Seeds of Life in Space (SOLIS) VIII. SiO isotopic fractionation and a
  new insight in the shocks of L1157-B1
  L1157-B1 is one of the outflow shocked regions along the blue-shifted outflow
driven by the Class 0 protostar L1157-mm, and is an ideal laboratory to study
the material ejected from the grains in very short timescales, i.e. its
chemical composition is representative of the composition of the grains. We
imaged $^{28}$SiO, $^{29}$SiO and $^{30}$SiO J = 2-1 emission towards L1157-B1
and B0 with the NOrthern Extended Millimeter Array (NOEMA) interferometer as
part of the Seeds of Life in Space (SOLIS) large project. We present here a
study of the isotopic fractionation of SiO towards L1157-B1. Furthermore, we
use the high spectral resolution observations on the main isotopologue,
$^{28}$SiO, to study the jet impact on the dense gas. We present here also
single-dish observations obtained with the IRAM 30m telescope and
Herschel-HIFI. We carried out a non-LTE analysis using a Large Velocity
Gradient (LVG) code to model the single-dish observations. From our
observations we can show that (i) the (2-1) transition of the main isotopologue
is optically thick in L1157-B1 even at high velocities, and (ii) the
[$^{29}$SiO/$^{30}$SiO] ratio is constant across the source, and consistent
with the solar value of 1.5. We report the first isotopic fractionation maps of
SiO in a shocked region and show the absence of a mass dependent fractionation
in $^{29}$Si and $^{30}$Si across L1157-B1. A high-velocity bullet in
$^{28}$SiO has been identified, showing the signature of a jet impacting on the
dense gas. With the dataset presented in this paper, both interferometric and
single-dish, we were able to study in great detail the gas shocked at the B1a
position and its surrounding gas.
