A search for Cyanopolyynes in L1157-B1
Abstract
We present here a systematic search for cyanopolyynes in the shock region L1157-B1 and its associated protostar L1157-mm in the framework of the Large Program "Astrochemical Surveys At IRAM" (ASAI), dedicated to chemical surveys of solar-type star forming regions with the IRAM 30m telescope. Observations of the millimeter windows between 72 and 272 GHz permitted the detection of HC3N and its 13C isotopologues, and HC5N (for the first time in a protostellar shock region). In the shock, analysis of the line profiles shows that the emission arises from the outflow cavities associated with L1157-B1 and L1157-B2. Molecular abundances and excitation conditions were obtained from analysis of the Spectral Line Energy Distributions under the assumption of Local Thermodynamical Equilibrium or using a radiative transfer code in the Large Velocity Gradient approximation. Towards L1157mm, the HC3N emission arises from the cold envelope (Trot=10 K) and a higher-excitation region (Trot= 31 K) of smaller extent around the protostar. We did not find any evidence of 13C or D fractionation enrichment towards L1157-B1. We obtain a relative abundance ratio HC3N/HC5N of 3.3 in the shocked gas. We find an increase by a factor of 30 of the HC3N abundance between the envelope of L1157-mm and the shock region itself. Altogether, these results are consistent with a scenario in which the bulk of HC3N was produced by means of gas phase reactions in the passage of the shock. This scenario is supported by the predictions of a parametric shock code coupled with the chemical model UCLCHEM.