Context. In the high-mass star-forming region G35.20-0.74N, small scale ( 800 AU) 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 500 K at constant H2 densities of 107 cm-3, 108 cm-3, and 109 cm-3. We tested the effect of varying the initial ice composition, cosmic-ray ionization rate (1.3 × 10-17 s-1, 1 × 10-16 s-1, and 6 × 10-16 s-1), warm-up time (over 50, 200, and 1000 kyr), and initial (10, 15, and 25 K) and final temperatures (300 and 500 K).
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 ×10-10 for CH2CHCN and 2 ×10-10 for C2H5CN with a fast warm-up time (52 kyr), while the lower limit for the observed abundance of C2H5CN toward source B3 is 3.4 ×10-10. 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 ( 1 × 10-16 s-1) at a high gas density (>109 cm-3). The abundances observed at the other positions in G35.20-0.74N also require a fast warm-up but allow lower gas densities ( 108 cm-3) and cosmic-ray ionization rates ( 1 × 10-17 s-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 (>200 kyr), and a lower gas density (tested down to 107 cm-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.
