Investigating the impact of X-rays on the molecular abundances of inner envelopes
and disks around low-mass protostars with ngVLA
Shota Notsu1 , Nami Sakai1 , Hideko Nomura2
1

Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research,
2–1 Hirosawa, Wako, Saitama 351–0198, Japan
2
National Astronomical Observatory of Japan,
2–21–1 Osawa, Mitaka, Tokyo 181–8588, Japan
shota.notsu@riken.jp

Abstract
Water destruction by X-rays has been proposed to explain the low water gas fractional abundances (< 10−6 ) inthe inner warm envelopes and disks around several low-mass protostars, but the detailed chemistry, including thenature of alternative oxygen carriers, is not yet understood. Recently, we computed the chemical composition oflow-mass protostellar envelopes using a gas-grain chemical reaction network, with various X-ray luminosities ofthe central protostars. We aimed to understand the impact of X-rays on the composition of low-mass protostellar
envelopes, focusing specifically on water and related oxygen bearing species. According to our calculations, Xray induced chemistry strongly aﬀects the abundances of water and related species such as O, O2 . In addition,
the fractional abundances of HCO+ and CH3 OH, which have been considered to be tracers of the water snowline,
significantly increase/decrease within the water snowline, respectively, as the X-ray fluxes become larger. Moreover,the fractional abundance of NH3 , one of the dominant Nitrogen carriers, is also aﬀected by strong X-ray fields,
especially within its snowline. Future molecular line observations with ngVLA will clarify the impact of X-rays,
which enable us to make a more detailed picture of chemistry in the inner envelopes and disks around protostars.
Key words: Astrochemistry – ISM: molecules – Stars: formation – Stars: protostars – Protoplanetary disks –1.

Introduction

Water is a key molecule in star and planet forming regions.
Recent water line observations toward several low-mass Class
0 and I protostars suggested low water gas fractional abundances (< 10−6 ) in the inner warm envelopes and disks (r < 102au, e.g., Persson et al. 2016; Jensen et al. 2019; Harsono etal. 2020; van Dishoeck et al. 2021). Water destruction by Xrays has been proposed to influence the water abundances inthese inner regions (Stäuber et al. 2005; Stäuber et al. 2006),
but the nature of alternative oxygen carriers is not yet understood. In addition, the chemical model that Stäuber et al.
(2005) and Stäuber et al. (2006) adopted was limited. Most notably, they did not include detailed gas-grain interactions andgrain-surface chemistry.
In our recent study (Notsu et al. 2021), we revisited the chemistry of water and related molecules in low-mass Class 0 protostellar envelopes, under various X-ray field strengths. We computed the chemical composition of the protostellar envelopes
using a 1D gas-grain chemical reaction network (mostly basedon Walsh et al. (2015) and Bosman et al. (2018)) with adding
X-ray induced chemical processes. We aimed to understand theimpact of X-rays on the composition of low-mass protostellar
envelopes, focusing specifically on water and related oxygen
bearing species.
On the basis of our calculations (Notsu et al. 2021), the Xrays from central protostars have strong eﬀects on the watergas abundances. The behavior of the water abundances inthe gas phase changes inside and outside the water snowline(T gas ∼ 102 K, r ∼ 102 au). Outside the water snowline, thewater gas abundance increases with X-ray luminosities. Inside
the water snowline, water maintains a high gas abundance of∼ 10−4 for low X-ray luminosities, with water and CO being
the dominant oxygen carriers. For high X-ray luminosities, thewater gas abundances significantly decrease just inside the water snowline (down to < 10−7 ) and in the innermost regionswith T gas ≳ 250 K (down to ∼ 10−6 ). For these cases, the fractional abundances of gas-phase O2 and O reach ∼ 10−4 within
the water snowline, and they become the dominant oxygen carriers. However, observationally investigating the inner warm
O2 and O abundances for many objects is diﬃcult (Yıldız et al.
2013; Taquet et al. 2018). This is partly because O2 does nothave electric dipole-allowed transitions.

2. Molecular line observations with ngVLA
According to our model calculations (see Section 1 andNotsu et al. 2021), the HCO+ and CH3 OH gas abundances,which have been considered to be tracers of the water snowline(e.g., van ’t Hoﬀ et al. 2018a; van ’t Hoﬀ et al. 2018b; Leemker
et al. 2021), significantly increase/decrease within the water snowline, respectively, as the X-ray fluxes become larger.
Moreover, the fractional abundances of NH3 are also aﬀectedby strong X-ray fields, especially within its own snowlines.
Future high-spatial molecular line observations with ngVLA
will be expected to constrain the inner gas abundances of these2[Vol. ,IRAS 4A, H2O and NH3 gas abundance
10−310−428

H2O, LX=1032 erg/s
H2O, LX=1028 erg/s
NH3, LX=1032 erg/s
NH3, LX=10 erg/s

10−4−510+
IRAS 4A, HCO and CH3OH gas abundances+28

HCO+, LX=1032 erg/s
HCO , LX=1028 erg/s
CH3OH, LX=1032 erg/s
CH3OH, LX=10 erg/s

10−610−8−6nX/nHnX/nH

1010−7−810−1010−1210−1410−91010−1010−1610−11102

103

104

10−18r [au]102103104r [au]Fig. 1. [Left panels]: The green dashed dotted line and the blue dotted line show radial profiles of H2 O gas fractional abundances with respect to totalhydrogen densities (nH2 O /nH ) for values of central star X-ray luminosities LX =1028 and 1032 erg s−1 , respectively. The red dashed line and the blacksolid line show radial profiles of NH3 gas fractional abundances (nNH3 /nH ). [Right panel]: The green dashed dotted line and the blue dotted line show
radial profiles of HCO+ gas fractional abundances with respect to total hydrogen densities (nHCO+ /nH ). The red dashed line and the black solid line show
radial profiles of CH3 OH gas fractional abundances (nCH3 OH /nH ). In this Figure, these profiles are based on calculations of NGC 1333-IRAS 4A envelopemodels in Notsu et al. (2021).

molecules. By combining molecular line observations withALMA (such as H2 O lines) and ngVLA, a more detailed picture of the X-ray eﬀects on the oxygen chemistry will be obtained. In addition, they will also open a window into the independent nitrogen chemistry. Here, we briefly summarize theimpact of X-rays on the abundances of CH3 OH, HCO+ , andNH3 , and consider the possible observations with ngVLA.
2.1. NH3
Left panel of Figure 1 shows the radial profiles of the waterand NH3 gas fractional abundances for values of central star
X-ray luminosities LX =1028 and 1032 erg s−1 , respectively. As
we explained in Section 1 and Notsu et al. (2021), the behavior of the water gas abundances changes outside and inside its
snowline (T gas ∼ 102 K, r ∼ 102 au).
NH3 is a dominant nitrogen-bearing molecule in molecular
cloud and pre-stellar cores (especially in the later stage of
their evolutions), in which NH3 ice abundances are as highas 10−5 (Mumma & Charnley 2011; Furuya & Persson 2018).
Considering the diﬀerences in binding energies, the NH3 snowline position locates outside the water snowline (≳ 2 × 102 au).
Zhang et al. (2018) discussed the possibility to observe the NH3line emission (the 23GHz 1,1 and 2,2 lines) with ngVLA as aproxy of the water snowline in disks.
According to our calculations, the NH3 gas abundances increase with X-ray luminosities outside its snowline. Inside theNH3 snowline, as the X-ray fluxes increase, the NH3 gas abundances also decrease from 10−5 to ≲ 10−7 , as in the case ofH2 O. Thus, with strong X-ray field, the NH3 abundance is nolonger dominant nitrogen carrier. In addition, it cannot be usedas the tracer of the water snowline position, since gas-phase
NH3 abundances are similar inside and outside the NH3 snowline. We note that by the destruction of NH3 , free nitrogenatoms are released into the gas phase, which will drive additional nitrogen chemistry there.
The NH3 line emission (e.g., the 23GHz (1,1), (2,2), and (3,3)
lines) have been observed toward protostellar disks and envelopes using VLA (e.g., Choi et al. 2007; Choi et al. 2010).
However, the spatial resolutions of such VLA observations (∼1.0”) were not suﬃcient to resolve the inner structures aroundthe protostars nearby (∼ a few hundreds pc). In addition, especially in the inner envelopes and disks around low-mass protostars, sensitivities of such VLA observations were insuﬃcient to detect higher J and K transitions, which can potentiallytrace regions close to the protostars. Future ngVLA observations of these NH3 lines with much higher resolutions (≲ 0.3”
at around 23 GHz) and much higher sensitivities will resolve
the NH3 gas emission within its snowline towards many protostars, and can also constrain the impact of X-rays on NH3 gas
abundances. Moreover, such observations will be important totrace the chemical history of nitrogen bearing molecules, since
NH3 is also one of key molecules in considering formation ofnitrogen-bearing complex organic molecules.
2.2. HCO+ and CH3 OH
Right panel of Figure 1 shows the radial profiles of theHCO+ and CH3 OH fractional abundances in the gas phase for
LX =1028 and 1032 erg s−1 , respectively. The fractional abundances of HCO+ and CH3 OH, which have been considered tobe tracers of the water snowline, significantly increase/decrease
within the water snowline, respectively, as the X-ray fluxes become larger (see also Notsu et al. 2021).
The HCO+ and CH3 OH lines have been observed toward several protostars with ALMA (e.g., van ’t Hoﬀ et al. 2018a; van ’t
Hoﬀ et al. 2018b; Hsieh et al. 2019). The HCO+ (1-0) 89 GHzand H13 CO+ (1-0) 87 GHz lines, and many CH3 OH lines areNo. ]
within the ngVLA frequency coverage. Since the dust opacities
in the frequencies of ngVLA are smaller than those of ALMA,
these observations will be useful to trace the inner gas abundances more precisely. Series of CH3 OH lines in 25 GHz band
have been detected toward NGC1333 IRAS 4A1, where almost
all of the molecular line emissions were absent in the ALMA
bands (De Simone et al. 2020).
3.

Summary

According to our calculations (Notsu et al. 2021), X-ray induced chemistry strongly aﬀects the abundances of water andrelated species, such as O2 , HCO+ , CH3 OH, and NH3 , especially in the inner regions, and can explain the observed low
water abundances in the inner protostellar envelopes. Future
molecular line observations with ngVLA will constrain the inner gas abundances of such molecules, creating a more detailed
picture of the oxygen and nitrogen chemistry. We note that thematerial in the protostellar envelopes accretes into disks, thus
the molecular abundances in protostellar envelopes determine
the initial abundances of chemical evolution in disks, whereplanet formation occurs (see e.g., Öberg & Bergin 2021). It
will be important to discuss how the X-ray induced chemistry
at protostar phases aﬀect the initial abundances and chemicalevolution in planet forming disks.
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