Colzi, L.L.ColziSipilä, O.O.SipiläRoueff, E.E.RoueffCaselli, P.P.CaselliFONTANI, FrancescoFrancescoFONTANI2025-02-182025-02-1820200004-6361http://hdl.handle.net/20.500.12386/36025Context. 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. <BR /> Aims: Recent chemical models predict that isotopic exchange reactions may lead to a depletion of <SUP>13</SUP>C in nitrile-bearing species, with <SUP>12</SUP>C/<SUP>13</SUP>C 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 <SUP>14</SUP>N/<SUP>15</SUP>N ratios with the double-isotope method, it is important to study carbon fractionation in detail to avoid incorrect assumptions. <BR /> 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 <SUP>12</SUP>C/<SUP>13</SUP>C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2 × 10<SUP>3</SUP> to 2 × 10<SUP>7</SUP> cm<SUP>-3</SUP>, respectively. <BR /> Results: We suggest a possible <SUP>13</SUP>C exchange through the <SUP>13</SUP>C + C<SUB>3</SUB> → <SUP>12</SUP>C +<SUP>13</SUP>CC<SUB>2</SUB> reaction, which does not result in dilution, but rather in <SUP>13</SUP>C 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 <SUP>12</SUP>C/<SUP>13</SUP>C ratios of nitriles are predicted to be a factor 0.8-1.9 different from the local <SUP>12</SUP>C/<SUP>13</SUP>C of 68 for high-mass star-forming regions. This result also affects the <SUP>14</SUP>N/<SUP>15</SUP>N 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 <SUP>12</SUP>C/<SUP>13</SUP>C ratios of nitriles by varying the cosmic-ray ionisation rate, ζ: the <SUP>12</SUP>C/<SUP>13</SUP>C ratios increase with ζ because of secondary photons and cosmic-ray reactions.STAMPAenArticle10.1051/0004-6361/2020382512-s2.0-85089734779https://www.aanda.org/articles/aa/full_html/2020/08/aa38251-20/aa38251-20.html2020A&A...640A..51CFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation