Issue |
A&A
Volume 678, October 2023
|
|
---|---|---|
Article Number | A120 | |
Number of page(s) | 13 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202347106 | |
Published online | 13 October 2023 |
Combined model for 15N, 13C, and spin-state chemistry in molecular clouds
1
Max-Planck-Institut für Extraterrestrische Physik (MPE),
Giessenbachstr. 1,
85748
Garching, Germany
e-mail: osipila@mpe.mpg.de
2
Centro de Astrobiología (CAB), CSIC-INTA,
Ctra. de Ajalvir Km. 4,
28850
Torrejón de Ardoz, Madrid, Spain
3
Sorbonne Université, Observatoire de Paris, PSL University, CNRS, LERMA,
5 place Janssen,
92190
Meudon Cedex, France
4
INAF – Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5,
50125
Florence, Italy
5
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory,
43992
Onsala, Sweden
Received:
6
June
2023
Accepted:
5
September
2023
We present a new gas-grain chemical model for the combined isotopic fractionation of carbon and nitrogen in molecular clouds. To this end, we have developed gas-phase and grain-surface chemical networks where the isotope chemistry of carbon and nitrogen is coupled with a time-dependent description of spin-state chemistry, which is important for nitrogen chemistry at low temperatures. We updated the rate coefficients of some isotopic exchange reactions considered previously in the literature, and here we present a set of new exchange reactions involving molecules substituted in 13C and 15N simultaneously. We applied the model to a series of zero-dimensional simulations representing a set of physical conditions across a prototypical prestellar core, exploring the deviations of the isotopic abundance ratios in the various molecules from the elemental isotopic ratios as a function of physical conditions and time. We find that the 12C/13C ratio can deviate from the elemental ratio to a high factor depending on the molecule, and that there are highly time-dependent variations in the ratios. The HCN/H13CN ratio, for example, can obtain values of less than ten depending on the simulation time. The 14N/15N ratios tend to remain close to the assumed elemental ratio within approximately 10%, with no clearly discernible trends for the various species as a function of the physical conditions. Abundance ratios between 13C-containing molecules and 13C+15N-containing molecules however show somewhat increased levels of fractionation as a result of the newly included exchange reactions, though they still remain within a few tens of percent of the elemental 14N/15N ratio. Our results imply the existence of gradients in isotopic abundance ratios across prestellar cores, suggesting that detailed simulations are required to interpret observations of isotopically substituted molecules correctly, especially given that the various isotopic forms of a given molecule do not necessarily trace the same gas layers.
Key words: astrochemistry / ISM: abundances / ISM: clouds / ISM: molecules
© The Authors 2023
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Open Access funding provided by Max Planck Society.
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