Volume 640, August 2020
|Number of page(s)||20|
|Section||Interstellar and circumstellar matter|
|Published online||10 August 2020|
Carbon isotopic fractionation in molecular clouds
Università degli studi di Firenze, Dipartimento di fisica e Astronomia,
Via Sansone 1,
Sesto Fiorentino, Italy
e-mail: firstname.lastname@example.org, email@example.com
2 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
3 Max-Planck-Institüt für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
4 Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France
Accepted: 4 June 2020
Context. Carbon fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and low-temperature isotopic exchange reactions.
Aims. Recent chemical models predict that isotopic exchange reactions may lead to a depletion of 13C in nitrile-bearing species, with 12C/13C ratios two times higher than the elemental abundance ratio of 68 in the local interstellar medium. Since the carbon isotopic ratio is commonly used to evaluate the 14N/15N ratios with the double-isotope method, it is important to study carbon fractionation in detail to avoid incorrect assumptions.
Methods. In this work, we implemented a gas-grain chemical model with new isotopic exchange reactions and investigated their introduction in the context of dense and cold molecular gas. In particular, we investigated the 12C/13C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2 × 103 to 2 × 107 cm−3, respectively.
Results. We suggest a possible 13C exchange through the 13C + C3 → 12C +13CC2 reaction, which does not result in dilution, but rather in 13C enhancement, for molecules that are formed starting from atomic carbon. This effect is efficient in a range of time between the formation of CO and its freeze-out on grains. Furthermore, the parameter-space exploration shows, on average, that the 12C/13C ratios of nitriles are predicted to be a factor 0.8–1.9 different from the local 12C/13C of 68 for high-mass star-forming regions. This result also affects the 14N/15N ratio: a value of 330 obtained with the double-isotope method is predicted to vary in the range 260–630, up to 1150, depending on the physical conditions. Finally, we studied the 12C/13C ratios of nitriles by varying the cosmic-ray ionisation rate, ζ: the 12C/13C ratios increase with ζ because of secondary photons and cosmic-ray reactions.
Key words: astrochemistry / methods: numerical / ISM: molecules / molecular processes
© ESO 2020
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