Ice chemistry in starless molecular cores
  Starless molecular cores are natural laboratories for interstellar molecular
chemistry research. The chemistry of ices in such objects was investigated with
a three-phase (gas, surface, and mantle) model. We considered the center part
of five starless cores, with their physical conditions derived from
observations. The ice chemistry of oxygen, nitrogen, sulfur, and complex
organic molecules (COMs) was analyzed. We found that an ice-depth dimension,
measured, e.g., in monolayers, is essential for modeling of chemistry in
interstellar ices. Particularly, the H2O:CO:CO2:N2:NH3 ice abundance ratio
regulates the production and destruction of minor species. It is suggested that
photodesorption during core collapse period is responsible for high abundance
of interstellar H2O2 and O2H, and other species synthesized on the surface. The
calculated abundances of COMs in ice were compared to observed gas-phase
values. Smaller activation barriers for CO and H2CO hydrogenation may help
explain the production of a number of COMs. The observed abundance of methyl
formate HCOOCH3 could be reproduced with a 1kyr, 20K temperature spike.
Possible desorption mechanisms, relevant for COMs, are gas turbulence (ice
exposure to interstellar photons) or a weak shock within the cloud core (grain
collisions). To reproduce the observed COM abundances with the present 0D
model, 1-10% of ice mass needs to be sublimated. We estimate that the lifetime
for starless cores likely does not exceed 1Myr. Taurus cores are likely to be
younger than their counterparts in most other clouds.
