pm$ 44 | 0.097 $\pm$ 0.007 | 0.064 $\pm$ 0.003
230 | upper | 1.2 $\pm$ 0.1 $\times$ 0.6 $\pm$ 0.1 | 41 $\pm$ 6 | 0.105 $\pm$ 0.003 | 0.071 $\pm$ 0.002
240 | upper | 1.6 $\pm$ 0.3 $\times$ 1.3 $\pm$ 0.5 |  24 $\pm$ 81 | 0.130 $\pm$ 0.012 | 0.068 $\pm$ 0.005
333(a) Deconvolved size (FWHM).

The continuum emission for each receiver and sideband is fitted with two-
dimensional (2D) Gaussians in the image plane, and the resulting deconvolved
size, position angle, integrated flux, and intensity peak are listed in Table
2. The continuum emission at 0.87 mm, previously detected with ALMA (Artur de
la Villarmois et al. 2018), showed the contribution of both protostellar
sources and a circumbinary disc, associated with a position angle of
54$\degr$, an extent of $\sim$620 AU, and a total integrated flux (both
sources and disc) of 0.30 Jy.

In the Rayleigh-Jeans limit of the spectrum, the sub-millimeter flux (Fν) has
a power-law dependence with the frequency, i.e. Fν $\propto$ $\nu$α, where
$\alpha$ is the slope of the sub-mm spectral energy distribution (SED) and is
related with the dust opacity spectral index ($\beta$) by $\alpha$ = $\beta$
\+ 2 (e.g. Beckwith & Sargent 1991; Testi et al. 2014). By fitting a power law
distribution to the integrated fluxes from Table 2 and the 0.87 mm value from
Artur de la Villarmois et al. (2018), an $\alpha$ index of 2.8 $\pm$ 0.3 is
found, and $\beta$ = 0.8 $\pm$ 0.3. The typical $\beta$-value of the
interstellar medium (ISM) is $\sim$1.7 (Natta et al. 2007), therefore, a lower
value suggests grain growth or optically thick dust emission. This is
consistent with the results from Jørgensen et al. (2007), who find an $\alpha$
index of between two and three for a sample of Class 0 protostars.

Table 3: Parameters of the detected molecular transitions.

Species | Transition | Frequency (a) |  Aij (a) |  Eu (a) |  ncrit (b)
---|---|---|---|---|---
|  | (GHz) | (s-1) | (K) | (cm-3)
CO- species
CO |  J=2$-$1 | 230.53800 | 6.92 $\times$ 10-7 | 17 | 2.2 $\times$ 104
13CO |  J=2$-$1 | 220.39868 | 6.07 $\times$ 10-7 | 16 | 2.0 $\times$ 104
C18O |  J=2$-$1 | 219.56035 | 6.01 $\times$ 10-7 | 16 | 1.9 $\times$ 104
C17O |  J=2$-$1 | 224.71439 | 6.42 $\times$ 10-7 | 16 | 2.1 $\times$ 104
H2CO | 30,3$-$20,2 (para) | 218.22219 | 2.82 $\times$ 10-4 | 21 | 3.4 $\times$ 106
H2CO | 32,2$-$22,1 (para) | 218.47563 | 1.58 $\times$ 10-4 | 68 | 1.4 $\times$ 106
H2CO | 32,1$-$22,0 (para) | 218.76007 | 1.58 $\times$ 10-4 | 68 | 3.2 $\times$ 106
H2CO | 31,2$-$21,1 (ortho) | 225.69778 | 2.75 $\times$ 10-4 | 33 | 5.7 $\times$ 106
Deuterated species
DCO+ |  J=3$-$2 | 216.11258 | 2.38 $\times$ 10-3 | 21 | 7.0 $\times$ 106
DCN | 3$-$2 | 217.23854 | 4.57 $\times$ 10-4 | 21 | 2.5 $\times$ 107
DNC | 3$-$2 | 228.91049 | 5.62$\times$ 10-4 | 22 | 4.9 $\times$ 106
S- species
CS | 5$-$4 | 244.93556 | 2.95 $\times$ 10-4 | 35 | 8.7 $\times$ 106
C34S | 5$-$4 | 241.01609 | 2.75 $\times$ 10-4 | 28 | 8.0 $\times$ 106
13CS | 5$-$4 | 231.22069 | 2.51 $\times$ 10-4 | 33 | 7.4 $\times$ 106
SO | 55$-$44 | 215.22065 | 1.20 $\times$ 10-4 | 44 | 1.8 $\times$ 108 (c)
SO | 65$-$54 | 219.94944 | 1.35 $\times$ 10-4 | 35 | 2.5 $\times$ 108 (c)
N- species
CN (d) |  N=2$-$1, J=3/2$-$3/2, F=1/2$-$1/2 | 226.28742 | 1.02 $\times$ 10-5 | 16 | 6.1 $\times$ 104
CN |  N=2$-$1, J=3/2$-$3/2, F=5/2$-$5/2 | 226.35987 | 1.62 $\times$ 10-5 | 16 | 5.8 $\times$ 105
CN |  N=2$-$1, J=3/2$-$1/2, F=1/2$-$3/2 | 226.61657 | 1.07 $\times$ 10-5 | 16 | 1.8 $\times$ 105
CN |  N=2$-$1, J=3/2$-$1/2, F=3/2$-$3/2 | 226.63219 | 4.27 $\times$ 10-5 | 16 | 2.6 $\times$ 105
CN |  N=2$-$1, J=3/2$-$1/2, F=5/2$-$3/2 | 226.65956 | 9.55 $\times$ 10-5 | 16 | 7.0 $\times$ 105
CN |  N=2$-$1, J=3/2$-$1/2, F=1/2$-$1/2 | 226.66369 | 8.51 $\times$ 10-5 | 16 | 6.0 $\times$ 105
CN |  N=2$-$1, J=3/2$-$1/2, F=3/2$-$1/2 | 226.67931 | 5.25 $\times$ 10-5 | 16 | 3.8 $\times$ 105
CN |  N=2$-$1, J=5/2$-$3/2, F=5/2$-$3/2 | 226.87419 | 9.55 $\times$ 10-5 | 16 | 2.4 $\times$ 106
CN |  N=2$-$1, J=5/2$-$3/2, F=7/2$-$5/2 | 226.87478 | 1.15 $\times$ 10-4 | 16 | 5.1 $\times$ 107
CN |  N=2$-$1, J=5/2$-$3/2, F=3/2$-$1/2 | 226.87590 | 8.51 $\times$ 10-5 | 16 | 1.2 $\times$ 106
CN |  N=2$-$1, J=5/2$-$3/2, F=3/2$-$3/2 | 226.88742 | 2.75 $\times$ 10-5 | 16 | 9.5 $\times$ 105
CN |  N=2$-$1, J=5/2$-$3/2, F=5/2$-$5/2 | 226.89213 | 1.82 $\times$ 10-5 | 16 | 5.4 $\times$ 105
13CN |  N=2$-$1, J=5/2$-$3/2, F=4$-$3 | 217.46715 | 1.01 $\times$ 10-4 | 16 |
Carbon-chain species
c-C3H2 | 33,0$-$22,1 (ortho) | 216.27876 | 2.57 $\times$ 10-4 | 19 | 2.0 $\times$ 108
c-C3H2 | 60,6$-$51,5 (para) | 217.82215 | 5.37 $\times$ 10-4 | 39 | 8.7 $\times$ 107
c-C3H2 | 51,4$-$42,3 (ortho) | 217.94005 | 3.98 $\times$ 10-4 | 35 | 6.3 $\times$ 107
c-C3H2 | 52,4$-$41,3 (para) | 218.16046 | 4.07 $\times$ 10-4 | 35 | 9.0 $\times$ 107
c-C3H2 | 72,6$-$71,7 (para) | 218.73273 | 8.91 $\times$ 10-5 | 61 | 2.9 $\times$ 107
c-C3H2 | 43,2$-$32,1 (ortho) | 227.16914 | 3.09 $\times$ 10-4 | 29 | 2.8 $\times$ 107
HC3N |  J=24$-$23 | 218.32472 | 8.32 $\times$ 10-4 | 131 | 4.7 $\times$ 107
HC3N |  J=25$-$24 | 227.41891 | 9.33 $\times$ 10-4 | 142 | 1.2 $\times$ 107
HC3N |  J=26$-$25 | 236.51279 | 1.05 $\times$ 10-3 | 153 | 3.2 $\times$ 107
444(a) Values from the CDMS database (Müller et al. 2001). (b) Calculated
values for a collisional temperature of 30 K and collisional rates from the
Leiden Atomic and Molecular Database (LAMDA; Schöier et al. 2005). (c) For a
collisional temperature of 60 K. (d) CN transitions with Aij higher than 1
$\times$ 10-5 s-1. The references for collisional rates of specific species
are presented in Table 4.

Figure 1:  CO emission above 5$\sigma$ ($\sigma$ = 0.04 Jy beam-1 km s-1). The
contours start at 5$\sigma$ and follow a step of 5$\sigma$. The numbers in
brackets indicate the velocity interval in units of km s-1 and the extent of
the circumbinary disc is represented by the black solid segment. The yellow
stars show the position of the sources. The synthesised beam and the outflow
direction from Bontemps et al. (1996) are represented in the upper-left panel
with a black filled ellipse and blue and red arrows, respectively.  Figure 2:
Emission of CO isotopologues and other bright molecular transitions. Channel
maps consist of velocity ranges of 1 km s-1 and moment 0 maps are integrated
over a velocity range of 8 km s-1. The contours start at 5$\sigma$ and follow
a step of 5$\sigma$ for 13CO, C18O, and CS (the remaining follow a step of
3$\sigma$). The two white crosses in the CS moment 0 map indicate the position
from which the spectra from Figs. 4 and 5 are extracted. The yellow stars show
the position of the sources and the solid segment represents the extent of the
circumbinary disc. The synthesised beam is represented by a solid black
ellipse in the right panels.

###  3.2 Molecular transitions

The observations covered multiple molecular transitions, such as CO
isotopologues, S-bearing species, deuterated species, and carbon-chain
molecules. The detected transitions are listed in Table 3 with their
spectroscopic parameters, while the lines without detection above a 3$\sigma$
level are listed in Table 4 in the Appendix.

####  3.2.1 CO isotopologues

The CO emission is shown in Fig. 1 for different velocity ranges. Most of the
CO emission is expected to be extended and thus, filtered out by the
interferometer due to the lack of short spacings. By comparing APEX data
(Lindberg et al. 2017) with convolved SMA C18O emission (see Fig. 12 and Table
5 in the Appendix), the recovered C18O flux with the SMA is less than 1$\%$.
Since CO is the more abundant isotopologue, and its emission is expected to be
more extended than C18O, the filtering out of emission is more significant for
CO and thus, the recovered CO flux is expected to be much lower than 1$\%$.
This implies that the SMA observations are probing the deepest and denser
regions, and the detected CO emission in Fig. 1 traces the more compact
structures. The emission is centred at the position of the system, probing
outer-envelope material, and knot-like structures are detected in red-shifted
emission between 1 and 2 km s-1 and blue-shifted emission between 3 and 4 km
s-1, which are consistent with the outflow direction found by Bontemps et al.
(1996) (see upper-left panel of Fig. 1).

Emission from 13CO, C18O, and C17O is shown in Fig. 2. 13CO presents a flatted
shape that follows the circumbinary disc structure and extends beyond it. C18O
follows a similar distribution as 13CO, and C17O stands out towards the
southern part of the disc. Another C17O transition (J=3$-$2) is analysed by
Artur de la Villarmois et al. (2018), where the emission traces the Keplerian
circumbinary disc, therefore, the differences seen between C17O J=3$-$2 and
C17O J=2$-$1 may be related to the sensitivity of the observations. A
rotational profile, perpendicular to the outflow direction, is clearly seen
for 13CO and C18O for velocities beyond $\pm$2 km s-1.

####  3.2.2 Sulphur-bearing species

Emission of CS and the brightest SO transition (65$-$54) is also shown in Fig.
2. The high dipole moment of CS makes this molecule a very good high-density
tracer (e.g. van der Tak et al. 2000). Its emission shows a large extent
($\sim$10$\arcsec$) and peaks at the edges of the circumbinary disc, showing a
rotational profile around the binary system. SO shows a more compact
distribution than CS, tracing the circumbinay disc material. SO 65$-$54 is
slightly brighter than SO 55$-$44 (see Fig. 8 in the Appendix) and a third SO
transition (12$-$21) was also targeted, however, no emission is detected above
a 3$\sigma$ level. This may be related with its low Aij, which is two orders
of magnitude lower than the value from the detected SO transitions (see Tables
3 and 4 in the Appendix).

Less abundant CS isotopologues, such as C34S and 13CS, are shown in Fig. 9 in
the Appendix, where C34S is more than a factor of two brighter than 13CS, in
agreement with their relative abundances with respect to CS (32S/34S=22 and
12C/13C=69; Wilson 1999). C34S follows the CS emission, where both species
peak at the edges of the circumbinary disc. 13CS, on the other hand, presents
isolated peaks that lack a clear correlation with the main isotopologue. The
emission does, however, have a low signal-to-noise level ($\leq$4$\sigma$).

####  3.2.3 H2CO

The observations include six H2CO transitions, associated with Eu values from
21 to 280 K. The transitions with the highest Eu (174 and 280 K) are not
detected (see Table 4) and the strongest lines are o-H2CO 31,2$-$21,1 (Eu = 33
K) and p-H2CO 30,3$-$20,2 (Eu = 21 K), shown in Fig. 2. The emission extends
beyond the circumbinary disc structure, peaks towards the edges of the disc,
and the intensity decreases towards the positions of the protostars. This is
similar to what is seen for CS, however, the H2CO emission stands out towards
the southern part of the circumbinary disc and the peak is slightly south from
the disc. It is interesting to notice that the southern part of the disc is
related with blue-shifted emission, while the blue component of the outflow is
seen towards the north-west.

Two H2CO transitions (32,2$-$22,1 and 32,1$-$22,0) show weak emission and are
presented in Fig. 9 in the Appendix. Their emission do not follow the
distribution from the brightest H2CO transition (31,2$-$21,1), however, both
of them are associated with higher Eu values of 68 K. As previously shown
(e.g. Mangum & Wootten 1993), and further discussed in Section 4.1, the
intensity ratios of these specific H2CO transitions are sensitive tracers of
the gas kinetic temperature and density.

####  3.2.4 Carbon-chain molecules

The C3H2 molecule has two isomeric forms: cyclic (c-C3H2) and linear (l-C3H2).
Although standard astrochemical models predict a cyclic-to-linear C3H2 ratio
of $\sim$1, observations show higher values ranging from 3 to 70 (Sipilä et
al. 2016). Our observations targeted nine c-C3H2 transitions, with Eu between
19 and 87 K, and three l-C3H2 transitions, with Eu between 66 and 80 K (see
Tables 3 and 4). However, the linear isomer is not detected. Figure 2 shows
the brightest c-C3H2 transition, where the emission stands out between $-$3
and $-$1 km s-1 and peaks south from the circumbinary disc. Other three c-C3H2
lines present weaker emission and are shown in Fig. 10 in the Appendix.

The brightest c-C3H2 transition (60,6$-$51,5) was also detected with APEX by
Lindberg et al. (2017) (see Appendix), and more than 92$\%$ of the flux is
filtered out by the interferometer, suggesting that the c-C3H2 distribution is
much more extended than what is seen in Fig. 2. In addition, c-C3H2 is the
only transition in the APEX data that peaks at 2.9 km s-1, and not at 4.2 km
s-1 like the rest of the detected molecular transitions (DCO+, H2CO, C18O, and
SO). The velocity of 2.9 km s-1 is consistent with the blue-shifted material
detected in the SMA observations, where the c-C3H2 emission stands out between
$-$3 and $-$1 km s-1 with respect to the binary system velocity.

Three HC3N transitions are detected and shown in Fig. 10 in the Appendix. The
emission peaks south of the circumbinary disc, which is consistent with the
c-C3H2 emission, suggesting that this region is rich in carbon-chain
molecules. On the other hand, the HC3N J=24$-$23 transition was observed with
APEX, but not detected, therefore, the emission may arise from a compact
component (i.e. the emission is suffering from beam dilution).

####  3.2.5 CN

Figure 3:  CN and deuterated species, DCN, DNC, and DCO+, moment 0 maps
integrated over a velocity range of 6 km s-1. The yellow stars show the
position of the sources and the solid segment represents the extent of the
circumbinary disc. The synthesised beam is represented by a black filled
ellipse in the upper-left panel.

CN is know as a photon-dominated region (or photodissociation region, PDR)
tracer, since its abundance increases when HCN photo-dissociates into CN due
to ultraviolet (UV) radiation from the star or the interstellar radiation
field (Willacy & Langer 2000; Aikawa & Herbst 2001; van Zadelhoff et al.
2003). Although gas-phase CN is expected to be abundant towards protostars, it
is often challenging to observe with interferometers at millimeter wavelengths
as it typically is extended and filtered out (e.g. Jørgensen et al. 2011).
Towards one Class 0 source, L483-mm, CN was found to probe material in the
boundary between the bulk protostellar envelope and its outflow cavity
(Jørgensen 2004). CN has also been detected towards a number of protostellar
discs around Class II sources (e.g. Dutrey et al. 1997; Qi 2001; Thi et al.
2004; Öberg et al. 2011), where it is found to be a good tracer of the disc
surfaces (e.g. van Dishoeck 2006).

Toward IRS67, ten of the twelve hyperfine transitions from CN 2$-$1 (see Table
3) are detected. The emission of one of these hyperfine transitions is shown
in Fig. 3. The brightest CN lines are blended with other CN hyperfine
transitions and their moment 0 maps are shown in Fig. 11 in the Appendix. The
CN emission stands out towards the southern region and peaks slightly offset
from the southern edge of the circumbinary disc, which is consistent with the
H2CO peak (see Fig. 2).

####  3.2.6 Deuterated species

Deuterated species are good tracers of cold regions, in particular, the outer
disc midplane where the temperature is $\leq$ 30 K (e.g. van Dishoeck 2006;
Jørgensen et al. 2011; Murillo et al. 2015; Öberg et al. 2015; Aikawa et al.
2018). The ion DCO+ is apparently formed in the gas phase, considering its
short destruction timescale and is expected to be abundant in regions with
temperature between 19 and 21 K (Aikawa et al. 2018). On the other hand,
neutral species as DCN can form in interstellar ice and later desorb in the
disc, thus tracing regions with higher temperatures than DCO+ ($\geq$ 30 K;
Jørgensen et al. 2004; Aikawa et al. 2018). Emission of DCN, DNC, and DCO+ is
detected and shown in Fig. 3. Both DCN and DNC peak towards the southern edge
of the protobinary disc, which is consistent with the CN peak, while DCO+
stands out close to the northern edge of the disc and no emission is detected
where DCN and DNC peak.

####  3.2.7 Spectra

Figure 4:  Spectra of the brightest molecular transitions (with the exception
of CN) taken at three different positions: the geometric centre (black), the
southern edge (blue), and the northern edge of the circumbinary disc (red; see
the white crosses in the moment 0 map of CS in Fig. 2). The zero velocity
value (dashed grey vertical line) corresponds to a system velocity of 4.2 km
s-1. The dashed grey horizontal line shows the value of 3$\sigma$ (see Table
1). Some of the spectra are multiplied by a factor of three, as indicated in
the top right corner.  Figure 5:  CN spectra taken at three different
positions: the geometric centre (black), the southern edge (blue), and the
northern edge of the circumbinary disc (red; see white crosses in the moment 0
map of CS in Fig. 2). The dashed green vertical lines represent the rest
frequency of the CN hyperfine transitions (see Table 3), while the dashed grey
horizontal line shows the value of 3$\sigma$.

Figure 4 shows the spectra of the species presented in Figs. 2 and 3, towards
three different positions: the geometric centre and the blue- and red-shifted
peaks from the CS emission, located at 2$\arcsec$ and 2.4$\arcsec$ from the
geometric centre, respectively (see white crosses in the moment 0 map of CS in
Fig. 2). CO and 13CO show a clear absorption future at the system velocity
(4.2 km s-1), mainly due to the interferometric filtering-out of emission from
large scales. C18O, CS, SO, and the two brightest H2CO transitions present
similar spectral features, with prominent emission between $-$3 and 3 km s-1
and a considerably symmetry between the blue- and red-shifted components.
These components are tracing the edges of the circumbinary disc, peak around
$\pm$2 km s-1 and are consistent with a rotational profile. This is in
agreement with the results from Artur de la Villarmois et al. (2018), where
velocities below $\pm$2.5 km s-1 are expected at a distance of 2$\arcsec$ from
the geometric centre and beyond. On the other hand, c-C3H2, DCN, DNC, and HC3N
show strong emission towards the southern edge of the circumstellar disc (blue
spectra), with a peak around $-$2 km s-1, tracing a different component than
the CO isotopologues, CS, SO and H2CO.

Since the observations include multiple CN hyperfine transitions, their
spectra are shown in Fig. 5, covering a frequency range from 226.356 to
226.896 GHz. The spectra of isolated lines show an absorption feature at the
system velocity, consistent with filtering-out of emission from large scales.
This agrees with the results from Jørgensen et al. (2011), where the CN
emission may be tracing the outer envelope towards IRAS 16293-2422 and is
filtered out by the SMA. Nevertheless, towards IRS 67 the CN emission peaks at
the southern edge of the circumbinary disc, with velocities around $-$2 km
s-1. The CN spectra resemble that of c-C3H2 (Fig. 4), suggesting that the two
species trace the same material.

##  4 Discussion

###  4.1 Structure of IRS 67

Figure 6:  Gas temperature estimation from the ratio between o-H2CO
30,3$-$20,2 and o-H2CO 32,2$-$22,1, following the results from Mangum &
Wootten (1993). Specific values of 60, 90, and 120 K are shown by blue, green,
and red contours, respectively. The grey region, where o-H2CO 32,2$-$22,1 is
not detected, indicates temperatures below 50 K. The yellow stars show the
position of the sources and the solid segment represents the extent of the
circumbinary disc. The synthesised beam is represented by a black filled
ellipse.

The ratio between H2CO 30,3$-$20,2 and H2CO 32,2$-$22,1 has previously been
shown to be a good tracer of the gas temperature (Mangum & Wootten 1993).
Figure 6 shows the estimated gas temperature for a H2 number density $\geq$
108 cm-3, following those results. The highest temperatures ($\geq$90 K) are
seen towards the outflow direction, while the southern edge of the
circumbinary disc is associated with temperatures between 50 and 60 K. This is
the region where the peak of emission is seen for DCN, DNC, CN, c-C3H2, and
HC3N, suggesting that these species are tracing high densities
(n${}_{\mathrm{H_{2}}}$ $\geq$ 108 cm-3) and lukewarm temperatures (50$-$60
K). The non-detection of H2CO 32,2$-$22,1 where H2CO 30,3$-$20,2 is detected
sets an upper limit of 50 K for the gas temperature (the grey region in Fig.
6).

CN and c-C3H2 are commonly used as PDR tracers, since their emission shows a
strong dependence on the UV radiation (e.g. Dutrey et al. 1997; van Dishoeck
2006; Öberg et al. 2011; Bergin et al. 2016; Murillo et al. 2018). The
circumbinary disc associated with IRS 67 may be strongly illuminated by UV
radiation from the binary system. Because discs are normally flared, is it
also likely that the southern region of the circumbinary disc is associated
with a PDR, in particular, the surface layer of the circumbinary disc. Other
common PDRs originate along the outflow cavity (e.g. Murillo et al. 2018),
however, this is unlikely for IRS 67 since the blue-shifted outflow component
is seen towards the north-west, and blue-shifted emission from c-C3H2 is
detected towards the south-east. The PDR originating at the surface layer of
the circumbinary disc is also consistent with the temperature structure
estimated from the H2CO 30,3$-$20,2/32,2$-$22,1 ratio (see Fig. 6).

The northern part of the circumbinary disc seems to be related with colder
regions than the southern part (see Fig. 6). DCO+ is expected to trace cold
regions ($\leq$30 K; Jørgensen et al. 2011; Aikawa et al. 2018) and its
emission peaks towards the north-east of the circumbinary disc (Fig. 3),
showing an anti-correlation with the PDR tracers (mainly CN and c-C3H2). The
DCO+ emission seems to be associated with cold regions from the inner envelope
at small scales ($\leq$1000 AU), however, the same DCO+ transition was
detected with APEX (see Fig. 12 in the Appendix), suggesting that more than
96$\%$ of the emission is filtered out by the interferometer. Therefore, DCO+
is expected to be present also at large scales (e.g. Jørgensen et al. 2011;
Murillo et al. 2018).

The system IRS 67 is particularly rich in molecular lines when it is compared
with other single Class I sources (Artur de la Villarmois et al. 2019),
possibly due to the mass content and extent of the circumbinary disc. One of
the species that is only detected towards IRS 67, among other 11 Class I
sources, is c-C3H2, where two transitions (55,1$-$44,0 and 55,0$-$44,1) are
seen (Artur de la Villarmois et al. 2018) and the emission is consistent with
the c-C3H2 lines analysed in this work. c-C3H2 may, therefore, be related with
PDRs (like outflow cavities and disc surface layers) but also with high gas
column densities expected towards Class 0 sources and the binary system IRS
67. This is in agreement with the non-detection of c-C3H2 towards a sample of
12 Class II discs (Öberg et al. 2010, 2011).

###  4.2 Non-detections

Together with the detection of multiple molecular transitions, the non-
detections may provide some clues about the physical parameters of the region.
The main non-detections at the 3$\sigma$ level are N2D+, the linear isomer
l-C3H2, SiO, and CH3OH.

N2D+ is a good tracer of cold regions ($\leq$30 K), where CO freezes out, and
usually correlates with DCO+ (e.g. Jørgensen et al. 2011; Aikawa et al. 2018;
Murillo et al. 2018). Since most of the DCO+ is filtered out by the
interferometer and its emission is expected to be present at large scales,
N2D+ may also be present at large scales. Therefore, the non-detection of N2D+
may be related to the lack of short-spacings in the observations.

The non-detection of the linear C3H2 isomer may be related with the high
cyclic-to-linear ratio or to an excitation effect, or a combination of both.
The cyclic-to-linear ratio has been proven to differ from the statistical
value of one for a variety of objects, ranging from 3 to 70 (Sipilä et al.
2016). On the other hand, the three l-C3H2 transitions covered in the
observations have Eu values between 66 and 80 K, while the brightest c-C3H2
line (see Fig. 2) has Eu = 39 K. In addition, c-C3H2 72,6$-$71,7 (Eu = 61 K)
shows weak emission and two c-C3H2 lines with Eu = 87 K are not detected. This
sets an upper limit of 60 K for the excitation temperature of the c-C3H2
emitting region. Therefore, a combination of both, high cyclic-to-linear ratio
and an excitation effect, may explain the non detection of the linear C3H2
isomer.

Typically, SiO is seen as a good tracer of shocks as it is present in the gas-
phase when refractory grains are destroyed (e.g. Bachiller & Perez Gutierrez
1997; Codella et al. 2014; Gusdorf et al. 2008b, a). Its non-detection towards
IRS 67 is consistent with other studies which suggest that Class I sources are
associated with less energetic outflows.

###  4.3 Chemical differentiation around IRS 67

Figure 7 shows a schematic representation of the environment associated with
IRS 67, where three main regions are distinguished: (i) a cold region (T
$\leq$ 30 K) beyond the extent of the circumbinary disc, traced by DCO+, (ii)
the circumbinary disc traced by CO isotopologues and sulphur-bearing species,
and (iii) a PDR, likely the surface layer of the circumbinary disc reached by
the UV radiation from the binary system, and traced by CN, DCN, and carbon-
chain molecules.

The chemistry towards IRS 67 shows some similarities with Class 0 sources,
possibly due to the high-gas column density, and with Class II discs, where
the UV radiation from the binary system may resemble the interaction between a
typical T-Tauri star and the surface layers of its disc. Cold gas tracers
(such as DCO+ and DCN), potential grain chemistry products (such as H2CO), and
species associated with carbon-chain chemistry (such as c-C3H2 and HC3N), are
commonly detected towards Class 0 sources (e.g. Jørgensen et al. 2005; Murillo
et al. 2018) and are also detected towards IRS 67. However, while H2CO and
c-C3H2 are mainly seen at the outflow cavities towards Class 0 sources,
towards IRS 67 they seem to trace the surface layers of the circumbinary disc.
On the other hand, photochemistry products such as CN are abundant towards
more evolved discs, associated with Class II sources (e.g. Öberg et al. 2010,
2011): CN will originate in the outer, very low density part of the disc,
which is completely exposed to UV radiation. The binary system IRS 67 may be
associated with a stronger UV radiation field than single Class I sources,
therefore, the CN emission towards IRS 67 appears to trace the same physical
component as in Class II sources, that is, the surface layers of the disc
exposed to UV radiation. Future observations of other Class I binary sources
will verify whether the chemical richness of IRS 67 is common towards these
stages, and higher angular resolutions will constrain the velocity profile and
the dynamics of the detected transitions.

Figure 7:  Schematic representation of the environment towards IRS 67 where
three main regions are distinguished. Cold regions traced by DCO+, the disc
structure proven by CO isotopologues and S-bearing species, and a PDR
associated with the surface layers of the disc, traced by CN, DCN, and carbon-
chain molecules. The outflow direction is taken from Bontemps et al. (1996).

##  5 Summary

This work presents SMA observations of the Class I binary system IRS 67 in the
Ophiuchus star forming region, with an angular resolution of
1$\aas@@fstack{\prime\prime}$7 $\times$1$\aas@@fstack{\prime\prime}$4
($\sim$260 $\times$ 210 AU). The continuum emission between 1.2 and 1.4 mm is
analysed, together with multiple molecular transitions that trace different
physics. The main results are summarised below.

  * •

The continuum emission is consistent with previous studies (Artur de la
Villarmois et al. 2018) and a power-law fitting results in a $\beta$-value of
0.8 $\pm$ 0.3, suggesting that dust grains have grown to larger sizes than the
ISM dust particles, or that the dust is optically thick.

  * •

The detected molecular transitions are tracing three main regions: cold
regions beyond the circumbinary disc extent, the circumbinary disc, and a PDR
likely related with the surface layers of the disc. DCO+ is tracing the cold
regions, while the CO isotopologues and the sulphur-bearing species are
probing the disc structure. In addition, CN, DCN, and carbon-chain molecules
are tracing the PDR.

  * •

The case of H2CO is special, as it traces both the circumbinary disc and the
PDR. The ratio between o-H2CO 30,3$-$20,2 and o-H2CO 32,2$-$22,1 has been
shown to be a good indicator of the gas temperature, where the temperature map
is consistent with the physical structure of IRS 67, that is, the warmer gas
follows the outflow direction, lukewarm temperatures are associated with the
PDR, and colder gas is related to the circumbinary disc.

  * •

IRS 67 shows chemical similarities with Class 0 sources, such as the detection
of sulphur-bearing species and carbon-chain molecules, while PDR tracers, such
as CN, are associated with Class II discs, where the UV radiation can reach
the surface layers of the disc. IRS 67 is, therefore, a chemical link between
these two stages.

This work shows the potential of the broad spectral coverage of the SMA,
allowing us to detect and analyse multiple molecules and transitions from the
same species. Similar observations of other Class I sources will provide more
statistical results and highlight if IRS 67 is a particular chemically-rich
system or it represents a general trend. In addition, higher angular
resolution observations will constrain the dynamics of the gas and possibly
resolve the individual circumstellar discs around each source, constraining
the link between circumstellar and circumbinary discs.

###### Acknowledgements.

We thank Johan Lindberg for sharing and discussing APEX data with us. This
paper is based on data from the Submillimeter Array: the Submillimeter Array
is a joint project between the Smithsonian Astrophysical Observatory and the
Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the
Smithsonian Institution and the Academia Sinica. The group of JKJ acknowledges
support from the European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (grant agreement No 646908)
through ERC Consolidator Grant “S4F”. Research at the Centre for Star and
Planet Formation is funded by the Danish National Research Foundation.

##  Appendix A Other molecular transitions

Other transitions and less abundant isotopologues of the species discussed in
Sect. 3.2 show weaker emission and are presented in Figs. 8, 9, and 10. In
addition, the brightest CN hyperfine transitions are blended with other CN
lines (see Fig. 5). Emission of these transitions is shown in Fig. 11.

Figure 8:  Emission of SO 55$-$44. Channel maps consist of velocity ranges of
1 km s-1 and the moment 0 map is integrated over a velocity range of 8 km s-1.
The contours start at 3$\sigma$ and follow a step of 3$\sigma$. The yellow
stars show the position of the sources and the solid segment represents the
extent of the circumbinary disc. The synthesised beam is represented in the
right panel by a solid black ellipse.

Figure 9:  C34S, 13CS, and H2CO (Eu = 68 K) moment 0 maps (colour scale)
integrated over a velocity range of 6 km s-1, overlapped with CS emission
(contours in C34S and 13CS maps) and H2CO 31,2$-$21,1 (contours in H2CO maps)
from Fig. 2. The yellow stars show the position of the sources and the solid
segment represents the extent of the circumbinary disc. The synthesised beam
is represented by a black filled ellipse.  Figure 10:  Emission of c-C3H2 and
HC3N. Channel maps consist of velocity ranges of 1 km s-1 and moment 0 maps
are integrated over a velocity range of 8 km s-1. The contours start at
3$\sigma$ and follow a step of 3$\sigma$ and 2$\sigma$ for c-C3H2 and HC3N,
respectively. The yellow stars show the position of the sources and the solid
segment represents the extent of the circumbinary disc. The synthesised beam
is represented by a black filled ellipse in the right panels.  Figure 11:  CN
moment 0 maps of the brightest transitions, which are blended with other
hyperfine lines. The integration is done over a velocity range of 11 and 8 km
s-1 for the left and right panels, respectively. The yellow stars show the
position of the sources and the solid segment represents the extent of the
circumbinary disc. The synthesised beam is represented by a black filled
ellipse in the left panel.

##  Appendix B Non-detections

Non-detected lines at the 3$\sigma$ level within the covered molecular
transitions are listed in Table 4, along with their spectroscopic parameters.

Table 4: Parameters of the non-detected molecular transitions.

Species | Transition | Frequency (a) |  Aij (a) |  Eu (a) |  ncrit (b)
---|---|---|---|---|---
|  | (GHz) | (s-1) | (K) | (cm-3)
CO- species
p-H2CO | 112,9$-$120,12 | 215.97616 | 3.16 $\times$ 10-7 | 280 | 3.3 $\times$ 104
o-H2CO | 91,8$-$91,9 | 216.56865 | 7.24 $\times$ 10-6 | 174 | 2.1 $\times$ 105
CH3OH | 50,5$-$40,4 | 241.70022 | 6.03 $\times$ 10-5 | 48 | 1.7 $\times$ 106
CH3OH | 50,5$-$40,4++ | 241.79143 | 6.03 $\times$ 10-5 | 35 | 4.6 $\times$ 105
CH3OH | 51,4$-$41,3 | 241.87907 | 6.03 $\times$ 10-5 | 56 | 1.5 $\times$ 107
CH3OH | 51,4$-$41,3– | 241.91583 | 6.03 $\times$ 10-5 | 50 | 5.5 $\times$ 107
S- species
SO | 12$-$21 | 236.45229 | 1.41 $\times$ 10-6 | 16 | 4.0 $\times$ 104 (c)
SO2 | 111,11$-$100,10 | 221.96522 | 1.15 $\times$ 10-4 | 60 | 8.6 $\times$ 106 (d)
SO2 | 123,9$-$122,10 | 237.06883 | 1.15 $\times$ 10-4 | 60 | 1.1 $\times$ 108 (d)
SO2 | 52,4$-$41,3 | 241.61580 | 8.51 $\times$ 10-5 | 24 | 9.4 $\times$ 106 (d)
SO2 | 140,14$-$131,13 | 244.25422 | 1.62 $\times$ 10-4 | 94 | 1.2 $\times$ 107 (d)
OCS |  J=18$-$17 | 218.90336 | 3.02 $\times$ 10-5 | 100 | 8.2 $\times$ 105
OCS |  J=19$-$18 | 231.06099 | 3.55 $\times$ 10-5 | 111 | 1.3 $\times$ 106
OCS |  J=20$-$19 | 243.21804 | 4.17 $\times$ 10-5 | 123 | 3.5 $\times$ 105
p-H2S | 22,0$-$21,1 | 216.71044 | 4.90 $\times$ 10-5 | 84 | 1.1 $\times$ 106
HCS | 60,6$-$50,5 | 241.68929 | 1.20 $\times$ 10-5 | 41 |
HSC | 61,6$-$51,5 | 239.05112 | 4.27 $\times$ 10-4 | 53 |
HSC | 62,5$-$52,4 | 243.32134 | 4.17 $\times$ 10-4 | 94 |
HSC | 60,6$-$50,5 | 243.36604 | 4.68 $\times$ 10-4 | 41 |
HSC | 62,4$-$52,3 | 243.52763 | 4.17 $\times$ 10-4 | 94 |
H2CS | 71,7$-$61,6 | 236.72702 | 1.91 $\times$ 10-4 | 59 | 5.5 $\times$ 106
H2CS | 70,7$-$60,6 | 240.26687 | 2.04 $\times$ 10-4 | 46 | 2.0 $\times$ 107
H2CS | 72,6$-$62,5 | 240.38205 | 1.91 $\times$ 10-4 | 99 | 4.0 $\times$ 106
H2CS | 72,5$-$62,4 | 240.54907 | 1.91 $\times$ 10-4 | 99 | 6.4 $\times$ 106
H2CS | 71,6$-$61,5 | 244.04850 | 2.09 $\times$ 10-4 | 60 | 4.1 $\times$ 106
Carbon-chain species
N2D+ | 3$-$2 | 231.32186 | 7.08 $\times$ 10-4 | 22 | 2.1 $\times$ 106 (e)
H2CN | 30,3$-$20,2 | 219.85186 | 3.39 $\times$ 10-4 | 21 |
H2CN | 31,2$-$21,1 | 227.43700 | 3.39 $\times$ 10-4 | 34 |
c-C3H2 | 82,6$-$81,7 (para) | 218.44883 | 1.48 $\times$ 10-4 | 87 | 6.7 $\times$ 107
c-C3H2 | 83,6$-$82,7 (ortho) | 218.44944 | 1.48 $\times$ 10-4 | 87 | 3.2 $\times$ 108
c-C3H2 | 32,1$-$21,2 (ortho) | 244.22215 | 5.89 $\times$ 10-5 | 18 | 1.2 $\times$ 107
l-C3H2 | 111,11$-$101,10 | 226.54857 | 1.07 $\times$ 10-3 | 79 |
l-C3H2 | 110,11$-$100,10 | 228.60835 | 1.12 $\times$ 10-3 | 66 |
l-C3H2 | 111,10$-$101,9 | 230.77801 | 1.15 $\times$ 10-3 | 80 |
C4H |  N=23$-$22, J=47/2$-$45/2 | 218.83701 | 4.47 $\times$ 10-5 | 126 |
C4H |  N=23$-$22, J=45/2$-$43/2 | 218.87537 | 4.47 $\times$ 10-5 | 126 |
C4H |  N=24$-$23, J=49/2$-$47/2 | 228.34862 | 5.13 $\times$ 10-5 | 137 |
C4H |  N=24$-$23, J=47/2$-$45/2 | 228.38696 | 5.13 $\times$ 10-5 | 137 |
C4H |  N=25$-$24, J=51/2$-$49/2 | 237.85974 | 5.75 $\times$ 10-5 | 148 |
C4H |  N=25$-$24, J=49/2$-$47/2 | 237.89806 | 5.75 $\times$ 10-5 | 148 |
Others
SiO |  J=5$-$4 | 217.10498 | 5.25 $\times$ 10-4 | 31 | 2.2 $\times$ 107
HDO | 31,2$-$22,1 | 225.89672 | 1.32 $\times$ 10-5 | 167 | 7.3 $\times$ 106
HDO | 21,1$-$21,2 | 241.56155 | 1.17 $\times$ 10-5 | 95 | 9.6 $\times$ 105
HNCO | 100,10$-$90,9 | 219.79827 | 1.48 $\times$ 10-4 | 58 | 4.4 $\times$ 108
HNCO | 110,11$-$100,10 | 241.77403 | 1.95 $\times$ 10-4 | 70 | 2.3 $\times$ 107
555(a) Values from the CDMS database (Müller et al. 2001). (b) Calculated
values for a collisional temperature of 30 K and collisional rates from the
Leiden Atomic and Molecular Database (LAMDA; Schöier et al. 2005).(c) For a
collisional temperature of 60 K.(d) For a collisional temperature of 100 K.
(e) Calculated for N2H+. The collisional rates of specific species listed here
and in Table 3 were taken from the following sources: CO isotopologues from
Yang et al. (2010), H2CO and H2CS from Wiesenfeld & Faure (2013), CH3OH from
Rabli & Flower (2010), DCO+ and N2D+ from Flower (1999), DCN and DNC from
Dumouchel et al. (2010), CS isotopologues and SO from Lique et al. (2006), SO2
from Balança et al. (2016), OCS from Green & Chapman (1978), H2S from Daniel
et al. (2011), CN from Kalugina & Lique (2015), C3H2 from Chandra & Kegel
(2000), HC3N from Faure et al. (2016), SiO from Dayou & Balança (2006), HDO
from Faure et al. (2012), and HNCO from Sahnoun et al. (2018).

##  Appendix C APEX data

The system IRS 67 was observed with APEX (Lindberg et al. 2017) around 220 GHz
and with an angular resolution of 29$\arcsec$. In order to compare SMA with
APEX data, the SMA observations are convolved with a 29$\arcsec$ beam and the
intensity unit (Jy beam-1) is converted into Kelvin. Figure 12 shows a
comparison between the two datasets for five transitions, highlighting the
strong filtering-out of extended emission by the interferometer. The APEX and
SMA intensities are presented in Table 5, together with the percentage of
emission that has been filtered out.

Table 5: Comparison between APEX (Lindberg et al. 2017) and convolved SMA
intensities.

Molecular transition | Intensity (K km s-1) | Filtering ($\%$) (a)
---|---|---
| APEX | SMA |
DCO+ J=3$-$2 | 0.317 | 0.0125 |  96
c-C3H2 60,6$-$51,5 | 0.177 | 0.0137 |  92
p-H2CO 30,3$-$20,2 | 0.431 | 0.0036 |  99
C18O J=2$-$1 | 3.198 | 0.0093 | ¿ 99
SO 65$-$54 | 0.316 | 0.0003 | ¿ 99
666(a) Percentage of the emission filtered-out by the interferometer.

Figure 12:  Spectra from APEX data (black) and convolved SMA data with a
29$\arcsec$ beam (red). For clarity, the SMA spectra are scaled with a factor
of 20. The blue curve represents a Gaussian fit and, for C18O, the curve shows
the sum of three Gaussian components.

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