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

Context. The isotopic ratio of nitrogen presents a wide range of values in the Solar System: from ~140 in meteorites and comets to 441 in the solar wind. In star-forming systems, we observe even a higher spread of ~150-1000. The origin of these differences is still unclear.
Aims: Chemical reactions in the gas phase are one of the possible processes that could modify the ¹⁴N/¹⁵N ratio. We aim to investigate if and how the passage of a shock wave in the interstellar medium, which activates a rich chemistry, can affect the relative fraction of nitrogen isotopes. The ideal place for such a study is the chemically rich outflow powered by the L1157-mm protostar, where several shocked clumps are present.
Methods: We present the first measurement of the ¹⁴N/¹⁵N ratio in the two shocked clumps, B1 and B0, of the protostellar outflow L1157. The measurement is derived from the interferometeric maps of the H¹³CN (1-0) and the HC¹⁵N (1-0) lines obtained with the NOrthern Extended Millimeter Array (NOEMA) interferometer as part of the Seeds of Life in Space (SOLIS) programme.
Results: In B1, we find that the H¹³CN (1-0) and HC¹⁵N (1-0) emission traces the front of the clump, that is the apex of the shocked region, where the fast jet impacts the lower velocity medium with an averaged column density of N(H¹³CN) ~ 7 × 10¹² cm^-2 and N(HC¹⁵N) ~ 2 × 10¹² cm^-2. In this region, the ratio H¹³CN (1-0)/HC¹⁵N (1-0) is almost uniform with an average value of ~5 ± 1. The same average value is also measured in the smaller clump B0e. Assuming the standard ¹²C/¹³C = 68, we obtain ¹⁴N/¹⁵N = 340 ± 70. This ratio is similar to those usually found with the same species 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 H¹³CN/HC¹⁵N can be reproduced by a non-dissociative, C-type shock with pre-shock density n(H) = 10⁵ cm^-3, shock velocity V_s between 20 and 40 km s^-1, and cosmic-ray ionization rate of 3 × 10^-16 s^-1; this agrees with previous modelling of other chemical species in L1157-B1.
Conclusions: Both observations and chemical models indicate that the rich chemistry activated by the shock propagation does not affect the nitrogen isotopic ratio, which remains similar to that measured in lower temperature gas in prestellar cores and protostellar envelopes.