Issue |
A&A
Volume 645, January 2021
|
|
---|---|---|
Article Number | A91 | |
Number of page(s) | 7 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202039609 | |
Published online | 19 January 2021 |
Seeds of Life in Space (SOLIS)
XI. First measurement of nitrogen fractionation in shocked clumps of the L1157 protostellar outflow
1
INAF, Istituto di Astrofisica e Planetologia Spaziali,
Via Fosso del Cavaliere 100,
00133 Roma,
Italy
e-mail: milena.benedettini@inaf.it
2
Leiden Observatory, Leiden University,
PO Box 9513,
2300 RA Leiden,
The Netherlands
3
Department of Physics and Astronomy, University College London,
Gower Street,
London, WC1E 6BT,
UK
4
INAF, Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5,
50125 Firenze,
Italy
5
Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France
6
Institut de Ciències de l’Espai (ICE, CSIC),
Can Magrans, s/n,
08193 Cerdanyola del Vallès, Catalonia,
Spain
7
Institut d’Estudis Espacials de Catalunya (IEEC),
08034 Barcelona,
Catalonia,
Spain
8
Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstrasse 1,
85748 Garching,
Germany
9
IRAP, Université de Toulouse,
9 avenue du colonel Roche,
31028 Toulouse Cedex 4,
France
11
Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint-Martin d’Hères, France
Received:
7
October
2020
Accepted:
28
November
2020
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 Vs 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.
Key words: ISM: jets and outflows / ISM: molecules / ISM: individual objects: L1157
© ESO 2021
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