Complex cyanides as chemical clocks in hot cores
  In the high-mass star-forming region G35.20-0.74N, small scale (about 800 AU)
chemical segregation has been observed in which complex organic molecules
containing the CN group are located in a small location. We aim to determine
the physical origin of the large abundance difference (about 4 orders of
magnitude) in complex cyanides within G35.20-0.74 B, and we explore variations
in age, gas and dust temperature, and gas density. We performed gas-grain
astrochemical modeling experiments with exponentially increasing (coupled) gas
and dust temperature rising from 10 to 500 K at constant H$_2$ densities of
10$^7$, 10$^8$, and 10$^9$ cm$^{-3}$. We tested the effect of varying the
initial ice composition, cosmic-ray ionization rate, warm-up time (over 50,
200, and 1000 kyr), and initial (10, 15, and 25 K) and final temperatures (300
and 500 K). 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 (CH$_2$CHCN and C$_2$H$_5$CN,
respectively) cannot be produced in abundances (versus H$_2$) greater than
5x10$^{-10}$ for CH$_2$CHCN and 2x10$^{-10}$ for C$_2$H$_5$CN with a fast
warm-up time (52 kyr), while the lower limit for the observed abundance of
C$_2$H$_5$CN toward source B3 is 3.4x10$^{-10}$. Complex cyanide abundances are
reduced at higher initial temperatures and increased at higher cosmic-ray
ionization rates. Reproducing the observed abundances toward G35.20-0.74 Core
B3 requires a fast warm-up at a high cosmic-ray ionization rate (1x10$^{-16}$
s$^{-1}$) at a high gas density (>10$^9$ cm$^{-3}$). G35.20-0.74 source B3 only
needs to be about 2000 years older than B1/B2 for the observed chemical
difference to be present. (This abstract has been shortened)
