# Silicon-bearing molecules in the shock L1157-B1: first detection of SiS around a Sun-like protostar

L. Podio, C. Codella, B. Lefloch, N. Balucani, C. Ceccarelli, R. Bachiller, M.

Benedettini, J. Cernicharo, N. Faginas-Lago, F. Fontani, et al.

► To cite this version:

L. Podio, C. Codella, B. Lefloch, N. Balucani, C. Ceccarelli, et al., Silicon-bearing molecules in the shock L1157-B1: first detection of SiS around a Sun-like protostar. Monthly Notices of the Royal Astronomical Society: Letters, 2017, 470 (1), pp.L16-L20. 10.1093/mnrasl/slx068. hal-02240912

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# Silicon-bearing molecules in the shock L1157-B1: first detection of SiS around a Sun-like protostar

L. Podio,<fnref n="1" />* C. Codella,<fnref n="1" /> B. Lefloch,<fnref n="2" /> N. Balucani,<fnref n="3" />,<fnref n="1" /> C. Ceccarelli,<fnref n="2" />,<fnref n="1" /> R. Bachiller,<fnref n="4" /> M. Benedettini,<fnref n="5" /> J. Cernicharo,<fnref n="6" /> N. Faginas-Lago,<fnref n="3" /> F. Fontani,<fnref n="1" /> A. Gusdorf<fnref n="7" /> and M. Rosi<fnref n="8" />

Accepted 2017 April 28. Received 2017 April 28; in original form 2017 February 23

## ABSTRACT

The shock L1157-B1 driven by the low-mass protostar L1157-mm is a unique environment to investigate the chemical enrichment due to molecules released from dust grains. IRAM-30m and Plateau de Bure Interferometer observations allow a census of Si-bearing molecules in L1157-B1. We detect SiO and its isotopologues and, for the first time in a shock, SiS. The strong gradient of the [SiO/SiS] abundance ratio across the shock (from ≥180 to ~25) points to a different chemical origin of the two species. SiO peaks where the jet impacts the cavity walls ([SiO/H$_2$] ~ $10^{-6}$), indicating that SiO is directly released from grains or rapidly formed from released Si in the strong shock occurring at this location. In contrast, SiS is only detected at the head of the cavity opened by previous ejection events ([SiS/H$_2$] ~ $2 \times 10^{-8}$). This suggests that SiS is not directly released from the grain cores but instead should be formed through slow gas-phase processes using part of the released silicon. This finding shows that Si-bearing molecules can be useful to distinguish regions where grains or gas-phase chemistry dominates.

**Key words:** astrochemistry – stars: formation – ISM: jets and outflows – ISM: molecules.

## 1 INTRODUCTION

The gravitational infall from which a new star is formed is accompanied by the ejection of highly supersonic jets. The shocks produced when the jet impacts the parental cloud deeply change the chemical composition of the shocked gas due to both gas heating and compression, and dust grain erosion and disruption. These processes produce a strong enhancement of the abundance of some molecules, which are either released off the dust grains or formed in the gas (e.g. Bachiller & Pérez Gutiérrez 1997).

Some molecules, e.g. H$_2$O, S-bearing molecules and methanol, are trapped into the icy mantles of dust grains and are released in gas phase when the dust is heated (e.g. in a hot corino) or eroded in a shock. At difference, silicon is stored mainly in the core of dust grains (and in much smaller quantities, ≤10 per cent, in the grain

mantle or in gas phase; Gusdorf et al. 2008b) and is likely released in gas phase only when the grain core is destroyed via gas–grain (sputtering) or grain–grain collisions (shattering and vaporisation) (e.g. Gusdorf et al. 2008a; Guillet, Pineau Des Forêts & Jones 2011). Therefore, Si-bearing molecules, such as SiO, are a selective and powerful probe of shocked regions. However, while SiO has been widely observed in star-forming regions and used as a signpost of Class 0 protostars, SiS has only been conclusively observed in the envelope of C-rich stars (e.g. Cernicharo, Guélin & Kahane 2000) in Sgr B2 (Morris et al. 1975; Dickinson & Kuiper 1981) and in Orion KL (Ziurys 1988, 1991; Tercero et al. 2011). The studies concerning Sgr B2 and Orion suggested that, similarly to SiO, SiS is produced in shocks but the lack of resolved observations prevented to confirm its association with shocks and to investigate its chemical origin.

An interesting target to study the chemistry of shocks at the early protostellar stage is the B1 shock along the L1157 outflow. This is produced by the high-velocity precessing jet driven by the low-mass Class 0 source L1157-mm ($L \sim 3 L_{\odot}$, $d \sim 250$ pc) (e.g. Gueth

<footnote n="1">¹INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy</footnote>

<footnote n="2">²Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France</footnote>

<footnote n="3">³Dipartimento di Chimica, Biologia e Biotecnologie, Via Elce di Sotto 8, I-06123 Perugia, Italy</footnote>

<footnote n="4">⁴IGN – Observatorio Astronómico Nacional, Calle Alfonso XII, 3, 28014 Madrid, Spain</footnote>

<footnote n="5">⁵INAF, Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, I-00133 Roma, Italy</footnote>

<footnote n="6">⁶Grupo de Astrofísica Molecular, Instituto de CC, de Materiales de Madrid (ICMM-CSIC), Sor Juana Inés de la Cruz 3, Cantoblanco, E-28049 Madrid, Spain</footnote>

<footnote n="7">⁷LERMA, Observatoire de Paris, École normale supérieure, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, F-75231 Paris, France</footnote>

<footnote n="8">⁸Dipartimento di Ingegneria Civile ed Ambientale, Via Duranti 93, I-06125 Perugia, Italy</footnote>

<footnote>* E-mail: lpodio@arcetri.astro.it</footnote>
et al. 1996; Podio et al. 2016). Recent observations highlight an unprecedented chemical complexity due to dust grain destruction and show that this region is an ideal laboratory to benchmark chemical models and to investigate the formation routes of ‘exotic’ molecular species (e.g. Fontani et al. 2014; Mendoza et al. 2014; Podio et al. 2014; Codella et al. 2015).

In this letter, based on IRAM-30m and PdBI observations, we present a survey of Si-bearing molecules in the protostellar shock L1157-B1. We report several lines from SiO and the first detection of SiS in a protostellar shock and, more generally, in a low-mass star-forming region. The fractionation and abundance of SiO and SiS are estimated. Then we discuss the chemical origin of SiO and SiS, which may be either directly released from dust grains in the shock or formed through gas-phase processes. This kind of study is a key step towards the understanding of the chemical complexity in the interstellar medium up to the formation of complex organic and pre-biotic molecules.

## 2 OBSERVATIONS

The L1157-B1 protostellar shock ($\alpha_{J2000} = 20^h 39^m 10.2, \delta_{J2000} = +68^\circ 01'10.5$) was observed with the IRAM-30m telescope as part of the ASAI Large Programme.<fnref n="1" /> The observations were obtained in 2011–2012 with the broad-band EMIR receivers and the FTS spectrometers in their 200 kHz mode (velocity resolutions of 0.17–0.75 km s⁻<fnref n="1" />). The IRAM-30m data were processed using GILDAS/CLASS90² software. The spectra were baseline-subtracted and resampled to resolution of 1.4 km s⁻<fnref n="1" /> for SiO and 2.2 km s⁻<fnref n="1" /> for SiS to increase the signal to noise. Line intensities were converted from antenna temperature, $T_a$, to main-beam brightness temperature, $T_{mb}$, using the main-beam efficiency, $\eta_{mb}$ tabulated on the IRAM website.

The single-dish data were complemented by high-angular resolution observations taken with the IRAM-PdB interferometer (project X058). The SiS 9–8 line at 163 376.78 MHz was observed using the WideX backend, which covers a 4 GHz spectral window with a spectral resolution of ~2 MHz, i.e. ~3.6 km s⁻<fnref n="1" />. The calibration was carried out with GILDAS-CLIC following standard procedures. The clean beam is $1.88 \times 1.77$ (PA = 48°) and the rms noise is ~10 mJy km s⁻<fnref n="1" /> beam⁻<fnref n="1" />. The comparison of the SiS 9–8 spectrum extracted from the PdB1 map over a region of 15′′ with the spectrum obtained with IRAM-30m (HPBW~15′′) indicates that with PdB1 about 75 per cent of the line flux is recovered (see Fig. A1 in Appendix A, Supporting Information).

## 3 RESULTS

The IRAM-30m spectra show emission from two Si-bearing molecules: SiO and its isotopologues $^{29}$SiO and $^{30}$SiO (seven transitions identified from the 2–1 to the 8–7, $E_{up}$ from 6 to 75 K) and, for the first time in a protostellar shock, SiS (five lines from the 5–4 to the 15–14, $E_{up}$ ~ 13–105 K). The spectra are shown in Figs 1 and 2 and the line properties are summarized in Table B1 in Appendix B (Supporting Information).

The SiS and SiO lines peak at blue-shifted velocity, ~-4 km s⁻<fnref n="1" /> with respect to systemic velocity ($V_{sys}$ = +2.6 km s⁻<fnref n="1" />), and show a similar profile at low velocities ($|V - V_{sys}| \le 12$ km s⁻<fnref n="1" />). However, the SiS lines are ~100–200 times weaker than the SiO lines arising

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Line profiles of SiS in the L1157-B1 shock. The line intensity is in main beam temperature ($T_{mb}$). The transition, frequency, upper level energy and HPBW are labelled in the top-left corner. The baseline and the systemic velocity (+2.6 km s⁻<fnref n="1" />) are indicated by the horizontal and vertical dotted lines.</figcaption></figure>

from upper levels with similar energies. Moreover, the SiO lines show a high-velocity wing extending up to ~20 km s⁻<fnref n="1" /> likely originating in the strong shock occurring where the jet impacts the cavity walls (see Section 3.2).

### 3.1 Si isotopic ratios and SiO optical depth

The ratio between $^{28}$Si and its isotopes $^{29}$Si and $^{30}$Si in the local ISM has been so far poorly constrained. Observations of $^{29}$SiO and $^{30}$SiO in the Galactic centre and in Orion (Penzias 1981; Tercero et al. 2011) indicate an isotopic ratio in agreement with the Solar system ($[^{29}\text{Si}/^{30}\text{Si}]_{\odot} = 1.5$; Anders & Grevesse 1989). The secondary-to-primary isotope ratios, $[^{29}\text{Si}/^{28}\text{Si}]$ and $[^{30}\text{Si}/^{28}\text{Si}]$, are more difficult to estimate because the lowest transitions of $^{28}$SiO are likely optically thick (Penzias 1981). Observations of $^{28}$SiS and $^{29}$SiS in Orion KL provided an average value of $^{28}\text{Si}/^{29}\text{Si} = 26 \pm 10$ (Tercero et al. 2011), which is consistent with the Solar isotope ratio within the large reported uncertainty ($[^{28}\text{Si}/^{29}\text{Si}]_{\odot} = 19.7$; Anders & Grevesse 1989). Moreover, Monson, Morris & Young (2017) recently developed a method to correct the $^{28}$SiO 1–0 line

<footnote n="1">¹ http://www.oan.es/asai, Lefloch et al. (in preparation)</footnote>

<footnote n="2">² http://www.iram.fr/IRAMFR/GILDAS</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Line profiles of SiO in the L1157-B1 shock. The line intensity in main beam temperature ($T_{mb}$) from $^{29}$SiO (red) and $^{30}$SiO (blue) is overplotted on the main isotopologue, $^{28}$SiO (grey), by multiplying for their solar isotopic ratios ($[^{28}\text{Si}/^{29}\text{Si}]_{\odot} = 19.7$, $[^{28}\text{Si}/^{30}\text{Si}]_{\odot} = 29.8$; Anders & Grevesse 1989). The transition, frequency, upper level energy and HPBW for the main isotopologue are labelled. The baseline and the systemic velocity (+2.6 km s$^{-1}$) are indicated by the horizontal and vertical dotted lines. The redshifted component that seems to be associated with $^{29}$SiO 2–1 is due to blending with the $^{13}\text{CH}_3\text{OH}$ 4$_{1,3}$–3$_{-2,2}$ line.</figcaption></figure>

for optical depth and showed that the Si isotope ratios are uniform across the Milky Way and shows only a modest increase of the secondary isotopes. To derive an estimate of the Si isotopic ratios in the L1157-B1 shock, we compute the line intensity ratios $^{28}\text{SiO}/^{29}\text{SiO}$ and $^{28}\text{SiO}/^{30}\text{SiO}$ in the 6–5 and 5–4 lines, which are likely opti-

cally thin for all the three isotopologues. For a given transition the frequencies, upper level energies and Einstein coefficients for the three isotopologues are comparable; therefore, the abundance ratio can be directly derived from the line ratio. The $^{28}\text{SiO}/^{29}\text{SiO}$ and $^{28}\text{SiO}/^{30}\text{SiO}$ ratios in the high-velocity wings of the 5–4 and 6–5 transitions ($V - V_{\text{sys}} = -17/ -7 \text{ km s}^{-1}$) are in excellent agreement with the solar isotopic ratio ($^{28}\text{SiO}/^{29}\text{SiO} = 18.8, 19.7$), while at low velocities the ratios are 10–15 per cent lower ($^{28}\text{SiO}/^{29}\text{SiO} = 16.5, 17.3$; see Fig. 2). This suggests that opacity effects are responsible for previous estimates of low $^{28}\text{SiO}/^{29}\text{SiO}$ and $^{28}\text{SiO}/^{30}\text{SiO}$ ratios relative to solar (Penzias 1981) and that Galactic chemical evolution has apparently not led to significant changes of the Si isotope ratios in the past 4.6 billion years in our region of the Galaxy. Based on the above analysis, we adopt Solar isotopic ratios and estimate the optical depth of the $^{28}\text{SiO}$ 2–1 line ($\tau \sim 0.7–1.9$).

## 3.2 SiO and SiS: distribution and abundance

The left-hand panel of Fig. 3 shows the structure of the blueshifted outflow driven by the L1157-mm Class 0 protostar in CO 1–0 and SiO 2–1 (Gueth et al. 1996, 1998). The high-velocity ($\sim 87 \text{ km s}^{-1}$) precessing jet first observed by Podio et al. (2016) (black solid line) excavated two main cavities, B1 and B2, located at about 18 000 au and 33 000 au distance from L1157-mm. These cavities are well traced by low velocity CO 1–0 emission (Gueth et al. 1996). The peaks of SiO 2–1, instead, probe the regions where the episodic jet impacted the cavity walls at dynamical time-scales $t = 1800, 1100, 950$ yr (black diamonds). The strong shocks occurring at these locations destroy the dust grains, releasing in gas phase the silicates in the cores, as well as the molecules frozen on the icy mantles (e.g. Bachiller & Pérez Gutiérrez 1997; Arce et al. 2008). The right-hand panels of Fig. 3 show a zoom-in of the clumpy structure of the B1 cavity both in SiO 2–1 (black contours,

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Left-hand panel: integrated map of the L1157 blueshifted outflow in CO 1–0 (colour scale from −3 to 9 Jy km s$^{-1}$ beam$^{-1}$, $V_{LSR} = -8/ + 2.4 \text{ km s}^{-1}$) and SiO 2–1 (white contours from 1 Jy km s$^{-1}$ beam$^{-1}$ with steps of 0.6 Jy km s$^{-1}$ beam$^{-1}$, $V_{LSR} = -18/ + 5 \text{ km s}^{-1}$) (Gueth, Guilloteau & Bachiller 1996, 1998). The magenta and black ellipses show the HPBW of CO (3′′.6 × 3′′.0, PA = 90°) and SiO (2′′.8 × 2′′.2, PA = 56°). The black diamonds along the precession pattern modelled by Podio et al. (2016) (solid black line) indicate the position of the driving source, L1157-mm, and of the spots where the episodic jet impacts on the B2 and B1 cavity walls. Right panels: zoom-in on the L1157-B1 cavity. The emission clumps are labelled according to Benedettini et al. (2007) (cyan crosses). Integrated SiS 9–8 emission (blue contours, −10/ + 2.6 km s$^{-1}$) is overplotted on SiO 2–1 (grey-scale and black contours) integrated over low velocities (LV: −8/ + 5 km s$^{-1}$) (centre) and over high velocities (HV: −18/ −10 km s$^{-1}$) (right). The first contour is at 5$\sigma$ with steps of 1$\sigma$ for SiS and 3$\sigma$ for SiO (1$\sigma$ = 0.01 Jy km s$^{-1}$ beam$^{-1}$ for SiS, and 0.15 Jy km s$^{-1}$ beam$^{-1}$ for SiO). The blue and black ellipses show the HPBW for SiS (1′.88 × 1′.77, PA = 48°) and SiO (2′.8 × 2′.2, PA = 56°), respectively. The magenta diamond and dashed circles show the pointing and minimum/maximum HPBW of the IRAM-30m observations (see Table B1). The solid magenta circle show the IRAM-30m HPBW for SiS 9–8.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Rotation diagram for SiS. The derived column densities and rotational temperature are labelled.</figcaption></figure>

Gueth et al. 1998) and SiS 9–8 (blue contours, this work). The maps indicate that SiS and SiO originate from different regions in the L1157-B1 cavity. While SiO peaks at the positions where the jet impacted the cavity walls (i.e. B1a and B0e), the bulk of SiS emission is located at the head of the cavity B1c, located ~2000 au south of B1a. This anticorrelation is even more evident when comparing SiS with the SiO high-velocity emission that is exclusively detected in the B1a and B0e clumps.

In order to investigate the origin of the observed SiS and SiO emission, we derive their column density and abundance. First, the rotational diagram of the SiS lines is used to obtain an estimate of the gas temperature and SiS average column density over the cavity. We assume LTE optically thin emission and that the emitting size is the same for all the SiS transitions, i.e. ~3 arcsec (see the PdBI map of SiS 9–8 in Fig. 3). We derive $T_{rot} \sim 24 \pm 2$ K and $N_{SiS} \sim (4 \pm 1) \times 10^{13} \text{ cm}^{-2}$ (see Fig. 4). The derived temperature is in agreement with the value estimated in the cavity from the analysis of other molecules (e.g. Lefloch et al. 2012; Mendoza et al. 2014; Podio et al. 2014).

However, as SiS and SiO probe different regions of L1157-B1, in order to investigate their relative abundance across the shock, we extract the SiS 9–8 and SiO 2–1 line intensities from the corresponding PdBI data cubes at the position of the B1a and B1c clumps. The extracted spectra are shown in Fig. C1 in Appendix C (Supporting Information). The beam-averaged SiS and SiO column densities at the B1a and B1c positions are derived from the SiS 9–8 and SiO 2–1 line intensity integrated over their whole velocity profile (–17/ +8 km s⁻¹) assuming optically thin and thermalized emission at $T_K \sim T_{rot} = 25$ K, as estimated from the rotational diagram. In the case of SiO, the derived column density is corrected for the optical depth across the line profile, i.e. $N_{SiO} = N_{thin} \times \tau/(1 - \exp^{-\tau})$. Then, the abundances of SiS and SiO are derived as $X_X = X_{CO} \times N_X/N_{CO}$, where the column density in the outflow cavity is $N_{CO} = 9 \times 10^{16} \text{ cm}^{-2}$ (Lefloch et al. 2012) and the abundance of CO with respect to H₂ is assumed to be $X_{CO} = N_{CO}/N_{H_2} = 10^{-4}$. Considering the uncertainty on the line fluxes (due to the rms noise as reported in Table B1, ≤10 per cent, and to calibration, ~20 per cent) and on the estimated gas temperature in the cavity ($T_K \sim 25–80$ K, see Lefloch et al. 2012), the derived column densities and abundances are affected by an error of ≤30 per cent. The derived SiS and SiO column densities and abundances are summarized in Table 1. Interestingly, the relative abundance of SiS with respect to SiO varies by about one order of magnitude across the L1157-B1 shock, i.e. SiO/SiS = 25 in the head of the cavity B1c, and >180 at the jet impact region B1a.

<table><caption>**Table 1.** Beam-averaged column densities and abundances of SiS and SiO in the clumps B1a and B1c. The last column reports the SiO/SiS abundance ratio.</caption><thead><tr><th>Clumps</th><th><i>N</i><sub>SiS</sub><br>(10<sup>13</sup> cm<sup>-2</sup>)</th><th><i>N</i><sub>SiO</sub><br>(10<sup>14</sup> cm<sup>-2</sup>)</th><th><i>X</i><sub>SiS</sub><br>(10<sup>-8</sup>)</th><th><i>X</i><sub>SiO</sub><br>(10<sup>-6</sup>)</th><th>SiO/SiS</th></tr></thead><tbody><tr><td>B1a</td><td>≤0.5</td><td>9.3</td><td>≤0.6</td><td>1.0</td><td>≥180</td></tr><tr><td>B1c</td><td>1.8</td><td>4.4</td><td>2.0</td><td>0.5</td><td>25</td></tr></tbody></table>

## 4 THE ORIGIN OF Si-BEARING MOLECULES

The different abundance and spatial distribution of SiO and SiS strongly suggest a different chemical origin. There is general consensus that in quiescent clouds most of silicon is trapped into the dust grains cores (only ≤10 per cent may be in the mantles or in gas phase in the form of Si or Si⁺, Gusdorf et al. 2008b). However, silicon can be released in shocks in the form of atomic Si and/or SiO due to gas–grain collisions (sputtering) and/or grain–grain collisions (shattering and vaporisation). The latter produces SiO early in the shock while sputtering is only the first step in the production of SiO through the reactions of Si with O₂ or OH (Gusdorf et al. 2008a; Guillet et al. 2011). It appears then clear that SiO is a natural product of the release from dust grains cores and mantles in the shock.

On the other hand, the origin of SiS is unclear. One possible scenario is that SiS is trapped in the core of dust grains and released in gas phase in the shock. Observations of evolved stars show that SiS depletes before SiO. Hence, at difference with SiO, the release of SiS would occur only in strong shocks which are fully destroying the grains. However, a direct release from dust cores is in contrast with our observations as SiS is not detected at the jet impact site B1a, where we have evidence of a strong shock, but at the head of the cavity in the B1c clump.

Alternatively, ion-neutral gas-phase processes involving Si⁺ may convert part of the released silicon into SiS. This hypothesis is supported by *Spitzer* observations showing Si⁺ emission in L1157-B1 (Busquet et al. in preparation). This suggests that not all the released silicon is converted into SiO and that Si⁺ is easily produced by C⁺ and SiO whose abundance is enhanced in the shock (C⁺ + SiO → Si⁺ + CO). According to the existing chemical networks, e.g. KIDA,<fnref n="3" /> the only reaction producing SiS is HSiS⁺ + e⁻ → H + SiS, where HSiS⁺ can be formed starting from Si⁺ through the following sets of reactions: (1) Si⁺ + OCS → SiS⁺ + CO, SiS⁺ + H₂ → HSiS⁺ + H or alternatively (2) Si⁺ + H₂ → SiH₂⁺, S + SiH₂⁺ → H + HSiS⁺. However, laboratory experiments show that SiS⁺ and Si⁺ do not react with H₂ (Wlodek, Fox & Bohme 1987; Wlodek & Bohme 1989). Hence, the formation of SiS through recombination of HSiS⁺ is unlikely.

Based on the above discussion, we suggest that SiS is likely formed through neutral–neutral gas-phase reactions involving atomic Si or molecules such as SiHn (with n = 1, 2, 3, 4), e.g. Si + S/S2/HS and SiHn + S/S2/HS. However, these reactions are not included in the existing chemical networks; hence, experimental determinations or accurate theoretical predictions of the reactions barriers and rates are required to verify their efficiency.

In conclusion, according to our observations the L1157-B1 shock consists of two ‘chemically’ distinct regions: the northern region where the jet impacts the cavity walls (B1a clump) is dominated by the release from grains of species formed in the grain cores and

<footnote n="3">³ http://kidaobs.u-bordeaux1.fr</footnote>
mantles, and the older cavity B1c excavated by previous ejection episodes, where new molecules such as SiS are produced by gas-phase processes.

## 5 CONCLUSIONS

In this letter, we report the first detection of SiS in a low-mass star-forming region and in particular in the L1157-B1 protostellar shock. Interferometric observations of the shock obtained with the PdBI show for the first time that SiO and SiS have a different spatial distribution. We speculate that this is due to the different chemical origins of the two molecules. SiO peaks where the high-velocity molecular jet reported by Podio et al. (2016) impact the cavity walls (the B1a clump) producing a maximum SiO abundance of ~$10^{-6}$. Therefore, SiO is a probe of silicates released from dust grains in shocks. The SiS emission, instead, is detected only at the head of the cavity in the B1c clump ($[SiS/H_2] \sim 2 \times 10^{-8}$). This points against a direct release of SiS from the grain cores and suggests that SiS should be produced by gas-phase reactions using the released species. The existing chemical data bases indicate only one formation route for SiS, the reaction $HSiS^+ + e^- \rightarrow H^+ + SiS$. However, laboratory experiments show that it is not possible to hydrogenate $Si^+$ and $SiS^+$ to form $HSiS^+$. Therefore, we propose that SiS is formed by neutral-neutral gas-phase reactions, which are currently not included in the existing chemical networks. The determination of their reaction rates and their implementation in the chemical network is crucial to understand the chemistry of Si-bearing molecules and will be the subject of a follow-up letter.

## ACKNOWLEDGEMENTS

This work was supported by the French program 'Physique et Chimie du Milieu Interstellaire' (PCMI) funded by CNRS and CNES and by a grant from LabeX Osug@2020 (Investissements d'avenir - ANR10LABX56). We acknowledge Gemma Busquet for showing us the unpublished *Spitzer* map of $Si^+$ and the referee, Mark Morris, for his helpful suggestions. This study is based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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