  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.
