# HCN/HNC chemistry in shocks: a study of L1157-B1 with ASAI

B. Lefloch1, G. Busquet1,2,3, S. Viti4,5, C. Vastel6, E. Mendoza7, M.
Benedettini8, C. Codella9,1, L. Podio 9, A. Schutzer1, P.R. Rivera-Ortiz1,
J.R.D. Lépine7, R. Bachiller10
1Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France
2Institut de Ciències de l’Espai (ICE, CSIC), Can Magrans, s/n, 08193
Cerdanyola del Vallès, Catalonia, Spain
3Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Catalonia,
Spain
4 Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, the
Netherlands
5Department of Physics and Astronomy, University College London, Gower Street,
London, WC1E 6BT, England
6IRAP, Université de Toulouse, CNRS, UPS, CNES, 31400 Toulouse, France
7Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de
São Paulo, São Paulo 05508-090, SP, Brazil
8INAF, Istituto di Astrofisica e Planetologia Spaziali, via Fosso del
Cavaliere 100, 00133 Roma, Italy
9INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125
Firenze, Italy
10IGN, Observatorio Astronómico Nacional, Calle Alfonso XII, 3 E-28004 Madrid,
Spain

(Accepted 2021 July 13. Received 2021 June 14; in original form 2021 April
14.)

###### Abstract

HCN and its isomer HNC play an important role in molecular cloud chemistry and
the formation of more complex molecules. We investigate here the impact of
protostellar shocks on the HCN and HNC abundances from high-sensitivity IRAM
30m observations of the prototypical shock region L1157-B1 and the envelope of
the associated Class 0 protostar, as a proxy for the pre-shock gas. The
isotopologues H12CN, HN12C, H13CN, HN13C, HC15N, H15NC, DCN and DNC were all
detected towards both regions. Abundances and excitation conditions were
obtained from radiative transfer analysis of molecular line emission under the
assumption of Local Thermodynamical Equilibrium. In the pre-shock gas, the
abundances of the HCN and HNC isotopologues are similar to those encountered
in dark clouds, with a HCN/HNC abundance ratio $\approx 1$ for all
isotopologues. A strong D-enrichment (D/H$\approx 0.06$) is measured in the
pre-shock gas. There is no evidence of 15N fractionation neither in the
quiescent nor in the shocked gas. At the passage of the shock, the HCN and HNC
abundances increase in the gas phase in different manners so that the HCN/HNC
relative abundance ratio increases by a factor 20. The gas-grain chemical and
shock model UCLCHEM allows us to reproduce the observed trends for a C-type
shock with pre-shock density $n$(H)= $10^{5}\hbox{\kern 1.99997ptcm${}^{-3}$}$
and shock velocity $V_{s}=40\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$. We conclude that the HCN/HNC variations across the
shock are mainly caused by the sputtering of the grain mantle material in
relation with the history of the grain ices.

###### keywords:

astrochemistry – methods: observational – ISM: jets and outflow – ISM:
molecules – ISM: abundances

††pubyear: 2021††pagerange: HCN/HNC chemistry in shocks: a study of L1157-B1
with ASAI–LABEL:lastpage

##  1 Introduction

Hydrogen cyanide (HCN) is one of the most simple interstellar molecules.
Thanks to its large dipole moment (2.99 Debye; Bhattacharya & Gordy 1960), its
rotation transitions are good probe of dense molecular gas in Galactic and
extragalactic environments. HCN and its isomer hydrogen isocyanide HNC are
thought to play an important role in the formation of more complex molecules,
like cyanopolyynes HC2n+1N, either in dark cloud cores (Suzuki et al., 1992)
or in more energetic regions, like protostellar shocks (Mendoza et al., 2018).
Since HCN and HNC have similar energy spectra and dipole moments, their
differences in spatial distribution is mainly related to the gas chemical
conditions. For this reason, they have often been used as probes of gas
chemical evolution in both dense cores and star-forming regions (Schilke et
al., 1992; Ungerechts et al., 1997; Daniel et al., 2013).

Many systematic studies of dark clouds and low-mass protostellar cores have
shown the HNC/HCN ratio to be close to unity (see e.g., Irvine & Schloerb,
1984; Hirota et al., 1998; Hily-Blant et al., 2010). No difference is observed
between the values measured in prestellar and protostellar cores, implying
that the evaporation of HCN and HNC from dust grains does not contribute
significantly to the observed emission in the cold envelope. On the contrary,
towards the high-mass star forming region OMC-1 in Orion, the HNC/HCN ratio
displays strong variations with especially low values $\approx 0.01$ towards
the hot core regions while it is of the order of 0.2 in adjacent ridge
positions. While the abundance of HCN is similar to that of dark cloud cores,
the HNC abundance is 2 orders of magnitude lower in the high-temperature gas
of the hot core (Schilke et al., 1992). A somewhat similar behaviour is
observed towards IRAS16293$-$2422 when looking at the high-excitation lines of
HCN and HNC (van Dishoeck et al., 1995). Recently, Hacar et al. (2020)
demonstrated the high sensitivity of the HCN/HNC $J$=1–0 line intensity ratio
to the gas kinetic temperature.

In his pioneering work on outflow shock chemistry Bachiller & Pérez-Gutiérrez
(1997) brought the first hints of HCN abundance enhancement in protostellar
shock region. At that time, only a low number of molecular transitions was
observed with the IRAM 30m telescope, hence preventing an accurate
determination of the excitation conditions and molecular abundances. From a
theoretical point of view, both the dependence of the HCN/HNC ratio to the
temperature and the sputtering of dust grains are two processes which could a
priori alter molecular gas abundances of HCN and HNC across a shock.

Many subsequent observational studies on the physical and chemical
characterization of L1157-B1 have been carried out both with (sub)millimeter
single-dish and interferometers, which led to a rather detailed, consistent
picture of the outflow shock while unveiling its chemical richness (see e.g.
Codella et al. (2010, 2017); Viti et al. (2011); Lefloch et al. (2012, 2016,
2017); Busquet et al. (2014); Mendoza et al. (2014, 2018); Podio et al. (2014,
2016, 2017); Gómez-Ruiz et al. (2015)). The spatial distribution of the HCN
emission in the L1157 southern outflow lobe was obtained at $\approx 5\arcsec$
scale for the first time by Benedettini et al. (2007) with the IRAM Plateau de
Bure interferometer. The emission of the rare 13C, D and 15N isotopologues was
investigated a few years later at a few arcsec resolution with the IRAM NOEMA
interferometer (Busquet et al., 2017; Benedettini et al., 2021). In
particular, the distribution of DCN was shown to result from a combination of
gas-phase chemistry that produces the widespread DCN emission, dominating
especially in the head of the bow-shock, and sputtering from grain mantles
toward the jet impact region.

These results incited us to revisit the emission of the HCN and HNC
isotopologues in L1157-B1 in a more comprehensive and accurate way than was
possible before. This study benefits from the unbiased and high-sensitivity
millimeter spectral line survey of the shock region carried out as part of the
IRAM 30m Large Program ASAI ("Astrochemical Surveys At IRAM", Lefloch et al.
(2018)). In order to better understand and constrain more precisely the impact
of the shock on the chemistry of HCN and HNC, we have also investigated the
properties of their isotopologue emission in the envelope of the Class 0
protostar L1157-mm, at about 1 arcmin away, which we took as a proxy of the
initial gas composition before the arrival of the shock. The origin of the
molecular emission was then derived from comparison of our observational
results with the predictions of the time-dependent gas-grain chemical and
shock model UCLCHEM (Holdship et al., 2017).

The paper has been organized as follows. In Sect. 2, we summarize the main
observational properties of the shock region L1157-B1. The observations are
described in Sect. 3. In Sect. 4, we present our results on the HCN spatial
distribution in the region and the gas properties (excitation temperature,
column density, abundance) obtained for the HCN and HNC isotopologues based on
a simple radiative transfer analysis of the line emission. We discuss in
Section 5 the behaviour of HCN and HNC across the shock and we analyse the
origin of the molecular emission from comparison with the results of our
modelling. Finally, we present our conclusions in Sect. 6.

##  2 The source

The GAIA mission has led to a revision of the distance to L1157 by several
groups Dzib et al. (2018) and Sharma2020, who found $(360\pm 32)\hbox{\kern
1.99997ptpc}$ and $(340\pm 21)\hbox{\kern 1.99997ptpc}$, respectively. In this
work, we will adopt the value of $(352\pm 19)\hbox{\kern 1.99997ptpc}$ derived
by Zucker et al. (2019), in agreement with Benedettini et al. (2021). Gueth,
Guilloteau, & Bachiller (1996, 1998) have studied at high-angular resolution
($\approx 3\arcsec$) the structure and dynamics of the southern lobe of the
outflow driven by the protostar L1157-mm from the emission of the CO $J$=1–0
and SiO $J$=2–1 lines observed with the IRAM Plateau de Bure interferometer.
These authors showed that the southern lobe of this molecular outflow consists
of two cavities, likely created by the propagation of large bow shocks due to
episodic events in a precessing, highly collimated, high-velocity jet. This
jet was detected by Tafalla et al. (2015) and imaged by Podio et al. (2016).
Based on higher-angular and high-sensitivity CO observations with NOEMA at
$0.3\arcsec$, Podio et al. (2016) refined the jet precession modelling by
Gueth, Guilloteau, & Bachiller (1996) and estimated an age of $\approx
1500\hbox{\kern 1.99997ptyr}$ for B1 (and $2500\hbox{\kern 1.99997ptyr}$ for
B2), adopting the new distance of $352\hbox{\kern 1.99997ptpc}$. Located at
the apex of the more recent cavity, the bright bow shock region B1 displays a
peculiar molecular complexity (e.g., Codella et al., 2010; Benedettini et al.,
2012), which makes it a benchmark for magnetized shock models (e.g., Gusdorf
et al., 2008a, b; Viti et al., 2011). Multi-transition analysis of the
emission of tracers such as CO (Lefloch et al., 2010, 2012), H2O (Busquet et
al., 2014) and CS (Gómez-Ruiz et al., 2015) has allowed to elucidate a
coherent scenario where the molecular emission appears to arise from four
physically distinct components, with specific excitation conditions:

  * •

Component $g_{1}$: Lefloch et al. (2012) and Benedettini et al. (2012)
evidenced a region of high excitation of $\approx 10\arcsec$ size, with a
kinetic temperature $T_{\rm kin}\approx 200\hbox{\kern 1.99997ptK}$ and gas
density $n(\hbox{H${}_{2}$})\simeq 10^{6}\hbox{\kern 1.99997ptcm${}^{-3}$}$.
This region is associated with the impact of the jet against the L1157-B1 bow
shock.

  * •

Component $g_{2}$ is tracing the outflow cavity associated with L1157-B1, for
which a kinetic temperature $T_{\rm kin}\approx 60\hbox{\kern 1.99997ptK}$ and
a gas density $n(\hbox{H${}_{2}$})\simeq$ (1–10)$\times 10^{5}\hbox{\kern
1.99997ptcm${}^{-3}$}$ were estimated.

  * •

Component $g_{3}$ is tracing the late outflow cavity associated with L1157-B2,
for which have been estimated $T_{\rm kin}\approx 20\hbox{\kern 1.99997ptK}$
and $n(\hbox{H${}_{2}$})\simeq 10^{5}\hbox{\kern 1.99997ptcm${}^{-3}$}$

  * •

A hot ($T_{\rm kin}\simeq 1000\hbox{\kern 1.99997ptK}$) and tenuous gas
component ($n(\hbox{H${}_{2}$})\simeq 10^{3}-10^{4}\hbox{\kern
1.99997ptcm${}^{-3}$}$) with a size of $2\arcsec$ -

##  3 Observations

###  3.1 ASAI

Observations of L1157-B1 and L1157-mm were carried out during several runs
between September 2012 and March 2015 as part of the Large Program ASAI
(Lefloch et al., 2018). The source nominal positions are $\alpha_{J2000}=$
20${}^{\text{h}}$ 39${}^{\text{m}}$ 10.${}^{\text{s}}$2 $\delta_{J2000}=$ +68∘
01′ 10′′ for L1157-B1 and $\alpha_{J2000}=$ 20${}^{\text{h}}$
39${}^{\text{m}}$ 06.${}^{\text{s}}$3 $\delta_{J2000}=$ +68∘ 02′ 15.8′′ for
L1157-mm. We made use of the broad-band EMIR receivers connected to Fast
Fourier Transform Spectrometers in the 200 kHz spectral resolution mode.

Detailed information on the observation and the data reduction procedures can
be found in Lefloch et al. (2018). The ASAI observations were carried out
using the "Wobbler Switching" (WSW) mode with a throw of $3\arcmin$.

###  3.2 On-The-Fly mapping

The ASAI dataset was complemented with a fully sampled map of the HCN $J$= 3–2
transition in the southern lobe of the L1157 outflow with the IRAM 30m
telescope in December 2013. The size of the map, $2\arcmin\times 2.5\arcmin$,
is large enough to encompass the whole southern lobe. Observations were
carried out with the receiver E230 and the FTS spectrometer in its 192 kHz
resolution mode, using the On-The-Fly mode. We chose a reference position
offset by $10\arcmin$ in right ascension from L1157-B1, and checked to be free
of emission in the HCN $J$=3–2 line. Atmospheric calibrations were performed
every 15 min and showed the weather to be stable, with 2.5 – 3.0 mm of
precipitable water vapour, and system temperatures in the range 450 – 550 K.
Pointing was monitored every hour and corrections were always found lower than
$3\arcsec$.

###  3.3 Data reduction

Figure 1: HCN $J$=3–2 velocity-integrated emission in the southern lobe of the
L1157 outflow. The emission is integrated over the velocity range indicated in
each panel (colour scale) to illustrate the separation of the emitting gas
into the B0 and B1 shocks and the B2 shock. The emission in the central panel
between $-2.7$ and $+4.3\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$
encompasses the emission from the ambient gas at $V_{lsr}$= $+2.7\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$. White contours represent the total
velocity-integrated emission of the HCN $J$=3–2 line. Contour levels range
from 3$\sigma$ to 15$\sigma$ in steps of of 3$\sigma$, where $\sigma=1.07$ K
km s-1 is the rms noise of the map. In each panel, green crosses mark the
nominal position of shocks B0, B1, and B2; a red cross marks the nominal
position of protostar L1157-mm (Gueth et al., 1997). The size (HPBW) of the
telescope main beam is draw by the white circle in the bottom right corner of
each panel.

The data reduction was performed using the GILDAS/CLASS
software111https://www.iram.fr/IRAMFR/GILDAS/. We have detected several
transitions of HCN, HNC, and their rare 13C-, D- , 15N- isotopologues, between
72 and 272 GHz, towards the shock region L1157-B1 and the protostar L1157-mm.
The line spectra are presented in Figs. 2–5. The resolution of the
spectrometer allowed us to resolve the hyperfine structure of the rotational
transitions of HCN, H13CN and DCN. The location of the hyperfine components
are indicated by red arrows in the spectra. The line intensities are expressed
in units of antenna temperature corrected for atmospheric attenuation and
rearward losses ($T_{A}^{\ast}$).

For subsequent analysis and radiative transfer modelling, fluxes were
expressed in main beam temperature units ($T_{\rm mb}$). The telescope and
receiver parameters (beam efficiency, $B_{\mathrm{eff}}$; forward efficiency,
$F_{\mathrm{eff}}$; Half Power beam Width, HPBW) were taken from the IRAM
webpage222http://publicwiki.iram.es/Iram30mEfficiencies/. The calibration
uncertainties are typically 10, 15, 20% at 3 mm, 2 mm, 1.3 mm, respectively.

The spectroscopic properties and the observational parameters of all the
detected transitions are summarized in Tables 1–2, for L1157-B1 and L1157-mm,
respectively. We extracted the observational properties of the line profiles
(velocity-integrated flux, Full Width at Half Maximum, emission peak velocity)
performing a simple gaussian fitting using CLASS whenever possible. The only
exceptions concern HCN and H13CN in L1157-B1, where the presence of several
components prevents from disentangling the emission of the hyperfine
satellites. In that case, the total flux (in $T_{A}^{\ast}\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$) was obtained from integrating over the
whole velocity range of emission.

Figure 2: Montage of line profiles of HCN and its rare isotopologues (H13CN,
DCN, HC15N), as observed with ASAI towards L1157-B1. Dashed-dotted lines
indicate the baseline and rest velocity, $V_{lsr}$= $+2.7\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$. All the spectra are smoothed to a
resolution of $1\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$. The best
fits obtained from the LTE analysis with CASSIS are displayed in red. The
velocity axis is associated to the reference frequency given in Table 2. Red
arrows mark the location of the hyperfine satellites on the velocity axis,
based on the frequencies given in the CASSIS database.

Figure 3: Montage of line profiles of HNC and its rare isotopologues (HN13C,
DNC, H15NC), as observed with ASAI towards L1157-B1. Dashed-dotted lines
indicate the baseline and rest velocity, $V_{lsr}$= $+2.7\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$. All the spectra are smoothed to a
resolution of $1\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$. The best
fits obtained from the LTE analysis with CASSIS are displayed in red. The
velocity axis is associated to the reference frequency given in Table 2. Red
arrows mark the location of the hyperfine satellites on the velocity axis,
based on the frequencies given in the CASSIS database.

Figure 4: Montage of line profiles of HCN and its rare isotopologues (H13CN,
DCN, HC15N), as observed with ASAI towards L1157-mm. Dashed-dotted lines
indicate the baseline and rest velocity, $V_{lsr}$= $+2.7\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$. The best fits obtained from the LTE
analysis with CASSIS are displayed in red. The velocity axis is associated to
the reference frequency given in Table 2. Red arrows mark the location of the
hyperfine satellites on the velocity axis, based on the frequencies given in
the CASSIS database.

Figure 5: Montage of line profiles of HNC and its rare isotopologues (HN13C, DNC, H15NC), as observed with ASAI towards L1157-mm. Dashed-dotted lines indicate the baseline and rest velocity, $V_{lsr}=+$2.7 km s-1. The best fit obtained from the LTE analysis with CASSIS are displayed in red. The velocity axis is associated to the reference frequency given in Table 2. Red arrows mark the location of the hyperfine satellites on the velocity axis, based on the frequencies given in the CASSIS database.  Table 1: Spectroscopic and observational parameters of the transitions of the HCN and HNC and their rare D, 13C and 15N isotopologues detected towards the protostellar shock L1157-B1. Note that the flux uncertainties reported here contain only the statistical rms noise. TAG is a 5-digit code which indicates the spectroscopic database used for CASSIS line identification and modelling.  Species | TAG | Transition | Frequency | $E_{u}$ | $A_{ul}$ | HPBW | $\eta_{mb}$ | $\int T_{A}^{*}dv$ | FWHM | $V_{lsr}$
---|---|---|---|---|---|---|---|---|---|---
|  |  | MHz | K |  $10^{-5}$ s-1 | ′′ |  | K km s-1 | km s-1 | km s-1
HCN | 27501 | 1 – 0 | 88631.602 | 4.3 | 2.4 | 27.8 | 0.85 | 31.2(1.5) | 4 .7(1) | -0.2(1.0)
|  | 3 – 2 | 265886.434 | 25.5 | 83.6 | 9.3 | 0.59 | 29.9(1.0) | 9.5(2) | -1.3(.7)
DCN | 28509 | 1 – 0 | 72414.905 | 3.5 | 1.33 | 34.0 | 0.86 | 0.12(.03) | 7.9(.5) | -0.2 (.1)
|  | 2 – 1 | 144828.001 | 10.4 | 12.7 | 17.0 | 0.79 | 0.19(.01) | 7.0(.3) | -0.3(.2)
|  | 3 – 2 | 217238.538 | 20.9 | 46.0 | 11.3 | 0.65 | 0.10(.01) | 6.5(.6) | -0.1(.3)
H13CN | 28501 | 1 – 0 | 86340.184 | 4.1 | 2.23 | 28.5 | 0.85 | 1.76(.03) | 4.8(1) | -0.4(.2)
|  | 2 – 1 | 172677.851 | 12.4 | 21.4 | 14.3 | 0.75 | 1.20(.01) | 7.2(4) | -0.3 (1)
|  | 3 – 2 | 259011.798 | 24.9 | 77.3 | 9.5 | 0.60 | 0.73(.02) | 6.2(1) | -0.34(.3)
HC15N | 28506 | 1 – 0 | 86054.961 | 4.1 | 2.2 | 28.6 | 0.86 | 0.35(.01) | 6.1(.1) | -1.0(.1)
|  | 2 – 1 | 172107.956 | 12.4 | 21.1 | 14.3 | 0.75 | 0.27(.01) | 5.6(.6) | -0.3(.3)
|  | 3 – 2 | 258157.100 | 24.8 | 76.5 | 9.5 | 0.60 | 0.13(.01) | 6.2(.7) | -0.1(.3)
HNC | 27502 | 1 – 0 | 90663.568 | 4.4 | 2.69 | 27.1 | 0.85 | 4.26(.02) | 4.5 (1) | 0.3(.1)
|  | 3 – 2 | 271981.142 | 26.1 | 93.4 | 9.0 | 0.58 | 2.36(.04) | 3.7(1) | 1.5(.3)
DNC | 28508 | 1 – 0 | 76305.701 | 3.7 | 1.60 | 32.2 | 0.86 | 0.010(.004) | 1.4(.6) | 0.3(.3)
|  | 2 – 1 | 152609.746 | 11.0 | 15.4 | 16.1 | 0.77 | 0.04(.02) | 1.7(.6) | 0.6(.4)
|  | 3 – 2 | 228910.481 | 22.0 | 55.7 | 10.7 | 0.64 | 0.012(.003) | 2.7(.7) | 2.1(.3)
HN13C | 28515 | 1 – 0 | 87090.825 | 4.2 | 2.38 | 28.3 | 0.85 | 0.093(.005) | 5.2(.3) | 0.8(1)
H15NC | 28006 | 1 – 0 | 88865.715 | 4.3 | 2.53 | 27.8 | 0.85 | 0.025(.008) | 3.4(.3) | 1.8(.1)
Table 2: Spectroscopic properties and observational parameters of the transitions of the HCN and HNC and their rare D, 13C and 15N isotopologues detected towards the protostar L1157-mm. Note that the flux uncertainties reported here contain only the statistical rms noise. TAG is a 5-digit code which indicates the spectroscopic database used for CASSIS line identification and modelling. Species | TAG | Transition | Frequency | $E_{u}$ | $A_{ul}$ | HPBW | $\eta_{mb}$ | $\int T_{A}^{*}dv$ | FWHM | $V_{lsr}$
---|---|---|---|---|---|---|---|---|---|---
|  |  | MHz | K |  $10^{-5}$ s-1 | ′′ |  | K km s-1 | km s-1 | km s-1
HCN | 27501 | 1 – 0 | 88631.602 | 4.3 | 2.4 | 27.8 | 0.85 | 3.40(.01) | 1.4(.1) | 2.2 (.1)
|  | 3 – 2 | 265886.434 | 25.5 | 83.6 | 9.3 | 0.59 | 1.86(.20) | 0.8(.1) | 2.2 (.1)
DCN | 28509 | 1 – 0 | 72414.905 | 3.5 | 1.3 | 34.0 | 0.86 | 0.49(.02) | 1.5(.2) | 2.4(.1)
|  | 2 – 1 | 144828.001 | 10.4 | 12.7 | 17.0 | 0.79 | 0.32(.01) | 1.0(.1) | 2.5(.1)
|  | 3 – 2 | 217238.538 | 20.9 | 46.0 | 11.3 | 0.65 | 0.10(.01) | 0.96(.04) | 2.8(.1)
H13CN | 28501 | 1 – 0 | 86340.184 | 4.1 | 2.2 | 28.5 | 0.85 | 0.27 (.02) | 1.3(.1) | 2.7(.1)
|  | 2 – 1 | 172677.851 | 12.4 | 21.4 | 14.3 | 0.75 | 0.10(.02) | 1.4(.3) | 2.4(.1)
|  | 3 – 2 | 259011.798 | 24.9 | 77.3 | 9.5 | 0.60 | 0.08(.01) | 1.3(.1) | 2.7(.1)
HC15N | 28506 | 1 – 0 | 86054.961 | 4.1 | 2.2 | 28.6 | 0.86 | 0.09(.02) | 1.5(.2) | 2.6(.1)
|  | 2 – 1 | 172107.956 | 12.4 | 21.1 | 14.3 | 0.75 | 0.06(.01) | 1.8(.4) | 2.0(.2)
|  | 3 – 2 | 258157.100 | 24.8 | 76.5 | 9.5 | 0.60 | 0.04(.01) | 1.5(.2) | 2.6(.1)
HNC | 27502 | 1 – 0 | 90663.568 | 4.4 | 2.7 | 27.1 | 0.85 | 3.1(.1) | 1.8(.1) | 2.4(.1)
|  | 3 – 2 | 271981.142 | 26.1 | 93.4 | 9.0 | 0.58 | 1.3(.1) | 0.99(.1) | 2.5(.1)
DNC | 28508 | 1 – 0 | 76305.701 | 3.7 | 1.60 | 32.2 | 0.86 | 0.90(.01) | 1.7(.1) | 2.7(.1)
|  | 2 – 1 | 152609.746 | 11.0 | 15.4 | 16.1 | 0.77 | 0.69(.01) | 1.0(.1) | 2.6(.1)
|  | 3 – 2 | 228910.489 | 22.0 | 55.7 | 10.7 | 0.64 | 0.26(.01) | 0.8(.1) | 2.7(.1)
HN13C | 28515 | 1 – 0 | 87090.825 | 4.2 | 2.4 | 28.3 | 0.85 | 0.29(.01) | 1.4(.1) | 2.4 (.1)
|  | 3 – 2 | 261263.310 | 25.1 | 82.8 | 9.4 | 0.60 | 0.079(.004) | 0.8(.1) | 2.3(.1)
H15NC | 28006 | 1 – 0 | 88865.715 | 4.3 | 2.5 | 27.8 | 0.85 | 0.063(.004) | 1.4(.1) | 2.7(.1)
|  | 3 – 2 | 266587.800 | 25.6 | 87.9 | 9.2 | 0.59 | 0.026(.005) | 1.5(.4) | 2.9(.2)

##  4 Results

###  4.1 Spatial distribution

In Fig. 1, we show in white contours the spatial distribution of the HCN
$J$=3–2 velocity-integrated emission between $-20$ and $+5\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$ over the southern outflow lobe. The
emission draws two extended features of different Parallatic Angles, which
coincide with the two CO outflow cavities identified by Gueth, Guilloteau, &
Bachiller (1996). The emission peaks at the nominal position of the outflow
shock position B1. Secondary emission peaks are found towards the shock
positions B0 and B2, at about $30\arcsec$ north and south of B1, respectively
(Bachiller et al., 2001). Hardly any emission is detected along the outflow
between B0 and the protostar (located North, red cross in Fig. 1). The
systemic velocity of L1157 is $+2.7\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ (Bachiller & Pérez-Gutiérrez, 1997). We show in color
scale in Fig. 1 the HCN $J$=3–2 emission integrated over the three velocity
intervals (in kms-1): [$-$10.2; $-$2.7], [$-2.7$; $+$4.3] and [$+$4.3;
$+$6.3], from left to right. We find that the low-velocity emission (middle
panel in Fig. 1) is associated with the bright shocked regions B0, B1, and B2,
as well as faint and extended emission tracing the whole outflow lobe. As
shown by Podio et al. (2016), the outflow is oriented almost in the plane of
the sky (inclination $i\sim 73\degr$). We propose that the HCN emission
between $-2.7$ and $+4.3\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$
could arise from low-velocity shocks associated with the cavity walls of the
outflow, which would explain the extended emission at blue- and red-shifted
velocities. This is consistent with the distribution of the HCN $J$=1–0
emission line as mapped by Benedettini et al. (2007) at $\approx 5\arcsec$
with the Plateau de Bure Interferometer. The high-velocity blueshifted gas
traces the young shocks B0 and B1 (left panel in Fig. 1), while the redshifted
emission is only detected toward the oldest B2 shock (right panel). These maps
indicate that the shocked gas in B0 and B1 emit at a velocity distinct from
that of B2. Interestingly, the brightest HCN $J$=3–2 emission coincides with
the apex of the outflow cavities, consistent with the high angular resolution
NOEMA images of H13CN and HC15N (Busquet et al., 2017; Benedettini et al.,
2021).

To summarize, HCN emission is detected throughout most of the southern outflow
lobe, from B0 to the tip of the outflow at position B2, and the brightest
emission regions are associated with outflow shocks. This is consistent with
the distribution of the HCN $J$=1–0 emission line as mapped by Benedettini et
al. (2007) at $\approx 5\arcsec$ with the Plateau de Bure Interferometer.

###  4.2 L1157-B1

All the transitions of the HCN isotopologues and most of the transitions of
the HNC isotopologues present in the spectral range of ASAI, between 72 and
272 GHz, were detected towards the shock region (see Table 1). Due to
instrumental (spectral bandwidth) and observational (rms noise) limitations,
only the ground state transitions of HN13C and H15NC were detected with a SNR
$>$ 3\. The profiles of the transitions display broad linewidths (up to
$\approx 10\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$). All the line
profiles peak at $V_{lsr}\approx 0\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ and trace blueshifted gas, in agreement with the other
molecular tracers reported in previous works (Lefloch et al., 2012; Codella et
al., 2012; Gómez-Ruiz et al., 2015).

The excellent rms of the data permits detection of emission up to $V_{\rm
LSR}\approx$ $-20\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$ in the
different transitions of HCN. Several transitions of DCN, H13CN and HC15N were
also detected towards L1157-B1 and their profiles display similar broad
linewidths, confirming the shock association (Fig. 2). The detection of bright
deuterated emission suggests that despite shock processing, the gas has
preserved part of the initial chemical conditions, an effect already pointed
out by Codella et al. (2012) and Fontani et al. (2014), who reported the
detection of singly deuterated isotopologues of H2CO, CH3OH, NH3. A small
systematic velocity shift (less than $0.5\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$) is measured between the emission peaks of the HCN and
HNC line profiles, suggesting they are tracing different regions inside the
shock.

We note that the HNC isotopologues (HN12C, HN13C, H15NC) display much weaker
lines intensities than the HCN isotopologues. Line intensities and fluxes of
the ground state transitions are weaker by one order of magnitude. There is
however one exception, which is the case of deuterated isotopologues DNC and
DCN, whose peak intensities are similar within a factor of 2.

Following the approach presented in Lefloch et al. (2012), we have fitted the
line profiles by an exponential law of the type $\exp(-V/V_{0})$, a procedure
which is obviously limited by the SNR of the data. In practice, we considered
the lower-$J$ transitions of HCN, H13CN and HNC. The results are displayed in
Fig. 6, where the spectra are displayed in a linear-logarithmic scale. The
spectral slope of the HCN (H13CN) line profiles considered can be fitted by an
exponential law of the type $\exp(-V/V_{0})$, with $V_{0}$= $3.8\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$ ($V_{0}$= $4.5\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$; Fig 6). The similar values obtained
for $V_{0}$ indicate that we are indeed probing the same gas in the different
transitions of the same molecular species. A small deviation with respect to
the fit is observed at $V<-10\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ in the HCN $J$=1–0 line profile. This is probably due to
the overlap between the contributions of the different hyperfine satellites.
Another possibility would be the presence of spatial gradients of emission
across the telescope beam (Benedettini et al., 2013).

The spectral slope $V_{0}$ of the HCN line profiles is very similar to that
found for the CO emission from the L1157-B1 outflow cavity (component $g_{2}$)
by (Lefloch et al., 2012), as indicated by the similarity of the fits and the
derived exponent values ($V_{0}=4.4\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$). This implies that in the velocity range between $-20$
and $-5\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$, both species are
probing the same gas component. This is consistent with the analysis of the
HCN $J$=3–2 spatial distribution obtained with the IRAM 30m telescope at
$9.3\arcsec$ (Sect. 4.1) and the HCN $J$=1–0 and H13CN $J$=2–1 with the IRAM
interferometer at a few arcsec resolution (Benedettini et al., 2007; Busquet
et al., 2017). As can be seen in Fig. 6, the HNC line profiles look very
different from those of HCN in a linear-logarithmic scale. The velocity range
of the emission is narrower and stops at $-10\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$, whereas the emission of HCN extends up to
$-30\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$. Quantitatively, the
HNC line profiles are well fitted by an exponential function $\exp(-V/V_{0})$
with $V_{0}$= $2.1\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$, i.e. a
value of $V_{0}$ half the value obtained for HCN. This supports the idea that
HNC and HCN are tracing different regions. We note that the $V_{0}$ value
obtained for HNC is actually similar to that found for the CO component
$g_{3}$ ($V_{0}=2.5\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$), which
Lefloch et al. (2012) showed to be associated with the L1157-B2 outflow
cavity.

Figure 6: Montage of emission line profiles observed towards L1157-B1
displayed in linear-logarithmic scale, from top to bottom: HNC $J=3-2$,
$J=1-0$, H13CN $J=2-1$, $J=1-0$, HCN $J=3-2$, $J=1-0$. In each panel, we have
superposed in red (blue) a fit to the spectral slope of the type
$T(v)\propto\exp(-V/V_{0})$ with $V_{0}\simeq$ 2.0 (4.0)kms-1, associated with
the component $g_{3}$ ($g_{2}$) of the outflow. The exponent $v_{0}$ is given
for each transitions in the corresponding panel.

In L1157-B1, the HCN line profiles follow an exponential signature
$\exp(-V/V_{0})$, with the same $V_{0}$ value for the rotational transitions
$J$= 1–0 and $J$= 3–2, so that the HCN line intensity ratio $J$= 3–2/$J$=1–0
is almost constant for velocities beyond $-6\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ (see Fig. 6). Hence, the HCN excitation conditions are
essentially independent of velocity, at least in this velocity range. Similar
results were previously reported for CO (Lefloch et al., 2012). The similarity
of the spectral slopes of HCN and CO between $-20$ and $-5\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$ implies that the HCN/CO line flux ratio
is constant, and since the excitation conditions of both species are
independent of velocity in this interval, it means that the relative abundance
ratio is also independent of velocity in this velocity range.

The HCN/H13CN total line flux ratio is $\approx 19$ and 42 for the $J$=1–0 and
$J$=3–2 transitions, respectively. Assuming similar excitation conditions for
both isotopologues and an elemental abundance ratio 12C/13C equal to 66 (see
below), close to the local ISM value (Milam et al., 2005), we then derive line
opacities $\tau_{10}\simeq 3$ and $\tau_{32}\simeq 0.8$ for the $J$=1–0 and
$J$=3–2, respectively, i.e. the gas is moderately optically thick. The
HNC/HN13C takes lower values, $\approx 11$ and $16$ for the $J$=1–0 and
$J$=3–2 transitions, respectively, which implies opacities of about 5 and 4
for the main isotopologue, respectively, still under the assumption of a
standard elemental abundance ratio.

###  4.3 L1157-mm

Emission from the HCN and HNC isotopologues was detected towards the
protostellar envelope of L1157-mm. All the detected molecular transitions
display narrow linewidths between 0.8 and $1.8\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$, as expected for cold and quiescent protostellar gas
(Figs. 4–5). This permits identification of the hyperfine satellites, whose
emission was hidden otherwise in the outflowing gas at position B1. The
signature of the protostellar outflow is detected in the HCN and HNC $J$=1–0
lines, as a broad, low intensity component with a typical linewidth of
$5\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$.

Overall, comparison of the HCN and HNC isotopologues emission shows that the
transitions from same rotational levels display similar similar intensities
for each pair of isomers, as can be seen in Figs. 4-5 (see also Table 4).
Since the corresponding line intensity ratios are only weakly dependent on the
excitation temperature, this suggests that both isomers and their rare
isotopologues should have similar abundances. Note that the situation differs
very much from L1157-B1, where much brighter emission is detected in the HCN
isotopologues.

The HCN/H13CN (HNC/HN13C) line flux ratio is 11 and 16 (12 and 23) for the
$J$=1–0 and 3–2 transitions, respectively. Adopting a standard elemental
abundance ratio 12C/13C = 66, we then derive HCN line optical depths
$\tau_{10}\simeq$ 5.5 and $\tau_{32}\simeq$ 2.3, for the $J$=1–0 and $J$=3–2
transitions, respectively. Similar results are obtained for HNC. Hence, both
the HCN and HNC emissions are optically thick. Therefore, we have used the 13C
isotopologues to determine the total gas column density of HCN and HNC.

To summarize, a simple comparison of the line intensities of the HCN and HNC
isotopologues measured towards L1157-mm suggests that both isomers are present
in the pre-shock phase with similar abundances. The pre-shock gas displays
hints of isotopic fractionation enrichment, in particular deuteration, which
is partially preserved through the shock. A strong differentiation is observed
in the shock between HCN and HNC with the emission of the former strongly
enhanced over the latter. Analysis of the spectral signature suggests that
both isomers are tracing different regions in the shock region.

###  4.4 Line excitation

The above analysis of the line profiles shows hints of chemical evolution in
the passage of the shock. In order to gain more insight on the shock impact on
nitrile chemistry, we have first determined the excitation conditions of the
different molecular species (excitation temperature, column density). The
detection of the rare isotopologues offers an opportunity to constrain
directly the evolution of isotopic fractionation through the passage of a
protostellar shock. In the case of L1157-mm, rather than a detailed physical
and chemical description of the structure of the envelope, our goal here is to
determine the relative abundances of these various species, which we take as
representative of the chemical composition of the pre-shock gas in the
L1157-B1 region.

In order to follow a systematic approach in the molecular abundance
determination, we decided to model the line profiles in the approximation of
Local Thermodynamical Equilibrium (LTE) using
CASSIS333http://cassis.irap.omp.eu/ (Vastel et al., 2015). In the case only
one transition was detected (HC15N, HN13C in L1157-B1), an excitation
temperature of $7\hbox{\kern 1.99997ptK}$ was adopted, in good agreement with
the other rare isotopologues. Line profiles towards L1157-B1 could usually be
reproduced with one single Gaussian component, of Full Width at Half Power
(FWHP) consistent with our determination of the line parameters (see Table 1).
In order to reproduce as accurately as possible the observed line profiles, we
made use of the CASSIS private database which takes into account the hyperfine
spectroscopic properties of HCN isotopologues. Towards L1157-mm, it was
necessary to introduce a second component of broad linewidth ($5\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$) and low intensity in order to
reproduce the outflow signature (see Sect. 4.3).

We have adopted a size of $60\arcsec$ for the envelope of L1157-mm, a value in
good agreement with the size of the envelope observed in N2H+ (Tobin et al.,
2013). In practice, this value allowed us to best simultaneously reproduce the
emission of the various transitions of a given molecular species under the
assumption of one single excitation temperature. We have adopted a total gas
column density of $2\times 10^{22}\hbox{\kern 1.99997ptcm${}^{-2}$}$, based on
the H2 column density maps of the region obtained in the Herschel Gould Belt
Survey (HGBS) of nearby star-forming molecular clouds (Di Francesco et al.,
2020). We note that this H2 column density is about a factor of 3 higher than
the determination obtained by Mendoza et al. (2018). The latter value
($6\times 10^{21}\hbox{\kern 1.99997ptcm${}^{-2}$}$) was derived from the 13CO
$J$=1–0 line in the ASAI spectrum of the protostar, adopting a canonical
abundance of $1.6\times 10^{-6}$. As a matter of fact, the low gas kinetic
temperature ($\sim 10\hbox{\kern 1.99997ptK}$) and the relatively high gas
density ($\sim$ a few $10^{4}\hbox{\kern 1.99997ptcm${}^{-3}$}$) of the
envelope are such that CO is most likely depleted at large scale in the
envelope by a factor of a few, as is commonly measured in cold cores (see
e.g., Bacmann et al., 2002). Both column density values can be reconciled if
CO is moderately depleted by a factor of about 3–4 over the envelope. We have
adopted a linewidth of $\sim 1\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ for the line profile modelling in L1157-mm, in agreement
with the results of our gaussian fits to the line spectra (see Figs. 4–5 and
Table 2).

In the case of L1157-B1, previous observational work showed that the molecular
gas emission arises from a region with a typical gaussian size (FWHM) in the
range $20\arcsec$ – $25\arcsec$ (Gueth, Guilloteau, & Bachiller, 1996; Lefloch
et al., 2012; Benedettini et al., 2013). This is consistent with the
distribution of the HCN $J$=3–2 emission (Fig. 1), which appears extended
between the protostar and the B1 shock position. The high-$J$ transitions of
the HCN and HNC isotopologues lie in a frequency range for which the main-beam
of the telescope is comparable to or smaller than the size of the shock
region, as estimated from interferometric observations (see Tables 1–2). For
these transitions, the emission can be considered as extended and the main-
beam temperature becomes a satisfying approximation to the molecular line
brightness temperature. The case of the ground state transitions of the HCN
and HNC isotopologues is more complicated for two reasons: first, the
telescope beamwidth (HPBW) is comparable to the molecular emission size;
second, the typical physical conditions ($n(\hbox{H${}_{2}$})\sim
10^{5}\hbox{\kern 1.99997ptcm${}^{-3}$}$; $T_{kin}$= $60\hbox{\kern
1.99997ptK}$–$90\hbox{\kern 1.99997ptK}$) are such than the rotational
transitions present in the millimeter bands are subthermally excited. The
$J$=1–0 excitation temperature is higher than that of the $J$=3–2 line, as can
be checked by a simple calculation using a radiative transfer code in the
Large Velocity Gradient approximation, like Madex (Cernicharo et al. 2012). In
the LTE approximation, the $T_{ex}$ of the $J$=1–0 line is underestimated by
the fitting procedure, and the approximation is no longer very satisfying.
However, the approximation of extended, or main-beam averaged, emission
compensates for the previous effect, which mainly affects the ground state
transitions, as explained above. We found that this approximation yields much
better quality LTE fits. The source-averaged column density is then obtained
by correcting the fitted (main-beam averaged) column density for the main-beam
filling factor.

The results of our LTE analysis (source-averaged column density $N$,
excitation temperature $T_{ex}$) and the corresponding molecular abundances
are summarised in Table 3. Due to the uncertainties in the derivation of the
HCN column densities from the 12C isotopologue, we have reported for HCN and
HNC the values obtained from the rare 13C isotopologues, adopting an elemental
abundance ratio 12C/13C = 66 (see below Sect. 5.1). They are indicated with a
(*) in Table 3.

The best fits to the individual line profiles obtained with CASSIS are
superimposed in red on the spectra in Figs. 2–5. The error bars on
$T_{\mathrm{rot}}$ and $N$ were estimated from LTE fits to the line profiles
taking into account the absolute flux calibration uncertainties (see Sect.
3.3) and the statistical rms noise. Our column density determinations for HCN
and HNC towards L1157-mm and L1157-B1 are in good agreement with those of
Bachiller & Pérez-Gutiérrez (1997). Recently, Benedettini et al. (2021)
observed the H13CN and HC15N $J$=1–0 line emission at $4\arcsec$ resolution
toward L1157-B1 using NOEMA, as part of the Large Program SOLIS. Their
radiative transfer modelling of both lines yielded $N$(H13CN) $\sim 7\times
10^{12}\hbox{\kern 1.99997ptcm${}^{-2}$}$ and $N$(HC15N) $\sim 2\times
10^{12}\hbox{\kern 1.99997ptcm${}^{-2}$}$. Our observational determinations
are in satisfying agreement with these values (see Table 3).

##  5 Discussion

In this section, we discuss the properties of the HCN and HNC isotopologues in
the pre-shock gas, based on the L1157-mm envelope properties, and their
evolution across the shock in L1157-B1. The molecular abundances and line
ratios of interest derived towards L1157-B1 and L1157-mm are presented in
Table 4. Using the time dependent gas-grain chemical and parametrised shock
model UCLCHEM (Holdship et al., 2017) we simulate the pre-shock abundances for
the gas and the solid phase, before the arrival of the shock, as well as
during and after the shock has passed. We discuss the modelling findings with
an emphasis of the behaviour of HCN and HNC.

Table 3: Excitation conditions (source-averaged column density $N$, rotational temperature $T_{\rm rot}$, abundance $X$) of the HCN and HNC isotopologues detected towards L1157-B1 and L1157-mm. Emission sizes of $60\arcsec$ and $20\arcsec$ were adopted for L1157-mm and L1157-B1, respectively. A total H2 column density of $2\times 10^{22}\hbox{\kern 1.99997ptcm${}^{-2}$}$ and $2\times 10^{21}\hbox{\kern 1.99997ptcm${}^{-2}$}$ was adopted for L1157-mm and L1157-B1, respectively (Lefloch et al., 2012; Di Francesco et al., 2020), in order to compute the molecular abundances. We follow the convention $a(b)=a\times 10^{b}$. The HCN and HNC parameters are marked with a * to indicate that they were obtained from the 13C isotopologue, adopting an abundance ratio 12C/13C= 66 (see Sect. 5.1).  | L1157-mm |  | L1157-B1
---|---|---|---
Species | $T_{\mathrm{rot}}$ | $N$ | $X$ |  | $T_{\mathrm{rot}}$ | $N$ | $X$
| (K) | (cm-2) |  |  | (K) | (cm-2) |
Main Body |  |  |  |  |  |  |
HCN(*) | 6.7${}^{+0.7}_{-0.5}$ | 4.6${}^{+0.6}_{-0.6}$(13) | 2.3${}^{+0.3}_{-0.3}$(-9) |  | 7.2${}^{+0.6}_{-1.0}$ | 6.6${}^{+1.1}_{-0.6}$(14) | 3.3${}^{+0.6}_{-0.3}$(-7)
H13CN | 6.7${}^{+0.7}_{-0.5}$ | 7.0${}^{+1.0}_{-1.0}$(11) | 3.5${}^{+0.5}_{-0.5}$(-11) |  | 7.2${}^{+0.6}_{-1.0}$ | 1.0${}^{+0.4}_{-0.1}$(13) | 5.0${}^{+2.0}_{-1.0}$(-9)
DCN | 4.9${}^{+0.2}_{-0.2}$ | 2.8${}^{+0.5}_{-0.3}$(12) | 1.4${}^{+0.3}_{-0.2}$(-10) |  | 6.5${}^{+0.5}_{-0.5}$ | 2.1${}^{+0.5}_{-0.5}$(12) | 1.1${}^{+0.2}_{-0.3}$(-9)
HC15N | 7.7${}^{+0.5}_{-0.4}$ | 1.2${}^{+0.3}_{-0.2}$(11) | 0.7${}^{+0.1}_{-0.1}$(-11) |  | 6.8${}^{+0.4}_{-0.5}$ | 2.0${}^{+0.5}_{-0.3}$(12) | 1.0${}^{+0.3}_{-0.2}$(-9)
HNC(*) | 6.8${}^{+0.3}_{-0.3}$ | 4.2${}^{+0.8}_{-0.5}$(13) | 2.1${}^{+0.4}_{-0.3}$(-9) |  | 7.0${}^{+1.3}_{-1.5}$ | 3.5${}^{+0.3}_{-0.6}$(13) | 1.7${}^{+0.2}_{-0.2}$(-8)
HN13C | 6.8${}^{+0.3}_{-0.3}$ | 6.3${}^{+1.2}_{-0.7}$(11) | 3.2${}^{+0.6}_{-0.4}$(-11) |  | 7.0${}^{+3.0}_{-1.7}$ | 5.3${}^{+0.6}_{-0.6}$(11) | 2.6${}^{+0.3}_{-0.2}$(-10)
DNC | 5.7${}^{+0.3}_{-0.2}$ | 2.5${}^{+0.5}_{-0.3}$(12) | 1.3${}^{+0.2}_{-0.2}$(-10) |  | 6.0${}^{+2.0}_{-2.0}$ | 3.2${}^{+1.8}_{-1.4}$(11) | 1.6${}^{+0.9}_{-0.7}$(-10)
H15NC | 7.7${}^{+0.5}_{-0.6}$ | 1.3${}^{+0.3}_{-0.4}$(11) | 0.7${}^{+0.1}_{-0.3}$(-11) |  | 7.0${}^{+3.0}_{-1.5}$ | 1.8${}^{+0.7}_{-0.6}$(11) | 8.5${}^{+3.5}_{-2.5}$(-11)
Outflow component |  |  |  |  |  |  |
HCN | 5.0 | 4.0(12) | 2.4(-8) |  | - | - | -
HNC | 5.0 | 1.5(12) | 0.9(-8) |  | - | - | -
Table 4: Isomer and isotopic abundance ratios of HCN and HNC in L1157-mm and L1157-B1. For each source, the ratios are computed adopting a canonical elemental 12C/13C abundance ratio of 66 (see also Sect. 5.1).  | L1157-mm |  | L1157-B1
---|---|---|---
HCN/H | 1.2${}^{+0.2}_{-0.2}$(-9) |  | 1.7${}^{+0.4}_{-0.2}$(-7)
HCN/H13CN | 66 |  | 66
HCN/HC15N | $383\pm 145$ |  | $330\pm 110$
HCN/DCN | $16\pm 5$ |  | $314\pm 110$
H13CN/HC15N | $5.8\pm 2.2$ |  | $5.0\pm 2.5$
HNC/HN13C | 66 |  | 66
HNC/H15NC | $323\pm 160$ |  | $194\pm 100$
HNC/DNC | $17\pm 7$ |  | $109\pm 70$
HN13C/H15NC | $4.8\pm 2.4$ |  | $3.0\pm 1.5$
HCN/HNC | $1.1\pm 0.4$ |  | $19\pm 6$
H13CN/HN13C | $1.1\pm 0.4$ |  | $19\pm 8$
HC15N/H15NC | $0.9\pm 0.5$ |  | $11\pm 6$
DCN/DNC | $1.1\pm 0.4$ |  | $6\pm 5$

###  5.1 12C/13C elemental abundance ratio

We have used the detected transitions of HC3N and its rare 13C isotopologues
observed towards L1157-B1 to estimate the elemental abundance ratio 12C/13C in
the L1157 star-forming region. The transitions of the rare isotopologues
$J$=10–9, $J$=11–10, $J$=12–11 of H13CCCN, HC13CCN and HCC13CN were presented
in Mendoza et al. (2018). The transitions of same quantum numbers of the
different 13C isotopologues are very close in frequency, separated by a few
GHz at most. These different transitions were observed simultaneously in the
ASAI survey, with the advantage of minimizing the calibration uncertainty,
which is then dominated by the rms noise of the spectrum. Mendoza et al.
(2018) showed that the emission of all the HC3N isotopologues detected in the
millimeter range is optically thin. Hence, the line flux ratio of the 12C to
the sum of the 13C isotopologues is equal to three times the elemental
abundance ratio 12C/13C. For the $J$=10–9 and $J$=12–11 transitions, we found
12C/13C= $66\pm 6$. For the $J$=11-10, we discarded the H13CCCN line whose
intensity is almost twice as bright as the lines of HC13CCN and HCC13CN, a
dissymmetry which is observed only in this transition and not in the $J$=10–9
and $J$=12–11. It then comes 12C/13C= $70\pm 10$, in agreement with the
determinations from the $J$=10–9 and $J$=12–11. This direct observational
determination of the 12C/13C ratio in the L1157 star forming region is
actually very close to the elemental abundance ratio in the local ISM, equal
to 68, (Lucas & Liszt, 1998; Milam et al., 2005) and the value assumed in our
previous works (see Sect. 2).

Both the HCN/H13CN and HNC/HN13C column density ratios display values well
below the canonical 12C/13C elemental abundance ratio in the ISM, equal to 68
(Lucas & Liszt, 1998; Milam et al., 2005). In particular, a low 12C/13C ratio
of $\approx 11\pm 4$ is measured towards towards the cold protostellar core
L1157-mm. We first note that models of 13C isotopic fractionation in cold
cores lead to an opposite effect, i.e. a H12CN/H13CN (HN12C/HN13C) ratio
higher than the 12C/13C elemental abundance ratio (Langer et al., 1984; Roueff
et al., 2015). A similar effect was reported by citetDaniel2013 in their
detailed study of nitrogen hydrides and nitriles in the Class 0 protostellar
core Barnard 1. The authors concluded that the apparently low H12CN/H13CN
ratio was most likely a bias caused by the simple hypothesis made in the
radiative transfer modelling of the line excitation and the geometry of the
astrophysical source. In particular, the excitation of the HCN and HNC ground-
state transitions also depends on the extended, low-density gas surrounding
the dense core, so that a detailed multi-dimensional radiative transfer model
of the source taking into account the full density and velocity fields is
needed in order to properly reproduce the line profiles .

From what precedes, we conclude that there is no convincing evidence for
12C/13C fractionation of HCN/HNC in the star-forming region. Because of the
high optical depth of the HCN and HNC lines, which cast some uncertainties on
the determation of the HCN and HNC total column densities, these were obtained
from the column densities of their optically thin 13C isotopologues and taking
into account our estimate of the elemental abundance 12C/13C= $66\pm 6$.

###  5.2 Isomers

In the pre-shock gas, we find similar molecular abundances of HCN and HNC,
close to $2\times 10^{-9}$, and a a relative abundance ratio HCN/HNC=
H13CN/HN13C= $1.1\pm 0.4$. These values were derived from a direct to the
L1157-mm line profiles with CASSIS, taking into account the line optical
depths. Similar results were obtained when considering the rare isotopologues
ratios DCN/DNC (= $1.1\pm 0.4$) and HC15N/H15NC (= $0.9\pm 0.5$). The
consistency between these values determined independently, makes us confident
about the reliability of our method. The molecular abundances of HCN and HNC
are similar to those reported in previous surveys of dark cloud cores and low-
mass star forming cores (e.g., Hirota et al., 1998; Hily-Blant et al., 2010;
Daniel et al., 2013), suggesting that the envelope of L1157-mm has kept memory
of the prestellar phase. Hirota et al. (1998) pointed out that since no
difference is observed between the values measured in prestellar and
protostellar cores, the evaporation of HCN and HNC from dust grains does not
contribute significantly to the observed emission in the cold gas.

Comparison of the molecular abundances of L1157-mm (pre-shock gas) and the
shocked region L1157-B1 (see Table 3) shows that the HCN and HNC isomers
behave differently across the shock. First, we observe an increase in the
abundance of all the HCN isotopologues. The rare H13CN and HC15N (and HCN)
abundances increase by a similar factor of 140 while the DCN abundance
increases much less, by a factor $\approx 8$. Unlike HCN, the abundances of
all the HNC isotopologues vary by a much smaller factor of a few ($\approx$
8–12). The abundance DNC remains almost unchanged. Overall, the abundances of
the HNC isotopologues seem barely affected by the passage of the shock. From a
ratio HCN/HNC with an initial value of 1 in the quiescent gas, it is now
$19\pm 6$ in the post-shocked gas. The isomer ratio displays values in the
range 11–19 which are consistent, within the uncertainties, for H13CN/HN13C
and HC15N/H15NC. The deuterated isomers seem to behave differently with a much
lower increase of the DCN/DNC ratio.

In the protostellar outflow near L1157-mm, the high sensitivity of the ASAI
data has permitted detection of the emission of the ground state transition of
HCN and HNC. This emission can be modelled as a broad component of $\approx
5\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$ and low excitation (Table
3). Interestingly, the HCN/HNC abundance (column density) ratio is close to 3,
suggesting that the relative abundance of HCN with respect to HNC is enhanced
in the formation of the outflow.

###  5.3 Isotopic fractionation

####  5.3.1 Deuterium

It is well established that strong Deuterium enrichment takes place in
molecular material of cold, dark and prestellar cores (Caselli & Ceccarelli,
2012). The first evidence of molecular deuteration in L1157-mm were brought by
Bachiller & Pérez-Gutiérrez (1997), with the detection of the deuterated forms
of HCN and HCO+, and a molecular D/H ratio of 0.018 in the cold envelope. Our
own determinations, which are based on the modelling of a larger number of
rotational transitions, yield a somewhat higher D/H ratio $0.06\pm 0.02$ for
both HCN and HNC, in rough agreement with Bachiller & Pérez-Gutiérrez (1997).

At the passage of the shock, the DCN abundance increases by a factor 7 whereas
DNC remains unchanged. Overall, the magnitude of these variations remain small
in front of those affecting the main isotopologues and indicate that, at first
order, the abundances of these deuterated species are only moderately affected
by the passage of the shock, with a different behaviour for DCN and DNC. As
for L1157-B1, previous estimates of the DCN/HCN ratio were obtained by Codella
et al. (2012), based on a reduced number of transitions of H13CN and DCN. Our
present determination of $(3.1\pm 1.0)\times 10^{-3}$ is consistent with their
previous estimate D/H = (0.5–3)$\times 10^{-3}$. More recently, Busquet et al.
(2017) reported similar deuterium fractionation ratios ($\approx 4.0\times
10^{-3}$) in the small-scale structures detected in the bow-shock, with no
significant spatial variation. Our unbiased analysis of the deuterated isomers
confirms that the molecular D/H ratio decreases across the shock and it allows
us to quantify the magnitude of the effect: a factor 26 and 8 for HCN and HNC,
respectively.

The behaviour of DCN/HCN in the B1 shock position has already been studied in
detail by Busquet et al. (2017) based on high-angular resolution observations
of H13CN $J$=2–1 and DCN $J$=2–1 combined with the UCLCHEM code and adopting
the deuterated network of Esplugues et al. (2013). The results from the model
show that the DCN/HCN abundance ratio varies with the passage of the shock.
The observed DCN/HCN ratio is well reproduced by the model in the post-shock
gas material, around $t\sim 1000$ yr, when the gas has cooled down to $\sim
80\hbox{\kern 1.99997ptK}$. The morphology of DCN together with the shock
model suggests that gas-phase chemistry is the dominant mechanism producing
the widespread DCN emission, which dominates in the head of the bow-shock.

####  5.3.2 Nitrogen

We do not find any evidence of 15N isotopic fractionation. Towards L1157-mm,
we obtain 13C/15N ratios of $5.8\pm 2.2$ and $4.8\pm 2.4$ for HCN and HNC,
respectively. Both values agree within the uncertainties, and they are also
consistent with the canonical elemental abundance ratio of 6.5 in the local
ISM (Milam et al., 2005; Marty et al., 2011). This translates into elemental
abundance ratios HC14N/HC15N of $380\pm 145$ and H14NC/H15NC $\approx 320\pm
160$ towards L1157-mm. Both values are consistent with the solar value in the
local ISM (440; Marty et al. 2011). Towards L1157-B1, the H13CN/HC15N is
almost unchanged ($5.0\pm 2.5$) whereas the HN13C/H15NC takes a lower value
($3.0\pm 1.5$), but still in rough agreement with the canonical value.

At the passage of the shock, the rare 15N isotopologues display the same
behaviour as HCN and HNC, as can be seen in Table 3. The H13CN and HC15N
abundances in L1157-B1 increase by a factor of 140 with respect to the values
measured towards L1157-mm. By comparison, HN13C and H15NC increase by a factor
($\sim 8$). Overall, these variations remain very modest. Therefore, our
analysis brings direct observational evidence that the 14N/15N elemental
abundance ratio is not affected by the passage of the shock.

Recently, Benedettini et al. (2021) have investigated the 14N/15N
fractionation of HCN in the two shocked clumps, B1 and B0, of the L1157
outflow using the NOEMA interferometer at $\sim 3\arcsec$ resolution, and
obtained 14N/15N$=340\pm 70$, in good agreement with our determination (see
Table 4). Based on the chemical shock modelling, Benedettini et al. (2021)
conclude that the rich gas chemistry activated by the passage of the shock
does not affect the 14N/15N ratio with respect to the local ISM value. This
conclusion is observationally confirmed by our results.

Figure 7:  Time-dependent fractional abundances (with respect to the number of
hydrogen nuclei) of HCN and HNC across a protostellar shock propagating at
$V=20\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$ (top) and
$V=40\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$ (bottom),
respectively. (left)"Long" pre-shock phase. (right) "Short" pre-shock phase.
The abundances of HCN (black) and HNC (magenta) in the gas phase and on the
dust grains are indicated by the solid and the dashed curve, respectively. The
temperature profile across the shock is drawn with the red dash-dot curve. The
fractional abundances measured towards L1157-B1 and their uncertainties are
indicated by the blue rectangles. The vertical dotted line at
$t=1500\hbox{\kern 1.99997ptyr}$ indicates the estimated age of the shock by
Podio et al. (2016).

###  5.4 Chemical modelling

As discussed in the previous section, a simple qualitative analysis of the
spectral line signature shows evidence for the shock impact on Nitrile gas
composition. The HCN isotopologue abundances have been deeply affected in the
shock, increasing by more than one order of magnitude, whereas the HNC
isotopologues seem to have barely noticed its presence.

Our goal is to identify the chemical processes responsible for the HCN and HNC
gas phase abundances in L1157-B1, before and during the passage of the shock.
The different studies of H2O, NH3, CH3OH, H2CO, HC3N, H2S, NO, PN, PO by our
group (Viti et al., 2011; Lefloch et al., 2016; Holdship et al., 2017;
Benedettini et al., 2013; Mendoza et al., 2018; Benedettini et al., 2013;
Busquet et al., 2017; Codella et al., 2018; Benedettini et al., 2021) have led
to a coherent picture of L1157-B1, in which the detected molecular emission is
accounted for by a C-type shock propagating into a pre-shock medium of density
$n(\hbox{H${}_{2}$})\simeq 5\times 10^{4}\hbox{\kern 1.99997ptcm${}^{-3}$}$
and shock velocity $V_{s}$ in the range $20-40\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$. We have adopted the same methodological approach as in
our previous studies, and have used the public time dependent gas-grain
chemical and parametrised shock model UCLCHEM code (Holdship et al., 2017) to
follow the evolution of the gas phase composition across a protostellar shock,
adopting a cosmic ray ionization rate of $\zeta=1.3\times 10^{-16}\hbox{\kern
1.99997pts${}^{-1}$}$, in agreement with Podio et al. (2014). We refer the
reader to the release paper of UCLCHEM for a detailed description of this
public code (Holdship et al., 2017). Briefly, the code is run in two phases,
where Phase I forms a dense core out of a diffuse, essentially atomic medium.
An initial density of $100\hbox{\kern 1.99997ptcm${}^{-3}$}$ is adopted here,
for the diffuse medium. During this phase, gas-phase chemistry, freezing on to
dust particles and subsequent surface processing, occurs.

We have considered two cases, depending on the duration on the pre-shock
phase. In the first case ("short” duration), one considers the extreme case
where the jet impacts the cloud as soon as the latter is formed; in the second
case (long” duration), one begins with a situation where 5 million years pass
after the cloud is formed and before the shock arrives. The density at the end
of Phase I is a free parameter, called from now on the pre-shock density. The
second phase computes the time-dependent chemical evolution of the gas and
dust during the passage of a magnetized (C-type) shock whose structure is at
steady-state.

Figure 7 presents the variations of the HCN and HNC molecular abundances
relative to the number of H nuclei as a function of time through the shock, as
predicted by our model for the shock velocities $V_{s}=20\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$ (top) and $V_{s}=40\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$ (bottom) and for the durations of the
pre-shock phase, long (left) and short (right). The HCN (HNC) abundance in the
gas and the solid phase are drawn in solid and dotted black (magenta),
respectively. In this figure, the shock is propagating to the left. Since the
shock is propagating at steady state, the time variations of the different
physical or chemical parameters (temperature, density, abundances) can simply
be converted into space variations, via the relation $z$= $V_{s}\times t$,
hence giving access to the shock structure itself. The observational
determinations of the gas phase abundances of HCN and HNC and their
uncertainties are indicated by the dark blue rectangles.

####  5.4.1 Pre-shock phase

Our modelling of the pre-shock phase is consistent with the physical and
chemical conditions in the envelope of L1157-mm, i.e., cold gas at
$10\hbox{\kern 1.99997ptK}$ of moderate density
($n(\hbox{H${}_{2}$})=10^{5}\hbox{\kern 1.99997ptcm${}^{-3}$}$), for which we
determined an abundance of $1.2\times 10^{-9}$ and $1.1\times 10^{-9}$ for HCN
and HNC, respectively, relative to H. The key observational constraint here is
that the abundances of HCN and HNC in the gas phase are similar. As can be
seen in Fig. 7, our chemical modelling succeeds in reproducing similar gas
phase abundances for HCN and HNC in the pre-shock phase. The HCN/HNC ratio
barely varies between the long duration ($\sim 1$) and the short duration
scenario ($\sim 2$). Both scenarios also predict correct gas phase abundances
relative to H, of the order of $10^{-9}$ (Table 3). The short duration
scenario predicts gas phase abundances in slightly better agreement with our
observational determinations, within the observational uncertainties. The
"long" duration scenario predicts abundances higher by a factor of 3. The gas
phase abundances of the rare isotopologues are also in good agreement with our
observational determinations and with the absence of 13C and 15N
fractionation, as discussed above in Sect. 5.3.

In the gas phase, HNC and HCN are efficiently produced by the dissociative
recombination (DR) of HCNH+ with a branching ratio $\approx 1$ (Herbst et al.,
2000; Mendes et al., 2012):

$\rm HCNH^{+}+e^{-}\rightarrow HNC+H,HNC+H.$

Several other routes involving neutral-neutral reactions can substantially
contribute to the formation of HCN, especially at higher densities (and hence
at later times). The dominant route of "destruction" for both species is
freeze-out on the dust grains.

We show in Fig. 7 (in magenta dashed) the abundances of HCN and HNC on the
dust grains (respectively s-HCN and s-HNC). The main difference between the
short and the long duration Phase 1 models lies in fact in the solid HCN/HNC
ratio, whereby the latter model predicts a ratio of $\sim$ 10 while the former
a ratio of more than 100. This is probably due to the very efficient formation
route for HNC in the gas phase, which then readily freezes out. On the surface
of grains, HCN can also continue to form via the efficient hydrogenation of
CN, while HNC is not produced nor efficiently destroyed on the grains. This
explains the larger ratio of solid HCN/HNC compared to the gas phase HCN/HNC.
Therefore, the duration of the pre-shock phase has a marked impact on the HCN
and HNC abundance in the solid phase.

####  5.4.2 Shock impact

In the models with $V_{s}$= $20\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ (top right and left panels in Fig. 7), the shock impact
on the HCN and HNC abundances is essentially the same. In the magnetic
precursor region, where ions and neutrals are kinematically decoupled, all the
material stored at the surface of the dust grains is sputtered and released
into the gas phase. This leads to a quick increase of both the HCN and HNC
abundances in the gas phase in a few years. Therefore, in the early years, the
gas phase abundance of HCN and HNC is mainly determined by the abundance of
both species on the dust grains and, at later times, by the different pre-
shock gas composition between the long and short Phase I models, as well as by
temperature-dependent gas-phase chemistry. The main differences between the
long-duration and the short-duration scenarii are the late time behaviours of
HCN and HNC, where both increase, in the latter case, much more sharply. The
reactions that seem to contribute to these increases are the two neutral-
neutral reactions of molecular H2 with CN (for HCN) and of atomic N with CH2
(for HNC). A full chemical analysis of this behaviour is beyond the scope of
this work, especially as the age of L1157-B1 is estimated to be $\sim
1500\hbox{\kern 1.99997ptyr}$ (Podio et al., 2016) but the shorter Phase I
duration certainly implies a richer gas phase at the beginning of Phase 2, and
hence more abundant nitrogen and CN. This is because of a less effective
freeze out during Phase I which would have not only depleted these two species
but also hydrogenated them into NH3 and HCN respectively.

The models with higher shock velocities ($V_{s}$= $40\hbox{\kern
1.99997ptkm\kern 1.99997pts${}^{-1}$}$) and hence higher maximum temperatures
(see bottom panels in Fig. 7), display qualitatively similar evolution for HCN
among the two models. Quantitatively, the "short" duration scenario predicts
HCN and HNC abundances both in agreement with our observational determinations
for time $t\simeq 2000\hbox{\kern 1.99997ptyr}$, which is also consistent with
the estimated age of B1 ($\sim 1500\hbox{\kern 1.99997ptyr}$; Podio et al.
(2016)). On the contrary, the "long" duration scenario results into an
overabundance of HCN (HNC) in the shocked gas by a factor of 10 (5) with
respect to our estimates, so that the HCN abundance remains essentially
constant with time. In both the "long" and the "short" duration scenarii, the
HNC abundance suddenly drops to values of a few $10^{-11}$ when the
temperature rises above a threshold value of $\approx 2000\hbox{\kern
1.99997ptK}$. This is probably due to the endothermic reaction C+ \+ HNC which
only becomes efficient at a temperature above $2000\hbox{\kern 1.99997ptK}$.

Therefore, our modelling favors a scenario with a "short" duration pre-shock
phase and a shock propagating at $V_{s}=40\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ into pre-density gas $n(\hbox{H${}_{2}$})=5\times
10^{4}\hbox{\kern 1.99997ptcm${}^{-3}$}$. It successfully accounts
quantitatively for the dramatic increase of HCN abundance with respect to HNC
on timescales consistent with the age of the shock. The dramatic increase of
HCN/HNC mainly appears as an effect of grain sputtering, which is already
efficient in low velocity shocks ($V_{s}=20\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$). This could explain why the HCN and HNC emission is so
extended along the outflow, while other species, like e.g. NH2CHO, require
more stringent shock conditions to be formed (Codella et al., 2017). We note
that these results are fully consistent with our previous studies of L1157-B1.

##  6 Conclusions

Using the IRAM 30m telescope in the framework of ASAI, we have carried out a
comprehensive observational study of the HCN/HNC isotopologues in the
protostellar shock region L1157-B1.

Based on the observations of the envelope of the protostar L1157-mm, we find
that the abundances of the rare HCN/HNC isotopologues in the pre-shock gas are
similar to those found in cold dark and prestellar cores. We find a ratio
HCN/HNC close to unity for all rare isotopologues and we derive an elemental
abundance ratio 14N/15N close to its elemental value in the ISM. There is no
evidence of elemental fractionation, except for Deuterium.

The impact of the shock is characterized by a strong increase of the HCN/HNC
abundance ratio and of the HCN abundance relative to H2. The 13C and 15N
isotopologues display the same qualitative and quantitative behaviour as the
main isopologues, i.e. the elemental abundance ratios remain unchanged in the
passage of the shock.

A chemical modelling using UCLCHEM successfully accounts for the properties of
HCN/HNC in the gas phase and their evolution in the shock. A very good
agreement with the observations is obtained for a steady-state C-shock
propagating with velocity $V_{s}=40\hbox{\kern 1.99997ptkm\kern
1.99997pts${}^{-1}$}$ into a pre-shock medium of density $\rm
n(H)=10^{5}\hbox{\kern 1.99997ptcm${}^{-3}$}$ at a time ($\sim 2000\hbox{\kern
1.99997ptyr}$) reasonably consistent with the age of the shock as estimated by
Podio et al. (2016). These results agree with our previous UCLCHEM studies on
the chemical composition of L1157-B1 (Viti et al., 2011; Lefloch et al., 2016;
Holdship et al., 2017; Benedettini et al., 2013; Mendoza et al., 2018;
Benedettini et al., 2013; Busquet et al., 2017; Codella et al., 2018;
Benedettini et al., 2021).

The HCN/HNC chemical evolution appears to be tighly connected to the history
and the composition of the pre-shock phase. In the case of L1157-B1, it is
mainly driven by grain sputtering, a process also efficient in lower velocity
shocks ($20\hbox{\kern 1.99997ptkm\kern 1.99997pts${}^{-1}$}$). We propose
this could explain the wide spatial extent of the HCN and HNC emissions in the
L1157-B1 outflow, while other species (e.g. NH2CHO) require more stringent
shock conditions to be formed.

## Data availability

The ASAI data are publicly available under the IRAM Data Archive webpage at
https://www.iram.fr/ILPA/LP007/. The other, complementary data underlying this
article will be shared on reasonable request to the corresponding author.

## Acknowledgements

Based on observations carried out as part of the Large Program ASAI (project
number 012-12) with the IRAM 30m telescope. IRAM is supported by INSU/CNRS
(France), MPG (Germany) and IGN (Spain). This work was supported by : (i) a
grant from LabeX Osug@2020 (Investissements d’avenir - ANR10LABX56), by the
French National Research Agency in the framework of the Investissements
d’Avenir program (ANR-15-IDEX-02), through the funding of the "Origin of Life"
project of the Univ. Grenoble-Alpes; (ii) the ERC Horizon 2020 research and
innovation programme "The Dawn of Organic Chemistry" (DOC), grant agreement No
741002; (iii) the ERC Horizon 2020 ITN Project “Astro-Chemistry Origins”
(ACO), grant agreement No 811312. SV acknowledges the European Research
Council (ERC) Advanced Grant MOPPEX 833460 E.M. and JL acknowledge support
from the Brazilian agency FAPESP (grant 2014/22095-6 and 2015/22254-0). G.B.
is supported by the State Agency for Research (AEI) of the Spanish MCIU
through the AYA2017-84390-C2-2-R grant.

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