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

## TL;DR

This study reports the first detection of SiS in a shock environment around a Sun-like protostar, revealing different chemical origins for SiO and SiS and demonstrating their use in probing dust grain processing and gas-phase chemistry.

## Contribution

First detection of SiS in a shock environment, highlighting its formation through gas-phase processes and differentiating it from SiO's grain-related origin.

## Key findings

- SiO peaks at jet impact sites, indicating direct grain release.
- SiS is detected at cavity head, formed via gas-phase chemistry.
- The [SiO/SiS] ratio varies significantly across the shock.

## Abstract

The shock L1157-B1 driven by the low-mass protostar L1157-mm is an 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/H2] ~ 1e-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/H2] ~ 2e-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.