Seed of Life in Space (SOLIS) XI. First measurement of nitrogen fractionation in shocked clumps of the L1157 protostellar outflow

Abstract

The isotopic ratio of nitrogen presents a wide range of values in the Solar System and in star forming system whose origin is still unclear. Chemical reactions in the gas phase are one of the possible processes that could modify the ${}^{14}$N/${}^{15}$N ratio. We aim at investigating if and how the passage of a shock wave in the interstellar medium, can affect the relative fraction of nitrogen isotopes. The ideal place for such a study is the L1157 outflow, where several shocked clumps are present. We present the first measurement of the ${}^{14}$N/${}^{15}$N ratio in the two shocked clumps, B1 and B0, of the protostellar outflow L1157, derived from the interferomteric maps of the H${}^{13}$CN(1-0) and the HC${}^{15}$N(1-0) lines. In B1, we find that the H${}^{13}$CN(1-0) and HC${}^{15}$N(1-0) emission traces the front of the clump, with averaged column density of $N$(H${}^{13}$CN) $\sim$ 7$\times$10${}^{12}$ cm${}^{-2}$ and $N$(HC${}^{15}$N) $\sim$ 2$\times$10${}^{12}$ cm${}^{-2}$. In this region the ratio H${}^{13}$CN(1-0)/HC${}^{15}$N(1-0) is quite uniform with an average value of $\sim$ 5$\pm$1. The same average value is also measured in the smaller clump B0e. Assuming the standard ${}^{12}$C/${}^{13}$C = 68, we obtain ${}^{14}$N/${}^{15}$N = 340$\pm$70, similar to those usually found in prestellar cores and protostars. We analysed the prediction of a chemical shock model for several shock conditions and we found that the nitrogen and carbon fractionations do not vary much for the first period after the shock. The observed H${}^{13}$CN/HC${}^{15}$N can be reproduced by a non-dissociative, C-type shock with parameters in agreement with previous modelling of L1157-B1. Both observations and chemical models indicate that the shock propagation does not affect the nitrogen isotopic ratio that remains similar to that measured in lower temperature gas in prestellar cores and in protostellar envelopes.

arXiv comment7 pages, 6 figures

References in corpus (25)

• The chemical composition of the Sun
• A computer program for fast non-LTE analysis of interstellar line spectra
• Astrochemical evolution along star formation: Overview of the IRAM Large Program ASAI
• L1157-B1, a factory of complex organic molecules in a Solar-type star forming region
• The CN isotopic ratios in comets
• The CHESS chemical Herschel surveys of star forming regions: Peering into the protostellar shock L1157-B1. I. Shock chemical complexity
• Isotope selective photodissociation of N2 by the interstellar radiation field and cosmic rays
• The 15N-enrichment in dark clouds and Solar System objects
• L1157-B1: Water and ammonia as diagnostics of shock temperature
• Detection of (15)NNH+ in L1544: non-LTE modelling of dyazenilium hyperfine line emission and accurate (14)N/(15)N values
• The clumpy structure of the chemically active L1157 outflow
• Astrochemistry at work in the L1157-B1 shock: acetaldehyde formation
• Nitrogen fractionation in high-mass star-forming cores across the Galaxy
• Nitrogen and hydrogen fractionation in high-mass star forming cores from observations of HCN and HNC
• The B1 shock in the L1157 outflow as seen at high spatial resolution
• Revised Models of Interstellar Nitrogen Isotopic Fractionation
• ${}^{14}$N/${}^{15}$N ratio measurements in prestellar cores with N${}_2$H${}^{+}$: new evidence of ${}^{15}$N-antifractionation
• Chemical nitrogen fractionation in dense molecular clouds
• The density structure of the L1157 molecular outflow
• The L1157-B1 astrochemical laboratory: testing the origin of DCN
• Deuterium and ${}^{15}$N fractionation in N${}_2$H${}^{+}$ during the formation of a Sun-like star
• A chemical study of carbon fractionation in external galaxies
• Nitrogen Fractionation in External Galaxies
• No nitrogen fractionation on 600 au scale in the Sun progenitor analogue OMC-2 FIR4
• Seeds of Life in Space (SOLIS) VIII. SiO isotopic fractionation and a new insight in the shocks of L1157-B1