Complex cyanides as chemical clocks in hot cores

Context. In the high-mass star-forming region G35.20-0.74N, small scale (similar to 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 (similar to 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 H-2 densities of 10(7) cm(-3), 10(8) cm(-3), and 10(9) cm(-3). We tested the effect of varying the initial ice composition, cosmic-ray ionization rate (1.3 x 10(-17) s(-1), 1 x 10(-16) s(-1), and 6 x 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. H-2) greater than 5 x 10(-10) for CH2CHCN and 2 x 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 x 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 (similar to 1 x 10(-16) s(-1)) at a high gas density (>10(9) 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 (similar to 10(8) cm(-3)) and cosmic-ray ionization rates (similar to 1 x 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 similar to 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