2018A&A...616A..67A

Context. In the high-mass star-forming region G35.20-0.74N, small scale (∼800AU) chemical segregation has been observed in which complex organic molecules containing the CN group are located in a small location (toward continuum peak B3) within an apparently coherently rotating structure.
Aims. We aim to determine the physical origin of the large abundance difference (∼4 orders of magnitude) in complex cyanides within G35.20-0.74 B, and we explore variations in age, gas/dust temperature, and gas density.
Methods. We performed gas-grain astrochemical modeling experiments with exponentially increasing (coupled) gas and dust temperature rising from 10 to 500K at constant H₂ densities of 10⁷cm^–3, 10⁸cm^–3, and 10⁹cm^–3. We tested the effect of varying the initial ice composition, cosmic-ray ionization rate (1.3x10^–17s^–1, 1x10^–16s^–1, and 6x10^–16s^–1), warm-up time (over 50, 200, and 1000kyr), and initial (10, 15, and 25K) and final temperatures (300 and 500K).
Results. Varying the initial ice compositions within the observed and expected ranges does not noticeably affect the modeled abundances indicating that the chemical make-up of hot cores is determined in the warm-up stage. Complex cyanides vinyl and ethyl cyanide (CH₂CHCN and C₂H₅CN, respectively) cannot be produced in abundances (vs. H₂) greater than 5 x10^–10 for CH₂CHCN and 2 x10^–10 for C₂H₅CN with a fast warm-up time (52 kyr), while the lower limit for the observed abundance of C₂H₅CN toward source B3 is 3.4x10^–10. Complex cyanide abundances are reduced at higher initial temperatures and increased at higher cosmic-ray ionization rates. Reaction-diffusion competition is necessary to reproduce observed abundances of oxygen-bearing species in our model.
Conclusions. Within the context of this model, reproducing the observed abundances toward G35.20-0.74 Core B3 requires a fast warm-up at a high cosmic-ray ionization rate (∼1x10^–16s^–1) at a high gas density (>10⁹cm^–3). The abundances observed at the other positions in G35.20-0.74N also require a fast warm-up but allow lower gas densities (∼10⁸cm^–3) and cosmic-ray ionization rates (∼1x10^–17s^–1). In general, we find that the abundance of ethyl cyanide in particular is maximized in models with a low initial temperature, a high cosmic-ray ionization rate, a long warm-up time (>200kyr), and a lower gas density (tested down to 10⁷cm^–3). G35.20-0.74 source B3 only needs to be ∼2000 years older than B1/B2 for the observed chemical difference to be present, which maintains the possibility that G35.20-0.74 B contains a Keplerian disk.