Complex cyanides as chemical clocks in hot cores

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.