Seeds of Life in Space (SOLIS): VIII. SiO isotopic fractionation, and a new insight into the shocks of L1157-B1

Context. Contrary to what is expected from models of Galactic chemical evolution, the isotopic fractionation of silicon (Si) in the Galaxy has recently been found to be constant. This finding calls for new observations, also at core scales, to re-evaluate the fractionation of Si.Aims. L1157-B1 is one of the outflow-shocked regions along the blue-shifted outflow that is driven by the Class 0 protostar L1157-mm. It is an ideal laboratory for studying the material ejected from the grains on very short timescales because its chemical composition is representative of the composition of the grains.Methods. 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 used the high spectral resolution observations on the main isotopologue, 28 SiO, to study the jet impact on the dense gas. We here also present single-dish observations obtained with the IRAM 30 m telescope and Herschel -HIFI. We carried out a non-local thermal equilibrium analysis using a large velocity gradient code to model the single-dish observations.Results. 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.Conclusions. 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 the gas that is shocked at the B1a position and its surrounding gas in great detail.