# Salt-bearing disk candidates around high-mass young stellar objects

Adam Ginsburg Department of Astronomy, University of Florida, P.O. Box 112055,
Gainesville, FL, USA  Brett A. McGuire Department of Chemistry, Massachusetts
Institute of Technology, Cambridge, MA 02139  National Radio Astronomy
Observatory, Charlottesville, VA 22903  Patricio Sanhueza National
Astronomical Observatory of Japan, National Institutes of Natural Sciences,
2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan  Department of Astronomical
Science, SOKENDAI (The Graduate University for Advanced Studies), 2-21-1
Osawa, Mitaka, Tokyo 181-8588, Japan  Fernando Olguin Institute of Astronomy
and Department of Physics, National Tsing Hua University, Hsinchu 30013,
Taiwan  Luke T. Maud ESO Headquarters, Karl-Schwarzchild-Str 2 85748 Garching,
Germany  Kei E. I. Tanaka Center for Astrophysics and Space Astronomy,
University of Colorado Boulder, Boulder, CO 80309, USA  National Astronomical
Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan  Yichen Zhang Department of Astronomy,
University of Virginia, Charlottesville, VA 22904, USA  RIKEN Cluster for
Pioneering Research, Wako, Saitama 351-0198, Japan  Henrik Beuther Max Planck
Institute for Astronomy, Königstuhl 17, 69117, Heidelberg, Germany  Nick
Indriolo AURA for the European Space Agency (ESA), Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

###### Abstract

Molecular lines tracing the orbital motion of gas in a well-defined disk are
valuable tools for inferring both the properties of the disk and the star it
surrounds. Lines that arise only from a disk, and not also from the
surrounding molecular cloud core that birthed the star or from the outflow it
drives, are rare. Several such emission lines have recently been discovered in
one example case, those from NaCl and KCl salt molecules. We studied a sample
of 23 candidate high-mass young stellar objects (HMYSOs) in 17 high-mass star-
forming regions to determine how frequently emission from these species is
detected. We present 5 new detections of water, NaCl, KCl, PN, and SiS from
the innermost regions around the objects, bringing the total number of known
briny disk candidates to 9. Their kinematic structure is generally disk-like,
though we are unable to determine whether they arise from a disk or outflow in
the sources with new detections. We demonstrate that these species are
spatially coincident in a few resolved cases and show that they are generally
detected together, suggesting a common origin or excitation mechanism. We also
show that several disks around HMYSOs clearly do not exhibit emission in these
species. Salty disks are therefore neither particularly rare in high-mass
disks, nor are they ubiquitous.

††software:  The source code underlying this work are available from github at
https://github.com/keflavich/saltmining/releases/tag/accepted-2022-11-04. This
work used CARTA (Comrie et al., 2021), Jupyter notebooks (Kluyver et al.,
2016). numpy (van der Walt et al., 2011; Harris et al., 2020), scipy (Virtanen
et al., 2020), astropy (Astropy Collaboration et al., 2013, 2018; The Astropy
Collaboration et al., 2022), spectral-cube (Ginsburg et al., 2019b), radio-
beam (Koch et al., 2021), and CASA-6
(https://casa.nrao.edu/casadocs/casa-5.6.0/introduction/casa6-installation-
and-usage). matplotlib (Hunter, 2007), astroquery (Ginsburg et al., 2019c),
and pyspeckit (Ginsburg et al., 2022).

doublespace

##  1 Introduction

Circumstellar accretion disks develop around forming new stars. While the
presence of disks around low-mass stars has been clear for decades, we have
only definitively demonstrated that accretion disks exist around high-mass
young stellar objects (HYMSOs) in the last decade (e.g., Beltrán & de Wit,
2016; Maud et al., 2017; Ilee et al., 2018; Motogi et al., 2019; Johnston et
al., 2020; Sanna et al., 2021; Moscadelli et al., 2021).

One limiting factor in the detection and subsequent characterization of HYMSOs
has been the lack of molecular emission lines that arise from the disk, but
are not confused with or absorbed by the surrounding molecular cloud. A select
few lines have recently been discovered that are uniquely produced in the
disks of some HMYSOs. Emission from salt molecules has been detected in the
surroundings of four stars in three star-forming regions: Orion Source I
(Ginsburg et al., 2019a), G17.64+0.16 (Maud et al., 2019), and a pair in IRAS
16547-4247 (Tanaka et al., 2020) (hereafter, SrcI, G17, and I16547,
respectively). Of these, SrcI and G17 are confirmed disks, while the I16547
pair remain candidates, and in all cases the salt emission comes from zones
within $\lesssim 100$ au of the central source. Each of these sources also
exhibits H2O emission from both the (candidate) disk and a slightly more
extended region, and thus we dub these objects ‘brinaries.’

Salts are detected in the atmospheres of AGB and post-AGB stars, giving some
clues as to the physical conditions needed to produce them. Salts have been
detected in CRL2688 (Highberger et al., 2003), IRAS+10216 (Cernicharo &
Guelin, 1987), IK Tauri, VY Canis Majoris (Milam et al., 2007; Decin et al.,
2016), VX Sgr (mentioned in passing in Danilovich et al., 2021), and
OH231.8+44.2 (Sánchez Contreras et al., 2022). Sánchez Contreras et al. (2022)
discovered a brinary disk surrounding the post-AGB mass transfer system
OH231.8+4.2, highlighting the similarity between birth and death among
moderately massive stars. However, there are also some non-detections in well-
surveyed sources, including the S-type AGB stars W Aql and $\pi^{1}$ Gru
(Homan et al., 2020; Danilovich et al., 2021).

The development of line lists in the infrared, and more complete lists in the
radio, has been driven in part by interest in salts as constituents of the
atmospheres of hot planets, in which these species are predicted to be
important components of upper cloud layers (Barton et al., 2014). There is
therefore some motivation to understand where salts occur in disks and in or
on dust grains.

Studies of (post)-AGB stars and models of planetary atmospheres provide some
clues about the physical conditions required to produce gas-phase salts. NaCl
is expected to change states (from solid to gas or vice-versa) around
$\sim$500–600 K at planetary atmospheric pressure (i.e., 1 bar; Woitke et al.,
2018). Decin et al. (2016) suggest that it comes off of grains at 100–300 K
based on the detection locations in supergiant stars. These studies provide a
first hint about where NaCl may come from if it precipitates out of cooling
gas, though it remains unclear if the same mechanisms apply in and around YSO
disks.

Motivated by the detection of salts in a few HMYSOs with ALMA in recent years,
we present a first search for salt-bearing disks in the ALMA archives. In
Section 2, we describe the ALMA observations we analyze. In Section 3, we
describe the analysis approach (3.1.1) and detections (3.3). We discuss the
chemical correlations observed in the sample and possible reasons for
(non)detections in Section 4, then conclude.

##  2 Data & Sample Selection

We utilize archival and new data from several projects. We select data sets
that have high angular resolution ($\lesssim 0.1$ ″) targeting high-mass star-
forming regions. Most of the sources in our sample come from the Digging into
the Interior of Hot Cores with ALMA (DIHCA; PI: Sanhueza) program, which is
surveying $\sim 30$ candidate disks. The main aims of the DIHCA survey are to
study the interior of massive hot cores to determine whether they form high-
mass stars collapsing monolithically or by fragmenting into binary (multiple)
systems and to search for accretion disks around high-mass stars. The DIHCA
targets were selected from the literature as regions with previous
interferometric (e.g., SMA) observations and having an expected flux of $>$0.1
Jy at 230 GHz. All clumps follow the empirical threshold for high-mass star
formation suggested by Kauffmann & Pillai (2010). We only examine a small
subset of the DIHCA sample here because not all data were available as of
March 2022. Because our sample is not uniformly selected, we can say little
about completeness; we can only search for general trends. Nevertheless, the
trends we find are interesting and suggest that observing a more uniform
sample in the future would be productive.

DIHCA observations of G335, G333.23, NGC6334, IRAS16562, G34.43mm1, G29.96,
and G351.77 were obtained during July 2019, and observations of G5.89, G11.92,
IRAS18089, and W33A were taken both in September 2017 and July 2019. The
observations were reduced using CASA (v5.4.0-70; McMullin et al., 2007). The
data were then phase self-calibrated in three steps with decreasing solution
intervals and the continuum subtracted following the procedure of Olguin et
al. (2021). Dirty cubes were produced with a Briggs weighting robust parameter
of 0.5 using the CASA tclean task. We use dirty image cubes for expediency, so
it is possible significant improved images of these objects could be obtained,
though we note that the targeted lines are generally faint and would not be
affected by cleaning with typical clean parameters.

Table 1: Observation Summary

Field | Source Name | $\theta_{\rm maj}$ | $\theta_{\rm maj}$ | $\theta_{\rm min}$ | PA | $\sigma$ | $f(>5\sigma)$ | $N_{\rm beams}$ | $\sigma_{\rm avg}$ |  $v$ ref. line | Distance
---|---|---|---|---|---|---|---|---|---|---|---
|  | $\mathrm{AU}$ | ′′ | ′′ | ∘ | $\mathrm{K}$ |  |  | $\mathrm{K}$ |  | $\mathrm{kpc}$
Orion | SrcI | 20 | 0.050 | 0.039 | 72.7 | 19.3 | 0.05 | 43 | 2.9 | NaCl J=18-17 v=0 | 0.4
S255IR | SMA1 | 40 | 0.036 | 0.027 | 5.6 | 46.9 | 0.04 | 19 | 10.6 | no clear disk | 1.6k
G351.77 | mm12 | 50 | 0.027 | 0.022 | -89.3 | 6.0 | 0.08 | 24 | 1.2 | H2O | 2.2c
G351.77 | mm2 | 50 | 0.027 | 0.022 | -89.3 | 6.0 | 0.05 | 21 | 1.3 | H2O | 2.2c
G351.77 | mm1 | 50 | 0.027 | 0.022 | -89.3 | 6.0 | 0.14 | 41 | 0.9 | H2O | 2.2c
G17 | G17 | 50 | 0.038 | 0.022 | 44.4 | 10.0 | 0.01 | 75 | 1.2 | H2O | 2.2c
NGC6334I | mm1d | 50 | 0.074 | 0.041 | 62.8 | 19.6 | 0.10 | 20 | 4.3 | H2O | 1.3d
NGC6334I | mm1b | 50 | 0.074 | 0.041 | 62.8 | 19.6 | 0.23 | 27 | 3.7 | H2O | 1.3d
NGC6334I | mm2b | 50 | 0.074 | 0.041 | 62.8 | 19.6 | 0.07 | 7 | 7.0 | H2O | 1.3d
NGC6334IN | SMA1b/d | 50 | 0.074 | 0.042 | 63.4 | 17.5 | 0.16 | 278 | 1.0 | H2O | 1.3d
NGC6334IN | SMA6 | 50 | 0.074 | 0.042 | 63.4 | 17.5 | 0.06 | 22 | 3.7 | H2O | 1.3d
IRAS18162 | GGD27 | 90 | 0.099 | 0.066 | -89.0 | 6.9 | 0.01 | 36 | 1.1 | SO $6_{5}-5_{4}$ | 1.3a
IRAS18089 | I18089-1732 | 100 | 0.064 | 0.045 | 65.8 | 18.4 | 0.33 | 57 | 2.4 | CH3OH | 2.3f
G34.43 | mm1 | 110 | 0.105 | 0.069 | 59.5 | 9.2 | 0.35 | 53 | 1.3 | no clear disk | 1.6l
IRAS16562 | G345.4938+01.4677 | 120 | 0.106 | 0.053 | 81.6 | 10.5 | 0.04 | 25 | 2.1 | H30$\alpha$ | 2.3g
I16547 | A | 130 | 0.065 | 0.043 | 35.7 | 23.2 | 0.19 | 7 | 8.4 | H2O | 2.9c
I16547 | B | 130 | 0.065 | 0.043 | 35.7 | 23.2 | 0.18 | 5 | 9.5 | H2O | 2.9c
G5.89 | mm15 | 130 | 0.063 | 0.043 | 66.3 | 19.2 | 0.00 | 85 | 2.1 | H2O | 3.0b
G335 | ALMA1 | 140 | 0.066 | 0.041 | 47.3 | 20.4 | 0.29 | 118 | 1.9 | no clear disk | 3.3i
W33A | mm1-main | 170 | 0.103 | 0.067 | -86.4 | 6.5 | 0.17 | 51 | 0.9 | H2O | 2.6k
G333.23 | mm1 | 220 | 0.069 | 0.041 | 54.1 | 18.4 | 0.02 | 70 | 2.2 | SO $6_{5}-5_{4}$ | 5.3h
G333.23 | mm2 | 220 | 0.069 | 0.041 | 54.1 | 18.4 | 0.08 | 45 | 2.7 | SO $6_{5}-5_{4}$ | 5.3h
G11.92 | mm1 | 220 | 0.101 | 0.066 | -86.9 | 6.6 | 0.16 | 103 | 0.7 | SO $6_{5}-5_{4}$ | 3.3e
G29.96 | submm1 | 530 | 0.099 | 0.071 | 65.0 | 9.6 | 0.22 | 369 | 0.5 | no clear disk | 7.4j

Observation properties. The ‘Field’ name indicates the region of the ALMA
pointing. The ‘Source Name’ is the identifier of the disk candidate examined.
$\theta$ gives the beam parameters, with $\theta_{maj}$ [au] providing the
physical size using the adopted distance. $\sigma$ is the average noise level
of the field, which is averaged down by $N_{beams}^{1/2}$ to give $\sigma_{\rm
avg}$, the noise level in the stacked spectrum. $f(>5\sigma)$ is the fraction
of the stacked spectrum that is above five times $\sigma_{avg}$; it is used as
a diagnostic of the line crowding in the spectrum covering 219.2–220.8 GHz,
which is high for complex-molecule-rich regions. The $v$ reference line is the
line used to create a velocity map to produce stacked spectra. Section 3.1.4
provides additional details. Distances come from the following sources: a
Añez-López et al. (2020), b Sato et al. (2014); Fernández-López et al. (2021),
c Beuther et al. (2017), d Chibueze et al. (2014), e Sato et al. (2014), f Xu
et al. (2011), g Guzmán et al. (2020), h Whitaker et al. (2017), i Peretto et
al. (2013), j Kalcheva et al. (2018), k Reid et al. (2014), l Kurayama et al.
(2011), m Ilee et al. (2018),

We use the SrcI data from Ginsburg et al. (2018, 2019a). We use G17 data from
Maud et al. (2019). We use G351.77 data from both DIHCA and Beuther et al.
(2019). We use I16547 data from both DIHCA and Tanaka et al. (2020). For each
of these data sets, we refer the reader to the cited papers for the data
reduction description. We include summary statistics of these observations in
Table 1. We give additional details about the physical resolution, distance to
the targets, and the line used as a velocity guide (see §3.1) in Table 1.

Finally, we use proprietary data toward Sh 255-IR SMA1 (hereafter S255IR) from
project 2019.1.00492.S (PI Ginsburg). We use the archive-produced data
products, which were cleaned with the ALMA pipeline, and imaged them with CASA
6.4.3.4. The images were cleaned to a depth of 10 mJy using Briggs robust=0
weighting.

##  3 Results and Analysis

We search for lines of NaCl, KCl, H2O, SiS, and H30$\alpha$ in each of the
target pointings (see Table 2). Since each target was selected for having a
high luminosity or a strong HMYSO disk candidate beforehand, we used the
literature identification of existing sources as our starting point. We cut
out cubes centered on the brightest continuum source in the ALMA images
(except G5.89; see §B.4). We also searched fainter continuum sources,
selecting small sub-regions around each of the compact continuum peaks that
could plausibly contain disks. Because the source selection is based on a by-
eye examination of the data over a limited field of view (in most cases, only
the inner 5–10 arcseconds of the ALMA field of view was imaged), the sample
presented here has unknown completeness - the conclusions we draw will
therefore be only suggestive, not conclusive.

Most of our line detections come from stacked spectra of resolved disk-like
objects. We describe in Section 3.1 the line stacking approach used to obtain
higher signal-to-noise ratio spectra that represent average values over the
candidate disk. Section 3.3 describes the detections in individual sources and
shows some of the extracted images and spectra. Additional images and spectra
are displayed in the appendices.

The main result is the detection of the ‘brinary’ lines toward the 9 sources
(of which 3 are tentative detections) shown in Figures 1 and 2. These initial
figures show moment maps of the NaCl lines as described in Section 3.1.3.

Figure 1: Moment-0 (integrated intensity) images of the resolved sources in
NaCl lines. For SrcI (top-left), this is the integrated intensity of the NaCl
$J$=18-17 $v$=0 line. For the remainder, G17 (top right), G351 mm1 (bottom
left), and NGC 6334I mm1b (bottom right), these are the average of the
$J$=18-17 and $J$=17-16 transitions of both the $v$=0 and $v$=1 states. The
coordinates are given in RA/Dec offset from the central position specified
under the abscissa. The scalebars show physical sizes as labeled. The ellipses
in the corners show the full-width half-maximum beam ellipse.

Figure 2: Integrated intensity (moment-0) images of the NaCl stacked lines
toward G351mm2 (top left), G351mm12 (top middle), NGC 6334I mm2b - which is
only a tentative detection (top right), I16547A (bottom left) and B (bottom
center), and W33A (bottom right). These disk candidates are either unresolved
or marginally resolved.

###  3.1 Line stacking extraction

From source candidate identification from the continuum data, the analysis
forks down two different paths. We start by searching by eye for emission
associated with the H2O line, which is quite bright in SrcI, in §3.1.1, which
we then use as a kinematic reference. If water is not detected, we use a
different line as our kinematic reference as described in §3.1.2.

####  3.1.1 Water-driven analysis

If the water line is detected, we use it to create a ‘velocity map’ following
this procedure:

  1. 1.

Cut out a cube containing the region around the estimated central $v_{\rm
LSR}\pm 20\>\textrm{km~{}s}^{-1}$ in velocity and encompassing only the
candidate disk region in space.

  2. 2.

Create a peak intensity map of the line in that region.

  3. 3.

Create a threshold mask including only the (by-eye) estimated significant
emission in the peak intensity map. Use one iteration of binary erosion and
three to seven iterations of binary dilation to remove isolated bright noise
pixels and fill back in the mask.111Binary dilation refers to mathematical
morphology operations in which any pixel having value False and a neighbor
with the value True is set to True. Binary erosion is the inverse operation.

  4. 4.

Create a volumetric threshold mask including only pixels above the estimated
noise level. Then, use one iteration of binary erosion followed by one to
three iterations of binary dilation to fill in the mask. As in the previous
step, this step is to eliminate isolated bright pixels, but because it is in
3D, a different threshold can be adopted.

  5. 5.

Create a moment-1 (intensity-weighted velocity) map of the spatially and
volumetrically masked data cube.

The erosion and dilation steps are performed to exclude isolated bright pixels
and to maximally include all pixels associated with source emission.

We then use the velocity map to stack the spectra obtained across the disk
candidate. Each spectrum from each spatial pixel in the cube that has a
measured velocity is shifted such that the line peak is moved to 0 km s-1. The
spectra are then averaged to produce the stacked spectrum.

This stacking process assumes that all spectra through the candidate disk will
have similar peak intensity and width but different central velocities; this
assumption held well in the SrcI spectrum (Ginsburg et al., 2018). We assume
that the kinematics of our selected line are the same as the lines of
interest; this assumption is justified by position-velocity diagrams in
§3.3.2.

We demonstrate the advantage of this line stacking approach in Figure 3. The
signal-to-noise in targeted lines is substantially increased, and faint lines
appear that are otherwise missed. In the example figure, the most obvious case
is KCl $v$=4 $J$=31-30, which is not apparent in either the peak intensity or
average spectrum, but is strongly evident in the stacked spectrum. Those
sources that are best-resolved - in our sample, G351 and G17 - show the most
improvement.

Figure 3: Demonstration of the utility of stacking analysis toward G17 in one
spectral window. The black spectrum is stacked by shifting each individual
spectrum to match the location of the peak intensity of the H2O line; it is
the same in both panels. The top panel shows the peak intensity spectrum taken
over the same area as the stacked spectrum, and the bottom shows the mean
spectrum in blue. It is clear that the stacked spectrum has clearer features
and better signal-to-noise ratios than either the peak or average spectrum.

We label the resulting averaged spectra with known NaCl and KCl transitions
and selected other lines, including those of SiO, SiS, H2O, H30$\alpha$, and
several prominent other molecules (see Table 2). We then use these plots to
populate the detection table, Table 3. We regard the lines as firm detections
only if they:

  * •

Are prominent, bright, and relatively isolated (e.g., H2O, as in Figure 4)

  * •

Exhibit an appropriately broad linewidth (H30$\alpha$ is expected to have
$\sigma_{v}\gtrsim 5$ km s-1 because it comes from hot plasma ($T\sim 10^{4}$
K), while molecular species are expected to be narrower, coming from gas at
$T<1000$ K).

For species for which we label the vibrationally excited transitions, we
consider only the $v$=0, $v$=1, and $v$=2 levels, as higher levels are
expected to be weaker if present.

There are several cases where a compelling detection of one line of a species
(NaCl or KCl) is detected, but an adjacent state is not detected. For example,
the $J$=18-17 $v$=1 line is detected, but the $J$=16-17 $v$=1 line is not. In
these cases, we regard the detection as tentative and note it in Table 3 with
an asterisk. These were cases in which the line may still be present, but may
be hidden by either blending with neighboring species or absorption by a line
in the surrounding core.

We report detections qualitatively rather than quantitatively because the
detections are generally obvious and high signal-to-noise. When a detection is
ambiguous, it is not because of high noise but because of confusion with
neighboring lines (e.g., 41KCl $J$=31-30 is confused with NaCl $v=1$
$J=18-17$) or absorption by non-disk material (e.g., KCl $v$=4 $J$=31-30).

####  3.1.2 Water nondetection-driven analysis

The path for water non-detections is less linear. If we are unable to clearly
identify emission in the H2O line, we search for other lines to use as the
basis for stacking. We first create a simple averaged spectrum over the
selected disk candidate’s emitting region by examining several different lines
in the cube. We then search for other plausible guiding lines, focusing on
those that exhibit gradients in the direction expected given known outflows
(i.e., we look at lines rotating perpendicular to outflows). We look at SiS,
SO, and, when truly desperate, $\textrm{CH}_{3}\textrm{OH}$ lines. If we are
able to identify a reasonably disk-like line from among these lines, we use it
to produce a velocity map as above (§3.1.1). If, after this search, we are
still unable to find a line that traces disk-like kinematics, we remove the
source from further consideration.

For both water- and non-water-driven stacking, we report the achieved noise
level in Table 1.

####  3.1.3 Cube stacking

When NaCl is detected, it is generally seen in multiple transitions. The NaCl
$v$=0 and $v$=1 $J$=18-17 and $v$=1 and $v$=2 $J$=17-16 transitions are
present in the DIHCA observational setups and are not too badly contaminated
by neighboring lines. We therefore create ‘stacked’ NaCl cubes by cutting out
cubes centered on each of those transitions, smoothing them to a common beam,
regridding them to a common spectral resolution, and then averaging the cubes.
We use these stacked cubes for further analysis of the NaCl lines, producing
both moment-0 images and position-velocity diagrams (except for SrcI, for
which the signal-to-noise ratio in individual lines was high enough to produce
moment maps without stacking). The stacked cubes have higher signal-to-noise
than the individual cubes and de-emphasize contaminant lines that are adjacent
to the target lines, since the contaminants arise at different relative
velocities for each NaCl line (for example, while the 41KCl 31-30 line is 13
km s-1 to the red of the NaCl $v=1$ $J=18-17$ line, there is no line 13 km s-1
to the red of the three other NaCl lines included here). This stacking
approach is not strictly necessary for use or analysis of the NaCl lines, but
it aesthetically improves the resulting images and makes visual inspection and
comparison more straightforward.

We created these cubes whether or not we first noted an NaCl detection in the
spectrum. In cases where no detection was apparent in the stacked spetrum, we
nevertheless checked the stacked NaCl cubes to see if extended emission at the
expected velocity was apparent. No additional detections were obtained through
this approach.

####  3.1.4 COMs

We identify the presence of complex organic molecules (COMs) in the spectrum
in a very broad-strokes manner. We do not identify specific species, though we
note that $\textrm{CH}_{3}\textrm{OH}$ and CH3CN are commonly detected, but
instead characterize the spectra by the richness of the ‘line forest.’ For
each observation, in the spectral range 219.2-220.8 GHz (which is COM-rich and
includes the CH3CN $J=12-11$ ladder), we measure the per-pixel noise
($\sigma_{\rm cube}$) by obtaining the standard deviation over the full field
of view over all pixels; this approach slightly overestimates the noise
because it includes signal in the noise estimate. For image cubes that were
not already continuum-subtracted, we estimate, and then subtract, the
continuum by performing pixel-by-pixel sigma clipping to 3-$\sigma$, then
taking the median across the spectral axis (i.e., as in Sánchez-Monge et al.,
2018). Then, for each extracted region around a candidate disk, we average the
spectra within that region, then determine what fraction of the spectrum
exceeds five times the expected noise level, where the expected noise level is
$\sigma_{\rm cube}n_{\rm beams}^{-1/2}$, where $n_{\rm beams}$ is the number
of beams included in the averaging area. Note that we only search for COMs in
emission; absorption by C- and O-bearing species is seen toward most sources,
but is not directly associated with the candidate disk, instead, it likely
comes from the surrounding envelope or molecular cloud.

Table 1 gives a summary of these statistics in addition to general properties
of the data.

###  3.2 Line Identification

We briefly discuss the key lines used for identification of NaCl, KCl, PN,
SiS, and SiO in this section. The summary of lines considered is in Table 2.
The NaCl $v$=0 and $v$=1 $J$=18-17 and $v$=1 and $v$=2 $J$=17-16 transitions
are present in the DIHCA observational setups. The $v=1$, $J=18-17$ line is
very close to the 41KCl 31-30 line, but the latter can be ruled out as a
contaminant because the 41KCl 29-28 is also included in the observations. In
Orion SrcI, the peak intensity ratio was NaCl $v=1$ $J=18-17$ $\approx
5\times$ 41KCl 29-28.

Table 2: Summary of spectroscopic lines used in this analysis

Line Name | Frequency | EU
---|---|---
| $\mathrm{GHz}$ | $\mathrm{K}$
SiS v=1 12-11 | 216.757603 | 1138.75
SiO v=0 5-4 | 217.104980 | 31.26
KCl v=4 29-28 | 217.228912 | 1733.21
41KCl 29-28 | 217.543178 | 156.71
SiS v=0 12-11 | 217.817644 | 67.95
NaCl v=2 17-16 | 217.979967 | 1128.38
H2CO $3_{0,3}-2_{0,2}$ | 218.222192 | 20.96
HC3N 24-23 | 218.324788 | 1084.99
CH3OH $4_{2,2}-3_{1,2}$ | 218.440063 | 1422.49
KCl v=3 29-28 | 218.579708 | 1345.06
C18O 2-1 | 219.560354 | 15.81
NaCl v=1 17-16 | 219.614936 | 614.51
HNCO $10_{10}-9_{9}$ | 219.798274 | 58.02
KCl v=2 29-28 | 219.936113 | 671.38
SO $6_{5}-5_{4}$ | 219.949440 | 34.98
K37Cl v=2 30-29 | 221.078543 | 948.25
41KCl v=1 31-30 | 231.088150 | 572.25
K37Cl 31-30 | 231.218839 | 177.68
H30$\alpha$ | 231.900928 | -
C30$\alpha$ | 232.016632 | -
KCl v=4 31-30 | 232.163002 | 1755.14
41KCl 31-30 | 232.499840 | 178.67
NaCl v=1 18-17 | 232.509950 | 625.67
Si33S 13-12 | 232.628545 | 78.16
H2O v2=1 $5_{5,0}-6_{4,3}$ | 232.686700 | 3461.91
K37Cl v=4 32-31 | 232.907553 | 1739.18
PN v=1 J=5-4 | 233.271800 | 1937.30
KCl v=3 31-30 | 233.605698 | 1367.12
NaCl v=0 18-17 | 234.251912 | 106.85
SiS v=1 13-12 | 234.812968 | 865.40
PN J=5-4 | 234.935663 | 33.83
KCl v=2 31-30 | 235.055578 | 687.16
K37Cl v=2 31-30 | 235.768235 | 970.53
SiS v=0 13-12 | 235.961363 | 79.28

Lines covered by one or more observations in this work. The frequency and
upper state energy levels are pulled from Splatalogue and refer either to
CDMS, SLAIM, or JPL values. For KCl v=3 and v=4 values, EU is drawn from the
modified version of the Barton et al. (2014) line list used in Ginsburg et al.
(2019a).

###  3.3 Salt detections

We detect salts in 9 sources (of which 3 are tentative) in 6 regions. Figure 1
gives an overview of those objects that are spatially resolved. In the
following sections, we describe the salt-bearing sources in more detail: G17
(§3.3.1), G351 mm1, mm2, and mm12 (§3.3.2), W33A (§3.3.4), NGC6334I mm1b and
mm2b (§3.3.3), and I16547 A and B (§3.3.5). The sixth region and disk, Orion
SrcI, is not discussed in detail here because the same analysis was already
done in Ginsburg et al. (2019a), but it is included in the discussion section
below.

Table 3: Summary of detections, tentative detections, and non-detections of
target species in the source sample

Source | disk | H2O | NaCl | KCl | SiO | RRL | COMs | SiS | SO | PN
---|---|---|---|---|---|---|---|---|---|---
Orion SrcI | yes | yes | yes | yes | yes | no | no | yes | yes | ?
G17 | yes | yes | yes | yes | yes | yes | no | yes | yes* | no
I16547A | yes-c | yes | yes* | no | yes | no | yes | yes | yes | yes
I16547B | yes-c | yes | yes* | no | yes | no | yes | yes | yes | yes
G351.77mm1 | yes-c | yes | yes | no | yes | no* | no | yes | no | yes
G351.77mm2 | unres | yes | yes* | no | no | yes | no | no | no | no*
G351.77mm12 | unres | yes | yes* | no | yes | no | no | yes | no | yes
W33A mm1-main | unres | yes* | yes* | no | yes | yes* | yes | yes* | yes | yes
NGC6334Imm1b | yes-c | yes | yes | yes | yes | no | yes* | yes | no* | yes*
G5.89 mm15 | cont | no | no | no | ? | no | no | no | no | no
IRAS18162 GGD27 | yes | no | no | no | ? | no | yes | no | yes | no
NGC6334IN SMA6 | no | yes* | no | no | no | no | yes | no | no | yes*
NGC6334IN SMA1b/d | no | no* | no | no | no | no | yes | no | no | no*
G11.92mm1 | yes | no | no | no | yes | no | yes | yes* | yes | yes*
IRAS18089 I18089-1732 | yes-c | no | no | no | yes | no | yes | yes* | yes | no*
IRAS16562 G345.4938+01.4677 | no | no | no | no | yes | yes | ext | no | ext | no
G333.23mm1 | no | no | no | no | no* | no | yes | no | yes | yes*
G333.23mm2 | yes* | no | no | no | no* | no | yes | no | yes | yes*
G335 ALMA1 | no | no | no | no | yes | no | yes | no | no* | no
G29.96 submm1 | no | no | no | no | yes | no | yes | no* | no* | no*
G34.43mm1 | no | no | no | no | no* | no* | yes | no | yes | no*
S255IR SMA1 | no* | no* | no | no | yes | yes | yes | no | yes | no
NGC6334Imm1d | yes* | yes* | no | no | no | no | no | no | no | yes*
NGC6334Imm2b | unres | yes | no* | no | yes | no | yes | yes | yes | yes*

We use ‘?’ to indicate “not observed”, ‘yes’ for a definitive detection, ‘no’
for definitive non-detections, ‘yes*’ for tentative detections, and ‘no*’ for
tentative nondetections, where this uncertainty can either be from line
confusion or low signal-to-noise, and ‘ext’ for those associated with the
envelope but not the disk. These all refer to detections in emission; COMs are
seen in absorption toward many sources, but we do not consider these. For SiO,
we do not attempt to distinguish between SiO from the outflow and from the
disk; in Orion SrcI, we know that SiO is present in both. For the ‘disk’
column, we either answer ‘yes’ for a clear disk detection from literature
kinematic characterization, ‘yes-c’ for a candidate disk toward which a
kinematic signature consisten with rotation has been observed, but for which
the kinematics have not yet been confirmed to be Keplerian, ‘continuum’ if a
disk-like (linear, $\lesssim 300$ au long) feature is seen in the continuum,
‘unresolved’ if the targeted source is too small for us to say, and ‘no’ if
neither of the above hold; ‘no’ does not indicate that no disk is present,
merely that we did not identify one. In many cases, we suspect a disk must be
present because there is an outflow, but we say ‘no’ if we can’t see it.

####  3.3.1 G17

The G17 disk is the best resolved source in our sample after SrcI (Fig 1; Maud
et al., 2019). It is a confirmed Keplerian disk (Maud et al., 2019). Because
of its high signal-to-noise and well-resolved structure, we investigate it in
somewhat more detail than the other sources in the sample. Figure 4 shows the
stacked spectrum based on the water line as described in Section 3.1.1.

Figure 4: Stacked spectra toward G17 from the Maud et al. (2019) data set. The
stacking was based on the H2O line. Line IDs are shown. Different colors are
used for targeted species with multiple transitions in-band: orange for SiS,
blue for KCl, red for NaCl, magenta for H30$\alpha$, purple for PN, and green
for H2O. The remaining species, with only one transition marked, are shown in
black.

The salt detections toward G17 resemble that toward SrcI, with both NaCl and
KCl tracing the same rotational structure. Figure 5 shows position-velocity
diagrams perpendicular to the outflow axis measured by Maud et al. 2018 at
position angle $\theta\approx 135^{\circ}$ based on the large-scale CO
outflow. Maud et al. (2019) measured the disk position angle to be 25.9∘,
which traces the direction of maximum gradient in the H2O disk and through the
emission peak of the continuum structure. This angle is nearly perpendicular
to the large-scale outflow; we adopt this angle as the disk PA. Maud et al.
(2019) measured the disk inclination to be $i=40\pm 4^{\circ}$ from the axis
ratio of the continuum image, so we adopt that inclination when overplotting
Keplerian curves. As with SrcI, both the water and salt lines trace out orbits
consistent with Keplerian rotation around a high-mass star. Figure 6 shows
integrated intensity (moment-0) and intensity-weighted velocity (moment-1)
maps of the G17 disk.

Figure 5: Position-velocity diagrams extracted across the G17 disk. (top left)
KCl grayscale with NaCl contours (top right) NaCl grayscale with Keplerian
rotation curves drawn for a 15, 30, 40 $M_{\odot}$ star with a $i=40^{\circ}$
disk (Maud et al., 2019) in orange, green, and blue, respectively. (bottom
left) Water (H2O) in grayscale with thin NaCl contours. In all cases, the
velocity gradient matches that of the disk identified in Maud et al. (2019).
(bottom right) Position-velocity diagram of the H30$\alpha$ line toward G17,
with NaCl contours overlaid. The H30$\alpha$ emission is clearly confined to
within the NaCl disk. While there is a hint of a velocity gradient in the
H30$\alpha$ line, it is unclear whether this gradient traces rotation.  Figure
6: Moment-0 (integrated intensity) and moment-1 (intensity-weighted velocity)
images of stacked NaCl (left) and H2O (right) for G17. The red and blue arrows
indicate the outflow direction from Maud et al. (2017). The dashed gray line
shows the direction the position-velocity diagram was extracted from (Figure
5).

G17 chemically resembles SrcI in several ways: the only lines seen directly
toward the source are H2O, NaCl, KCl, SiS, and SiO. There is little
‘contamination’ in the spectrum from COMs. As in SrcI, KCl lines are detected
at about half the peak brightness of NaCl lines with similar $E_{\rm U}$; no
$v$=0, $v$=1, or $v$=2 transitions of KCl are covered by the observations
(except KCl v=2 J=29-28, which is blended with SO $6_{5}-5_{4}$ and cannot be
clearly identified).

We measure enough lines to produce both rotational and vibrational diagrams
for KCl, but only a rotational diagram for NaCl (Figure 7). These plots
provide temperature and column density measurements of the target molecules if
the observed transitions are in local thermodynamic equilibrium (LTE). As in
SrcI, the rotational temperatures are much cooler ($T_{rot}\sim 35-60$ K) than
the vibrational temperatures ($T_{vib}\sim 900$ K), indicating that non-LTE
effects are important. We have yet to determine the underlying mechanism, but
several possibilities are discussed in Ginsburg et al. (2019a). We defer
further discussion of the excitation to a future work in which we will
integrate additional transitions.

Figure 7: Rotation diagrams from the fitted lines toward G17. (left) KCl
rotation diagram (right) NaCl rotation diagram

A key difference between SrcI and G17 is that G17 exhibits radio recombination
line (RRL) emission, while SrcI does not. Figure 5 shows the H30$\alpha$ RRL
in grayscale with NaCl contours on top, showing that the RRL comes from a
smaller region contained within the NaCl-bearing disk.

Both SrcI and G17 have central ‘holes’ in which there is no NaCl emission. In
SrcI, Ginsburg et al. (2019a) inferred the presence of this hole from the gas
kinematics, as the _apparent_ hole seen in Figure 1a is an observational
effect caused by high optical depth in the edge-on dust disk. In G17, which is
less inclined, the hole is directly observed (Figure 1b). Since the radiation
field is more energetic in G17 than in SrcI, it is possible that the salt hole
is related to the gas temperature.

####  3.3.2 G351.77-0.54

Beuther et al. (2017) and Beuther et al. (2019) published high-resolution
observations of the G351.77-0.54 region, which we use here over the lower-
resolution DIHCA data. We focus on the two brightest mm sources, mm1 and mm2,
which both exhibit H2O and salt line emission. G351mm2 is a substantially
fainter source, but similarly exhibits brine lines. Spectra of the G351
sources and subsequent sources are presented in the Appendices.

We identify several lines in mm1, including NaCl $v$=1 and $v$=2 $J$=18-17 and
SiS $v$=0 and $v$=1 $J$=13-12 (these latter were clearly detected, and used
for disk kinematic study, in Beuther et al. (2019), but were listed as
‘unidentified’). Because of the slightly different spectral configuration
adopted in these observations as compared to the DIHCA observations, both the
$J$=12-11 and $J$=13-12 lines of SiS $v$=0 are covered and detected.

The morphology and range covered by NaCl and SiS are very similar (Appendix
A.1), though the gap in the center for SiS is more pronounced than for NaCl.
These molecules arise in similar, but not identical, regions.

The velocity gradients in NaCl, H2O, and SiS in mm1 appear to trace a bipolar
outflow. Beuther et al. (2019) discussed the SiS emission lines in detail,
comparing the velocity gradient observed in this line to that seen in SiO. The
SiO $J$=5-4 and CO $J$=6-5 lines both appear in an extended redshifted lobe to
the northwest of the source. The outflow is asymmetric and truncates at the
position of mm1, suggesting that mm1 is the source (Beuther et al., 2017).
Since the redshifted lobe occurs on the redshifted side of the observed SiS
velocity gradient, Beuther et al. (2019) interpreted the lines as part of an
outflow rather than a disk. While there is a blueshifted component opposite
the redshifted flow, it is detected in only two channels and is only seen in
the beam adjacent to the central source, so its direction cannot be determined
independent from the elongated red lobe. They also noted the presence of a
velocity gradient perpendicular to the outflow direction in CH3CN, suggesting
that CH3CN traces the disk in a disk-outflow system. The observed velocity
gradient in briny lines is perpendicular to the CH3CN disk, indicating either
that the emission comes from outflow or that the disk changes orientation with
scale.

We re-examine the kinematics of the now-identified lines here. Figure 8 shows
moment-0 and moment-1 maps of both NaCl (stacked) and H2O. The kinematics
resemble a disk, with kinematics consistent with orbital motion around a $\sim
20/\sin i$ $M_{\odot}$ central potential, However, the extended SiO / CO
$J$=6-5 outflow feature is parallel, rather than perpendicular, to the axis of
the velocity gradient. Assuming the SiO structure is tracing an outflow, the
velocity structure seen in Figure 8 is the base of the outflow. The lack of
any gradient perpendicular to the outflow direction suggests that the briny
lines are not tracing a disk wind in G351mm1, as they are in SrcI (Hirota et
al., 2017), since they would retain that rotation signature for at least some
distance above the disk. However, the lines are marginally resolved and
extended in the direction perpendicular to the outflow, suggesting that the
emission arises from an area at least comparable to be beam size ($\sim 50$
AU). The extended launching region is difficult to reconcile with the lack of
disklike kinematics. We are not able to definitively conclude on the nature of
the velocity gradient in mm1 and suggest that it should be studied further at
high resolution, particularly to trace the kinematics of the disk-outflow
system(s). Nevertheless, we note that the briny emission is limited to a
region $<100$ AU across.

Figure 8: Moment-0 (integrated intensity) and moment-1 (intensity-weighted
velocity) images of stacked NaCl (left) and H2O (right) for G351 mm1. The
position-velocity diagrams in Figure 14 are taken along position angle
$130^{\circ}$, parallel to the velocity gradient seen in the lower panels
here. The red arrow shows the direction of the outflow from Beuther et al.
(2019) that extends $\gtrsim 0.2$ pc to the northwest. A compact blueshifted
feature was also weakly detected bin SiO opposite the redshifted flow, but it
is unresolved. The blue side of the NaCl, H2O, and (in Appendix A.1) SiS and
PN corresponds with the SiO blueshifted lobe seen in Fig. 7 of Beuther et al.
(2019), but since its extent is limited to the $\lesssim 100$ AU scale shown
in this figure, we cannot confirm whether it is comprised of outflowing
material.

By contrast to mm1, mm2 has H2O emission (Appendix A.2), but only tentative
NaCl emission (the $v$=1 line is detected at $\sim{\mathrm{few}}-\sigma$, but
the $v$=2 line is below the noise). No SiS emission is detected. However,
H30$\alpha$ emission is fairly clearly detected. There is a velocity gradient
along the NW-SE axis in the H2O line (Appendix A.2). Curiously, this is
perpendicular to the direction of the gradient shown in Figure 9 of Beuther et
al. (2019), which shows the unidentified 231.986 GHz line, suggesting that
there may be perpendicular gradients here tracing outflow and disk. However,
no outflow is known toward this source, and we do not see clear signs of
outflow in any of the lines studied here, including SiO. While SiO is detected
in emission and absorption toward mm1, it is detected only in absorption
toward mm2.

Despite the presence of many KCl transitions in band, there are no detections
- but our limits on these lines are relatively weak, as all of the $v$=0 and
$v$=1 transitions tend to land in confused regions or come from doubly-rare
isotopologues.

A third source, G351mm12, also has salt detected. Appendix A.3 shows the
standard suite of figures for this detection. This source is barely resolved.
Its moment maps indicate a hint of velocity gradient, but the gradient is at
the limit of our sensitivity and may be spurious.

####  3.3.3 NGC6334I

There are three sources within the NGC 6334I region that may exhibit brine
emission. Sources mm1b and mm2b are both notable for being faint continuum
sources adjacent to bright sources identified in lower-resolution data (the
‘b’ designation indicates that there are sources mm1a and mm2a that are
brighter).

mm1b exhibits clear signatures of H2O, multiple NaCl lines, SiS, and several
likely KCl detections (Appendix A.4). The H2O line shows hints of rotation
perpendicular to the outflow axis (Fig. 8 of Brogan et al. (2018) shows the
outflow, which is aligned to $\mathrm{PA}\approx 0^{\circ}$ to $-5^{\circ}$,
close to straight north-south), though the other lines do not as clearly
exhibit this signature. Brogan et al. (2018) note several other less prominent
outflows centered on this source, however, which hints that this object cannot
be interpreted as a single disk-outflow system. In the PV diagram of NaCl
(Fig. 9), we show curves at 10, 20, and 30 $M_{\odot}$ for an edge-on
Keplerian disk. Since the emission is confined to lower velocities than the 10
$M_{\odot}$ curve, it appears that this source is $<10$ $M_{\odot}$. However,
it may also be significantly inclined to the line of sight, in which case it
may be more massive. KCl is tentatively detected toward mm1b, with reasonably
strong peaks appearing in the KCl $v$=2 $J$=29-28 and K^37Cl $v$=0 $J$=31-30
lines. Some others that might be expected to be bright, e.g., KCl $v$=3
$J$=29-28 and $v$=3 $J$=31-30, are ambiguous or blended.

Figure 9: Position-velocity diagram of NaCl in the NGC 6334I mm1b disk.
Keplerian velocity curves for edge-on 10, 20, and 30 $M_{\odot}$ central
potentials are overplotted; the central potential here appears to have $M\sin
i<10$ $M_{\odot}$.

The other sources are less clear detections and are therefore considered only
candidate brinary sources. Similar to mm1b, mm2b has a reasonably clear H2O
detection and strong signs of SiS $v$=0 $J$=12-11 emission, but it has no
clear detection of any salt line (Appendix A.4). No clear outflow is present.
The SiS line profile is broad and somewhat different from that of water, so it
is not obvious that they trace the same kinematics.

mm1d has only has marginal SiS and H2O detections. We do not include it in the
figures or detection statistics. It is a strong candidate for follow-up
observations.

####  3.3.4 W33A

We marginally detect H2O, H30$\alpha$, and NaCl toward the bright source at
the center of W33A (Appendix A.5). At the current resolution, the emitting
region is unresolved. The detection of any of these lines individually is
tentative because we have only one firm line detection for each of these
molecules and they are potentially blended with other emission lines. The PN
5-4, H2O, and NaCl v=1 18-17 lines are the most prominently detected. The NaCl
v=1 and v=2 J=17-16 lines are too weak to confirm the NaCl detection. While we
detect only one PN line, it is isolated enough that confusion is unlikely to
affect it, so it is a reasonably firm detection. This source is a prime
candidate for further followup.

####  3.3.5 I16547

I16547A and B were reported to have salt, water, and SiS emission in Tanaka et
al. (2020). We confirm their detections both with their original data and with
coarser-resolution observations from the DIHCA program. Despite the clear
detection of those three briny species (see spectra in Appendix A.6 and A.7),
there is no sign of KCl in the data. The non-detection is in part driven by
confusion, in that many of the lower-$J$ transitions of KCl lay atop
transitions from other molecular species that are spatially extended and not
filtered out. Line stacking was not very helpful for these two targets because
they are only marginally resolved (Fig. 2). Nevertheless, velocity gradients
consistent with rotation are apparent in position-velocity diagrams extracted
along the direction of the maximum gradient (Appendices A.6 and A.7), which is
perpendicular to the outflow direction (Tanaka et al., 2020), suggesting that
these are both likely disks around high-mass YSOs.

###  3.4 Unsalted sources

The remainder of our targets do not have salt or water detections. We describe
them in slightly more detail in Appendix B. We note here that several of
these, i.e., G11.92, GGD27, and I18089, have clear extended disks that are
detected in other molecules (e.g., CH3CN) but not in brinary lines.

##  4 Discussion

###  4.1 Are briny lines disk-only tracers?

Many of our targets are only candidate disk sources, in that no resolved
Keplerian rotation curve has been observed. We therefore consider the
question: Could the briny emission be from outflows? Under the assumption that
outflows are driven by either a wind from a disk or from accretion from a disk
onto a star, the presence of an outflow still indicates the presence of a
disk, but that does not mean the lines we observe necessarily arise within
that disk.

In SrcI, it is clear that the water is partly outflowing, but it arose from a
disk wind and had a small observable scale height ($h<40$ au; see Fig. 10 of
Ginsburg et al., 2018). Even in that case, the water line was dominated by
disk kinematics, not outflow.

In most of the observed sources, a velocity gradient across the lines of
interest was observed. Such a velocity gradient can be produced by either
outflow ejection or disk rotation. In the best-resolved cases, G17 and G351,
the emission forms a complete ring, which is expected of a disk or disk wind.
The circular extent of the briny emission shows that it is not tracing a
collimated jet feature. However, in G351mm1, the direction of the gradient is
along the known outflow, suggesting that the briny lines do not trace a disk
or a disk wind; this source remains difficult to interpret.

While we cannot definitively determine the general origin of briny emission,
we observe here that it is restricted to radii $<300$ au in our full sample,
such that it is always consistent with arising in either the disk or the very
inner portion of the outflow.

###  4.2 Chemical similarities between the salt disks

We examine the general chemical properties of the brinary disks. Each has
several properties in common, but several differences. Table 3 lists the
detections and non-detections toward each examined source. We observe that
NaCl, H2O, and SiS lines are often detected toward the same sources, and
generally if one is absent, all are. In other words, they exhibit comparable
brightness when they are observed. This correlation hints that they come from
similar regions within disks or outflows, either because of excitation or
chemical (formation/destruction) conditions.

Figure 10: Cross-correlation plot made from Table 3. We cross-correlate purely
on the boolean value; those with a ‘yes’ are marked True, and those without a
‘yes’ are marked False. The rows are sorted in order of correlation with NaCl.

Figure 10 shows the cross-correlation of the boolean value (“yes” or “no”)
encoded in table 3; the tentative detections marked with ‘*’s are assigned to
their corresponding ‘yes’ or ‘no’ prefix. It shows that the presence of NaCl,
SiS, and H2O are strongly correlated. The presence of COMs in the spectra is
moderately anti-correlated with these ‘brinary’ species. RRLs are strongly
anti-correlated with COMs. The correlations among the other molecules (and
RRLs) are less pronounced.

We note that our data are extremely incomplete, as we explicitly select
against COM-bearing targets by selecting H2O-bearing disks. There are many
fainter disk candidates in the fields of view of our observations that we did
not include in our sample or in this plot. Most of these sources do exhibit
COM emission and do not exhibit RRL emission, though, so adding them to this
plot would generally strengthen the trends shown.

Because of the incompleteness of our sample, we caution against over-
interpretation. A repeat of this analysis with a consistently-selected sample
will be needed to draw firm conclusions. However, the correlations in the top-
left of the plot, between H2O, NaCl, and SiS appear firm - these species
coexist in this sample of disk candidates.

####  4.2.1 The low detection rate of KCl is an observational effect

The KCl detections are not perfectly correlated with NaCl detections, but we
argue here that this is an observational selection effect. Most likely, KCl is
better-correlated with NaCl than is apparent in Figure 10.

In the targeted band, there are no $v$=1 or $v$=0 transitions of the most
common isotopologue (39K-35Cl) of KCl, such that all detections are of higher-
excitation, lower-brightness transitions. The targeted KCl lines are also
partly hidden by confusion with other lines. The KCl lines are strongly anti-
correlated with the presence of COMs, which is primarily (or entirely) because
of confusion: the KCl lines in this band are weaker than NaCl when they are
detected, so they are more difficult to distinguish from the molecular line
forest present in dense spectra. They land inconveniently near bright COM
lines in the 215-235 GHz range more often than the NaCl lines.

The comparison to SrcI illustrates some of these effects. In SrcI, the peak
brightness temperature of the NaCl and KCl lines with E${}_{U}<1000$ K were
similar, with $T_{\rm B,max}\sim 100-200$ K. The NaCl $v$=0 lines were no more
than twice as bright as the KCl $v$=0 lines. The SrcI data set was targeted on
outflow-tracing lines, including 12CO $J$=2-1 and SiO $v$=1 $J$=5-4, which
both have nearby $v$=0 and $v$=1 KCl transitions, while the DIHCA observations
and others presented here chose to target the CH3CN ladder at 220.4 GHz and
therefore did not cover these low-$v$ transitions.

####  4.2.2 PN in the brine

The PN J=5-4 line appears to be detected in several of our sources. The PN v=1
J=5-4 transition, at 233.27182 GHz, is not detected. For the line-poor
brinaries, there is little confusion around this line, and its velocity lines
up perfectly with that of salts, so this identification is reasonably certain.
For the rest of the sample, the case is less clear; while there are some
tentative detections with clear lines at this velocity, the spectra are so
rich that we cannot definitively identify PN as the carrier species. The
presence of PN in the same regions as the highly-excited salt lines may
suggest that PN occupies a similar location within and binding energy to dust
grains. Rivilla et al. (2020) suggest that PN is present in the cavity walls
of an outflow toward AFGL 5142, but that it is released to the gas phase as
PH3, and the PN molecules are subsequently formed under the influence of the
star’s UV radiation. Given the lack of UV photons in these sources, as
indicated by the lack of correlation between PN and RRLs in Figure 10, our
data may indicate that PN is present in the grains and not formed in the gas
phase.

Figure 11: The integrated intensity (moment-0) maps of NaCl shown in Fig. 1
and 2, but now with each image scaled to the same physical resolution. The 100
AU scalebar, shown on the W33A mm1-main panel, applies to all panels. The
intensity scales are arbitrary, as the intent is only to show the disk
candidate structure.

####  4.2.3 RRLs

Hydrogen recombination lines are likely to be produced in the ionized regions
surrounding accreting high-mass young stars once they have contracted onto the
main sequence. In our sample, few RRLs are detected. Their presence, or
absence, is only weakly anti-correlated with the presence of COMs and PN, but
has little correlation with other molecules. While we might expect RRL
emission to become detectable from accreting HMYSOs toward the end of their
accretion phase, as the ionization rate is able to overtake the accretion rate
of fresh neutral material, our data provide little evidence for this process.

###  4.3 Excitation

At least in the best-resolved cases, G17 and G351, vibrationally excited
states of NaCl are detected ($v$=1, and 2). This feature is in common with
SrcI, where states up to $v$=6 were convincingly detected. The $v$=2
detections in particular, with $E_{\rm U}\gtrsim 1000$ K, suggest that an
excitation pattern similar to that in SrcI, in which $T_{\rm vib}>T_{\rm
rot}$, is common. This general feature is also common in evolved stars that
exhibit salt emission, suggesting that these salt lines only appear in regions
with strong radiative backgrounds in the infrared. Figure 7 highlights the
high excitation, though we defer deeper analysis of the excitation properties
to a future work.

###  4.4 Spatial Resolution

Our observed sample has non-uniform spatial resolution (Table 1), which helps
explain several of the non-detections. We show a version of Figure 1 with all
images resized to the same physical scale shown in Figure 11, highlighting
that the detected NaCl disks are small. We did not detect NaCl toward any
sources observed with beam size $>300$ au, which included four of our targeted
fields. Five of the fifteen targeted fields included detections. We performed
a logistic regression of the detection of salt against resolution and found
that the likelihood of salt detection in our sample is $>50\%$ for resolution
$<120$ au, peaking at $\sim 80\%$ at infinite resolution. While our haphazard
sample selection prevents drawing strong conclusions, these results suggest
that salty regions are limited to small scales ($\lesssim 300$ au) and are
challenging to observe if they are not well-resolved. This observational
limitation highlights the need for more extensive extremely high-resolution
observations with ALMA’s longest-baseline configurations to obtain dynamical
mass measurements for high-mass YSOs.

It is possible that more sensitive observations with poor physical resolution
may detect brinary lines now that we know to look for them. However, the main
difficulty in detecting these lines is not raw sensitivity, but confusion with
other lines. It will be beneficial to search other parts of the spectrum for
more isolated brinary lines, which might be expected to be more common at
lower frequencies. It may be possible to determine the presence or absence of
brinary lines by stacking the various transitions across species to average
down the ‘contaminant’ noise provided by other lines; we leave investigation
of this possibility to future work.

##  5 Conclusions

  * •

We have substantially increased the number of high-mass protostellar objects
with detection of salt, hot water, and SiS, increasing the number from three
to nine published detections across six regions.

  * •

Salt, water, and SiS tend to coexist. PN may also coexist with these species.
When any of these molecules is detected in a HMYSO disk, all of them are
likely to be.

  * •

Brinary sources are line-poor compared to typical hot cores. The lack of COMs
in the briny regions suggests that the chemistry of these regions is different
from hot cores, even when the central objects are surrounded by hot cores.

  * •

These emission lines do not come from the same volume as ionized hydrogen.
While some of the HMYSO candidates targeted exhibit both RRL and brine
emission, the presence of an RRL is a poor predictor of whether NaCl and H2O
are detected. The resolved case of G17 shows that the ionized gas comes from a
smaller radius than the NaCl.

  * •

With the nine brinaries presented here, we demonstrate that salt emission is
not rare. The primary reason it is not often detected is resolution: the
emission comes exclusively from small ($\lesssim 100$ au) size scales,
confirming that either chemistry or excitation restricts the millimeter lines
described here to disk-sized regions. Line confusion limits our ability to
detect these lines even when they are present, though confusion can be
alleviated with high spatial resolution.

  * •

However, salt and water emission is also not ubiquitous in HMYSO disks. Some
of the most compelling Keplerian disks around HMYSOs in the literature, such
as GGD27 and G11.92mm1, show no sign of salt emission despite their
similarities to other disks and superior data quality.

There is clearly substantial future work to do with these data and expanded
samples of brinaries. Some of the more obvious questions about brinary lines
include:

  * •

Are they correlated with source luminosity or stellar mass?

  * •

What disks, or outflows, produce them?

  * •

Do they occur around low-mass YSOs?

  * •

Why are highly vibrationally excited lines ($v>3$) detected?

Many of these questions will require establishing less biased, more systematic
samples of YSO candidates. Others will require more detailed, multi-line
studies toward a limited sample.

Acknowledgements We thank the anonymous referee for a thorough and
constructive report that significantly improved the paper, particularly in
terms of structure and readability. AG acknowledges support from NSF AAG
2008101 and NSF CAREER 2142300. HB acknowledges support from the European
Research Council under the European Community’s Horizon 2020 frame-work
program (2014-2020) via the ERC Consolidator Grant ‘From Cloud to Star
Formation (CSF)’ (project number 648505). HB also acknowledges support from
the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) –
Project-ID 138713538 – SFB 881 (“The Milky Way System”, subproject B01). P.S.
was partially supported by JSPS KAKENHI grant Nos. JP18H01259 and JP22H01271.
K.E.I.T. acknowledges support by JSPS KAKENHI grant Nos. JP19K14760,
JP19H05080, JP21H00058, and JP21H01145.

This paper makes use of the following ALMA data:
ADS/JAO.ALMA#2016.1.01036.S, ADS/JAO.ALMA#2017.1.00237.S,
ADS/JAO.ALMA#2016.1.00550.S, ADS/JAO.ALMA#2019.1.00492.S,
ADS/JAO.ALMA#2017.1.00098.S, ADS/JAO.ALMA#2018.1.01656.S, and
ADS/JAO.ALMA#2013.1.00260.S. ALMA is a partnership of ESO (representing its
member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST
and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the
Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and
NAOJ. In addition, publications from NA authors must include the standard NRAO
acknowledgement: The National Radio Astronomy Observatory is a facility of the
National Science Foundation operated under cooperative agreement by Associated
Universities, Inc.

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##  Appendix A Additional figures for salted sources

For the salted sources, we present moment maps and spectra with lines
identified in the following Appendices.

###  A.1 G351.77mm1

We show additional moment maps, spectra, and position-velocity diagrams from
G351.77mm1 in this section. This object is one of the most compelling, high
signal-to-noise objects in the sample, yet its kinematics are perplexing,
exhibiting disk-like rotation in the same direction as the larger-scale
outflow rather than rotating perpendicular to the outflow, as we would expect.
Figure 12 shows the stacked spectra. Figure 8 shows moment-0 and moment-1 maps
of NaCl and H2O. Figure 13 shows the same for SiS and PN. All four molecules
exhibit similar morphology and kinematics, including a prominent central hole,
which is strongly suggestive of a disk. Figure 14 shows moment-0 maps and
position-velocity diagrams, and Figure 15 more position-velocity diagrams,
illustrating that rotation is a plausible explanation for the observed
kinematics.

Figure 12: Stacked spectra from G351.77mm1 from the Beuther et al. (2019) data
set. The stacking was based on the H2O line. Line IDs are shown; no KCl
detections are clear. Different colors are used for targeted species with
multiple transitions in-band: orange for SiS, blue for KCl, red for NaCl,
magenta for H30$\alpha$, purple for PN, and green for H2O. The remaining
species, with only one transition marked, are shown in black. The PN line
identification should be taken with a grain of salt since it is the only
transition we observe from PN.  Figure 13: Moment-0 and 1 images as in Figure
8, but for the SiS v=0 J=13-12 and PN 5-4 lines of G351 mm1.

Figure 14: Comparison of NaCl and SiS moment-0 and position-velocity maps of
G351.77mm1. (top left) NaCl stack moment 0 image, as seen in Figure 1. (top
right) SiS 13-12 moment 0 image, showing similar morphology. (bottom left)
NaCl stack cube position-velocity diagram extracted along the direction of
maximum gradient. (bottom right) SiS 13-12 position-velocity diagram extracted
along the direction of maximum gradient. Keplerian rotation curves assuming an
edge-on central source with the listed mass are shown in colored lines; these
curves are not fits to the data and do not account for inclination, they are
just provided to guide the eye. Furthermore, as discussed in Section 3.3.2,
the velocity gradient shown here may trace an outflow rather than a disk.

Figure 15: Two more position-velocity diagrams of G351 mm1, showing the 235
GHz PN 5-4 line (newly identified here) and the H2O line. The common structure
seen in these diagrams, and in Figure 14, justifies our assumption that these
have common kinematics. Keplerian rotation curves assuming an edge-on central
source with the listed mass are shown in colored lines; these curves are not
fits to the data and do not account for inclination, they are just provided to
guide the eye. Furthermore, the direction of maximum gradient, along which
these diagrams are extracted, points in the direction of the outflow.

###  A.2 G351.77mm2

Figure 16 shows the stacked spectra of G351 mm2. Figure 17 shows the moment-0
and moment-1 maps of NaCl and H2O, which are only marginally resolved.

Figure 16:  The G351 mm2 disk candidate stacked spectrum. Like mm1 (Fig. 12),
the stacking was done on the H2O line. However, other lines are at most weakly
detected; the NaCl v=1 J=18-17 line is evident, but no other clear detections
are present in this or other bands. In all spectra, but particularly in
spectral window 0, much of the spectrum is absorbed by material unassociated
with the disk; we cut off the absorption features to emphasize emission
features here.  Figure 17: Moment-0 (integrated intensity) and moment-1
(intensity-weighted velocity) images of stacked NaCl (left) and H2O (right)
for G351 mm2.

###  A.3 G351.77mm12

Figure 18 shows the stacked spectra of G351 mm12. This source is unresolved,
as shown in Fig 2. Figure 19 shows the moment-0 and moment-1 maps of NaCl and
H2O, which are only marginally resolved.

Figure 18:  The G351 mm12 disk candidate stacked spectrum. See Figure 12 for
additional description.  Figure 19: Moment-0 (integrated intensity) and
moment-1 (intensity-weighted velocity) images of stacked NaCl (left) and H2O
(right) for G351 mm12.

###  A.4 NGC6334I

We show additional figures of NGC6334I, including spectra (Fig 20 and moment
maps (Fig 21. For mm2b, we show only the stacked spectrum (Fig 22), since the
source is unresolved

Figure 20: The NGC6334I mm1b disk stacked spectrum. See Figure 12 for
additional description.  Figure 21: Moment-0 and 1 images as in Figure 8, but
for NGC 6334I mm1b. The outflow noted by Brogan et al. (2018) is shown at
PA=-5∘ with green arrows, as it appears to be primarily in the plane of the
sky. The solid gray line shows the orientation from which the position-
velocity diagram (Figure 9) is extracted.

Figure 22: The NGC6334I mm2b disk stacked spectrum. See Figure 12 for
additional description.

###  A.5 W33A

The labeled, stacked spectrum from W33A is shown in Figure 23. The four-panel
moment map is not shown for this source because it is unresolved and shows no
structure; it is consistent with a point source.

Figure 23: The W33A disk stacked spectrum. See Figure 12 for additional
description.

###  A.6 I16547A

The labeled, stacked spectrum from I16547A is shown in Figure 24. Moment-0 and
moment-1 images are shown in Figure 25. A position-velocity diagram, with
overlaid Keplerian curves for an edge-on orbit with masses labeled, is shown
in Figure 28. We show these to provide order-of-magnitude mass estimates, but
note that we have no constraint on the disk inclination and have not attempted
to model the extent of the disk emission. No outflow is observed toward A
(Tanaka et al., 2020).

Figure 24: The I16547A disk stacked spectrum. See Figure 12 for additional
description.  Figure 25: Moment-0 and 1 images as in Figure 8, but for IRAS
16547A The solid gray line shows the orientation from which the position-
velocity diagram (Figure 28) is extracted, based on the angle determined in
Tanaka et al. (2020). The radio jet at PA=-16∘ is shown with arrows (Tanaka et
al., 2020).

###  A.7 I16547B

The labeled, stacked spectrum from I16547B is shown in Figure 26. Moment-0 and
moment-1 images are shown in Figure 27. A position-velocity diagram, with
overlaid Keplerian curves for an edge-on orbit with masses labeled, is shown
in Figure 28. We show these to provide order-of-magnitude mass estimates, but
note that we have no constraint on the disk inclination and have not attempted
to model the extent of the disk emission. An SiO outflow is observed toward B,
perpendicular to the gradient in the PV diagram (Tanaka et al., 2020).

Figure 26: The I16547B disk stacked spectrum. See Figure 12 for additional
description.  Figure 27: Moment-0 and 1 images as in Figure 8, but for IRAS
16547B The solid gray line shows the orientation from which the position-
velocity diagram (Figure 28) is extracted, based on the angle determined in
Tanaka et al. (2020). The red and blue arrows indicate the direction of the
SiO outflow noted in Tanaka et al. (2020).

Figure 28: Position-velocity diagrams of the I16547 disks in the stacked NaCl
lines. The overplotted curves show Keplerian rotation around central point
sources with masses indicated in the caption, assuming an edge-on inclination.
As noted in Tanaka et al. (2020), the disks appear to be counter-rotating
along similar position angles.

###  A.8 Continuum

We show Figures 1 and 2 again, but this time with continuum contours overlaid,
in Figures 29 and 30.

Figure 29: Moment-0 (integrated intensity) images of the resolved sources in
NaCl lines as described in Figure 1. The red ellipse shows the continuum beam
corresponding to the contours, while the black ellipse shows the line image
beam. The contours are: SrcI $\sigma=0.21$ mJy/beam, contours at 25, 50, 75,
100, 125, 150 $\sigma$, G17 $\sigma=0.12$ mJy/beam, contours at 50, 100, 150,
200 $\sigma$, G351mm1 $\sigma=0.08$ mJy/beam, contours at 50, 75, 100, 125
$\sigma$, NGC6334Imm1b $\sigma=1$ mJy/beam, contours at 10, 15, 20, 25
$\sigma$.

Figure 30: As in Figure 2, but with continuum contours overlaid. The red
ellipse shows the continuum beam corresponding to the contours, while the
black ellipse shows the line image beam. The contours are: G351mm2
$\sigma=0.08$ mJy/beam, contours at 50, 75, 100, 125 $\sigma$, G351mm12
$\sigma=0.08$ mJy/beam, contours at 10, 20, 30 $\sigma$, NGC6334Imm2
$\sigma=1$ mJy/beam, contours at 2, 3, 4 $\sigma$, I16547A $\sigma=0.08$
mJy/beam, contours at 100, 150, 200, 250, 300 $\sigma$, I16547B $\sigma=0.08$
mJy/beam, contours at 75, 100, 125, 150 $\sigma$, W33A $\sigma=0.3$ mJy/beam,
contours at 20,40,60,80 $\sigma$.

##  Appendix B Unsalted Sources

###  B.1 I16562

IRAS16562-3959 is also known as G345.4938+01.4677. This source shows SO in
absorption against a continuum disk. It exhibits RRL emission that shows a
very tiny gradient; Guzmán et al. (2020) reported the detection of an ionized
disk in this system based on RRL emission. No other emission lines are
associated with the RRL possible disk. There is extensive CH3CN emission
around this source with kinematics more complicated than a simple Keplerian
disk.

###  B.2 G333

We adopt naming from Stephens et al. (2015), though their resolution was only
$\sim 2.5\arcsec$.

G333.23mm1 is the bright central region. Morphologically, it looks like the
hub at the center of several converging filaments. There is no obvious sign of
a disk in any of the lines we examined. There is no hint of brinary lines.

G333.23mm2 shows some sign of a line gradient in $\textrm{CH}_{3}\textrm{OH}$,
hinting that a disk is present. However, no brinary lines are seen.

###  B.3 G335

Olguin et al. (2022) performed an extensive study of this system using the
data presented here. G335-ALMA1 is the bright central source that shows signs
of rotation, but no clear disk. Instead, it appears dominated by inflowing
accretion filaments. The spectrum is extremely rich, and we adopt the same
$\textrm{CH}_{3}\textrm{OH}$ line as in Olguin et al. (2022) as our velocity
reference. No brinary lines are detected.

###  B.4 G5.89

The G5.89 image is dominated by an extended HII region. The only disklike
source in the imaged field of view is mm15, so our cutout centered on that
source. It apparently exhibits _no_ line emission at all in the present data
set. We examined the other millimeter peaks in the region, but found no
obvious signatures of disks or brinary lines.

###  B.5 GGD27

GGD27mm1 (otherwise referred to here as GGD27) is the driving source of
HH80/81 (Girart et al., 2017, 2018; Añez-López et al., 2020). It is also known
as IRAS 18162-2048. It has a clear, well-defined, massive ($M\sim 5M_{\odot}$)
disk orbiting a $\sim 20$ $M_{\odot}$ star (Añez-López et al., 2020). There is
no sign of NaCl, H2O, KCl, or SiS in the spectrum of the disk in our data or
previous observations (Girart et al., 2017). It is bright in SO $6_{5}-5_{4}$,
which we used to stack spectra to perform a deeper search.

This source is the archetype of non-salt-bearing disks. Its well-defined
Keplerian line profiles, high central mass, and line-poor spectrum demonstrate
that not all HMYSO disks exhibit salt emission. The system is very similar to
SrcI in terms of its central stellar mass, disk size (though GGD27’s is
2-4$\times$ larger in radius), and luminosity, suggesting that none of these
features are critical for releasing salt into the gas phase. As the driver of
the HH80/81 outflow, it is also clear that the simple presence of an outflow
does not determine whether brinary lines are produced. However, one notable
difference is that the HH80/81 jet is highly collimated (Rodríguez-Kamenetzky
et al., 2017; Qiu et al., 2019), while SrcI drives a broader disk wind (Hirota
et al., 2017; Tachibana et al., 2019), hinting that the driving mechanism of
the outflow may have a role in determining when salts are detectable. The
comparison of this source to SrcI and others in this sample will be useful for
future understanding of the origin of salts.

###  B.6 IRAS18089

Sanhueza et al. (2021) observed this source at high resolution, though they
focused on the magnetic field. Previously, Beuther et al. (2004, 2005) showed
that this source was line-rich with SMA observations. We used a
$\textrm{CH}_{3}\textrm{OH}$ as the stacking line because the kinematics were
similar across all bright lines, including the SO $6_{5}-5_{4}$ line, but the
SO line was affected by strong absorption toward the inner disk. The line
kinematics were reasonably disk-like, but not consistent with a single
Keplerian disk; the velocity structure in this source requires more
sophisticated modeling. Nevertheless, there is no sign of brinary lines in the
averaged or stacked spectra.

###  B.7 G34.43mm1

There is a structure toward the center of G34.43mm1 with a clear line
gradient, but it does not trace disk-like kinematics and is quite extended. On
the larger scale, G34.43mm1 drives a powerful outflow (Sanhueza et al., 2010).
While this is region is quite nearby (1.56 kpc Xu et al., 2011), it is not
among the salt-bearing candidates. Despite its relatively near distance, the
observed spatial resolution is only $\sim 400$ au, which is much larger than
the detected disks. The overall appearance of the inner region suggests that
streamers are feeding in to a central region. The chemically rich inflowing
material results in a very spectrally dense spectrum (e.g., Sanhueza et al.,
2012; Liu et al., 2020a), which likely prevents detection of brinary features
even if they are present. This is a good candidate for followup at higher
angular resolution.

###  B.8 G29.96

Beuther et al. (2007) and Beltrán et al. (2011) studied this region at
$\sim$arcsecond resolution. The target region is centered on the brightest few
sources at the center. The distance to this object is unclear, with Beltrán et
al. (2011) reporting 3.5 kpc and Kalcheva et al. (2018) reporting 7.4 kpc.
There is no clear signature of any of the lines of interest, nor is there a
clear signature of rotation. There is a tentative detection of SiS v=0 12-11,
but neither of the SiS v=1 lines (13-12 or 12-11) appear, so this detection is
uncertain.

###  B.9 S255IR

Sh 2-255 IR SMA1 (S255IR) was the recent ($<10$ years) site of a major
accretion outburst. As such it is a strong candidate for being actively heated
over its ‘normal’ level.

No disk is obvious in the data cube from rotational signatures. While several
authors (Zinchenko et al., 2015, 2020; Liu et al., 2020b) have noted bulk
rotation in the molecular gas around S255IR, these signatures are not evident
on the smaller ($\sim 70$ au) scales probed here.

There is, however, a hint of both H2O and H30$\alpha$ emission along the
innermost part of the outflow, within about one resolution element of the
central source. There is also a hint of SiS here. There is clearly SiO
emission with a velocity gradient along the outflow axis, but curiously it is
not very bright along the continuum jet. This source warrants followup at
higher resolution and sensitivity to try to identify the location of the
actual disk, though it is not yet clear which lines to use for this search.

###  B.10 NGC6334IN

The NGC6334IN region contains several bright sources. The brightest in our
observations are SMA6 and SMA1 b/d (names from Sadaghiani et al., 2020).
Neither source shows clear signs of rotation in any line. SMA1b/d contains
several sources that may exhibit some hint of H2O emission, so we use that
line as the basis for stacking We do not detect any other lines. Notably, both
SO and SiO appear absent toward these sources. SMA6 also shows a hint of a
water line, but similarly shows no sign of any of the other targeted lines.

###  B.11 G11.92mm1

Ilee et al. (2018) reported on the disk G11.92mm1, showing a much larger
($r\sim 800$ au) size than most of the sources in our sample. We use SO
$6_{5}-5_{4}$ as the guide line for stacking, since it shows clear disk
kinematics. No brinary lines are detected.