Observing the NH3 snowline in protoplanetary disks with ngVLA
Kenji Furuya, 1 Hideko Nomura, 1 Yuri Aikawa, 2
1National Astoronomical Observatory of Japan, 2–21–1 Osawa,
Mitaka, Tokyo 181–8588
2
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japankenji.furuya@nao.ac.jp

Abstract
The snowline is the boundary where molecules freeze-out/sublimate in a protoplanetary disks. The elemental
compositions of gas and solid (ice + dust) change significantly across the snowlines of major volatiles such as H2 O,
NH3 , CO, etc., Then the detection of the snowlines and revealing the compositional changes across the snowlines are
essential for understanding planetary composition. In this article, we discuss the observability of the NH3 snowline
in Class II protoplanetary disks with ngVLA. NH3 is a key species to trace the chemical evolution of nitrogen duringstar- and planet formation and is also useful as a proxy to trace the H2 O snowline. We find that the NH3 snowline isdetectable in a disk around a Herbig Ae/Be star with ngVLA, while the detection is challenging for a disk around a
T-Tauri star.
Key words: protoplanetary disks — ISM: molecules — ISM: abundances

1. Introduction
Ammonia (NH3 ) is one of the major reservoirs of nitrogen instar-forming regions, accounting for ∼10 % of overall nitrogen(e.g., Öberg et al. 2011). The significant fraction of remaining nitrogen is likely present as N2 (e.g., Furuya et al. 2018),
which is not directly observable. As N2 is more volatile andchemically stable than NH3 , NH3 could be a raw material of
N-bearing complex organic molecules (Daranlot et al. 2012),
including amino acids. Taken together, NH3 is a key species totrace the chemical evolution of nitrogen during star- and planet
formation.
The bulk of NH3 in star-forming regions forms in molecular clouds and cores as ice. The NH3 evolution in the laterstages of star-formation, i.e., after protostellar formation, is
still largely unknown due to the lack of high spatial resolution observations at cm wavelengths, where NH3 has rotationinversion transitions. Gas-phase NH3 has been detected in
the warm (>100 K) gas of a deeply embedded protostellar
source with VLA (Choi et al. 2010), where NH3 ices havesublimated. To the best of our knowledge, there is only one
detection of NH3 in class II protoplanetary disks in the literature (Salinas et al. 2016). The observed gas-phase NH3by the Herschel Space Observatory traces the cold outerregions of the disk, where only a fraction of NH3 ice desorbes via photodesorption. In order to trace the bulk NH3
in disks, the observations of gas-phase NH3 inside the NH3snowline is necessary. It is known that NH3 is poor incometary ices with the abundance ratio of NH3 /H2 O <∼1%(Mumma & Charnley 2011 and references therein), which ismuch lower than that in interstellar ices, ∼5 % (e.g., Öberg
et al. 2011). Recent detection of ammonium salts in comet
67P/Churyumov-Gerasimenko by the ROSINA instrument on
Rosetta (Altwegg et al. 2020) implies that NH3 is largely converted into ammonium salts during star- and planet-formation.

ngVLA is the unique instrument that allows us to detect NH3
inside its snowline and thus to reveal the NH3 evolution.
NH3 would be also useful to locate the water snowline inprotoplanetary disks. The detection of water snowline by observing water emission lines with ALMA would be challenging, if the inner disk regions, where water snowline locates, are
optically thick for dust continuum at mm wavelengths (e.g.,
Banzatti et al. 2015). Even when dust continuum is opticallythick at mm wavelengths, it is likely optically thin at cm wavelengths. In addition, laboratory experiments of the sublimationof interstellar/cometary ice analogs have shown that at leastsignificant fraction of NH3 ice sublimates together with H2 O
ice (Martı́n-Doménech et al. 2014). Then, NH3 is suggested to
be a good proxy to trace the water snowline (e.g., Zhang et al.
2018).
2. Sensitivity estimate
Here we discuss the observability of the NH3 snowline in
a Class II protoplanetary disk with ngVLA. For this sake, we
conduct the line radiation transfer calculations, using a genetic
disk model.
2.1. Numerical setup
For the physical and chemical structures of a disk, we consider a disk around a Herbig Ae/Be star with the mass of 2 M⊙ .
We adopt the simulation results from Furuya et al. (in prep.),
in which the viscous evolution of gas, the dust evolution (collisional growth and radial drift), and gas-phase and grain surface
chemistry are considered in a time-dependent manner. In this1D radial model, temperature is calculated from the balance
between heating by stellar irradiation and viscous dissipation
and radiative cooling, using initial disk conditions. The initial
disk gas mass is 0.05 M⊙ with the dust-to-gas mass ratio of∼0.01. The binding energy of NH3 is set to be the same as that2[Vol. ,

Fig. 1. Upper panel) Column density of gas-phase NH3 as a function of
the distance from the central star in the disk model at t = 0 yr (blue line)
and 3 × 105 yr (orange line). Lower panel) Temperature distribution in
the disk model.

of H2 O, so that the snowlines of NH3 and H2 O are co-locatedin the disk model. It is assumed that the initial chemical composition of the disk is similar to that of dense molecular clouds
for simplicity, with the total (gas+ice) initial NH3 abundanceof ∼10−5 with respect to hydrogen nuclei, corresponding to
the NH3 /H2 O abundance ratio of ∼10 %.
The upper panel of Figure 1 shows that the column density
distribution of gas-phase NH3 in the model at t = 0 yr and at
3 × 105 yr, while the lower panel shows the temperature distribution adopted in the simulations. We chose the two specifictime to explore the impact of the NH3 column density distribution on the NH3 emission. The NH3 column density sharply
increases at ∼10 au, corresponding to the snowline of NH3 .
The NH3 column density inside the snowline is higher at thelater time, because of the radial drift of ice-coated dust grains
and sublimation of NH3 ice at the snowline. Outside the NH3snowline, the majority of NH3 is present as ice coatings of dust
grains, but some amount of NH3 still exist in the gas-phase dueto the gas-phase chemistry, which converts N2 to NH3 , andphotodesorption of NH3 ice. The NH3 column density outside
the snowline also increases with time, because the total surface
area of dust grains, which allows freeze-out of gas-phase NH3 ,reduces with time due to the coagulation and radial drift of dust
grains.
Figure 2 shows that the optical depth for dust continuum at
1 mm and at 1.3 cm in the disk model. The sharp increasesof the dust optical depth is seen at <∼5 au, corresponding to

Fig. 2. Dust optical depth at 1 mm (upper panel) and at 1.3 cm (lower
panel) as a function of the distance from the central star in the disk
model at t = 0 yr (blue line) and 3 × 105 yr (orange line).

the accumulation of dust grains there. This is caused by theassumption that the threshold fragmentation velocity of bare
dust grains is lower than that of dust grains coated by morethan one monolayer of water ice (e.g., Banzatti et al. 2015).
The dust opacity is calculated with the dsharp Opac package (Birnstiel et al. 2018), assuming the minimum and maximum dust sizes of 0.005 µm and 1 mm, respectively, with thepower law of -3.5. The dust absorption coefficients are 2.7
cm2 /g at 1 mm and 6.6 × 10−3 cm2 /g at 1.3 cm, respectively,in our models. Dust continuum at 1 mm wavelength (300 GHz)is optically thick at <∼ 10 au, while at 1.3 cm wavelength (23GHz), the dust continuum is optically thin in the whole region
in the disk. This comparison demonstrates that ngVLA is suitable to observe molecular lines from the midplane of the innerdisk regions.
As the adopted disk structure model is a 1D radial model,
we have to assume the vertical disk structure. For simplicity,
we assume that gas is vertically isothermal, the gas distribution
is in the hydrostatic equilibrium, the dust-to-gas mass ratio isvertically uniform (but not uniform radially), and NH3 gas ispresent only below one scale height of the gas. In reality, thedistribution of NH3 gas inside the snowline would depend on
the vertical temperature structure and the strength of verticalmixing. Such details are beyond the scope of this article.
The line radiation transfer calculations of para-NH3 inversion transitions are performed with LIME (Brinch &No. ]
Hogerheijde 2010), assuming local thermal equilibrium (LTE).
Molecular data are taken from the Leiden Atomic andMolecular Database (LAMBDA; Schöier et al. 2005). The assumption of LTE is valid as we focus on inner disk midplane,
where the gas density is very high (>1010 cm−3 ). For simplicity, we neglect hyper-fine structures of NH3 transitions here,
which should be explored in the future work. We assume thatthe distance of the disk from the Earth is 140 pc and the inclination angle of the disk is 30◦ .
2.2. Results
We performed the line transfer calculations to predict emission in (1,1)-(5,5) rotation-inversion transitions at 23-25 GHz
for two snapshots (t = 0 yr and 3 × 105 yr) of the disk structure model. We found that the results for the two snapshots are
similar, and thus we here only show the results at t = 3 × 105
yr. The left panel of Figure 3 shows the moment 0 map of the
NH3 (5, 5) emission model obtained by LIME with the spatial
resolution of 0.004” per pixel, while the right panel shows themoment 0 map of the emission model convolved with the 0.1”
Gaussian beam. As the intensity sharply drops at the snowline
position, one can locate the snowline from the moment 0 map if
the spatial resolution is high enough (<<10 au). Considering
the sensitivity of ngVLA for line observations, the beam sizeof ∼0.1” (∼14 au in our simulations) would be the minimum
for the detection of NH3 inversion transitions with reasonable
observing time (see below). Then, the NH3 snowline would be
only marginally resolved.
Even when the snowline is not spatially resolved, one canconstrain the snowline location from the spectrally resolved
observations (e.g., Notsu et al. 2017). Figure 4 shows the
spectral of the NH3 (5,5) emission extracted from inner ∼20
au. The peak brightness temperature is ∼50 K. Then, assuming a beam size of 0.1” and velocity resolution of 1 km/s,>
we can detect the emission with >∼10 σ (∼5 σ) for 10 hour
(2.5 hour) integration time to locate the NH3 snowline (cf.
https://ngvla.nrao.edu/page/performance). We do not see significant deference between (5,5) and (1,1) transitions (Fig. 4),
although the upper energy levels are very different (295 K versus 23 K). A higher transition would be better to avoid blending between the main components and the satellite components(note again that our calculations do not consider hyper-finestructures). As the NH3 emission from inside the snowline is
optically thick, it is hard to constrain the NH3 column density.
The observations of rarer isotopologoues, 15 NH3 , spectrally resolving the hyperfine components, may be helpful to constrain
the column density.
In our calculations, we consider a disk around a Herbig
Ae/Be star. The detection of the NH3 snowline in a T-Tauri
disk would be challenging due to the smaller size of the NH3snowlines (at ∼1 au) (Zhang et al. 2018).
3. Conclusion
In this article, we discussed the observability of the NH3snowline in a Class II protoplanetary disk. We performed theline radiation transfer of rotation-inversion transition of NH3 ,using a genetic disk model of Furuya et al. (in prep.). We
found that the NH3 snowline can be detected in a disk around3
a Herbig Ae/Be star with ngVLA with reasonable integration
time, while the detection is challenging for a disk around a TTauri star. The 14 NH3 emission lines are most likely opticallythick. Then the observations of rarer isotopologoues, 15 NH3 ,would be required to constrain the amount of NH3 inside its
snowline. This article focuses on NH3 , while Furuya et al.
disk model predicts that complex organic molecules (COMs),
such as CH3 OH and CH3 OCH3 , are accumulated in inner disk
regions inside water snowline. These COMs would also beinteresting targets for ngVLA.
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4[Vol. ,

Fig. 3. Left panel) Moment 0 map of the NH3 (5, 5) emission model obtained by line radiative transfer code LIME. Right panel) The NH3 (5, 5) emission
model convolved with the 0.1” Gaussian beam.

Fig. 4. Spectra of the NH3 (1, 1) emission (blue line) and the NH3 (5,5) emission (red line).