Gas-phase chemistry in dense interstellar clouds including grain surface molecular depletion and desorption
E. A. Bergin, W. D. Langer, P. F. Goldsmith;
ApJ, 1995, 441, 222
ABSTRACT:We present time-dependent models of the chemical evolution of molecular clouds which include depletion of atoms and molecules onto grain
surfaces and desorption, as well as gas-phase interactions.
We have included
three mechanisms to remove species from the grain mantles: thermal
evaporation, cosmic-ray-induced heating, and photodesorption.
A wide range of
parameter space has been explored to examine the abundance of species present
both on the grain mantles and in the gas phase as a function of both position in
the cloud (visual extinction) and of evolutionary state (time).
The
dominant mechanism that removes molecules from the grain mantles is cosmic-ray
desorption.
At times greater than the depletion timescale, the abundances of some
simple species agree with abundances observed in the cold dark cloud
TMC-1.
Even though cosmic-ray desorption preserves the gas-phase chemistry at
late times, molecules do show significant depletions from the gas
phase.
Examination of the dependence of depletion as a function of density shows that when
the density increases from 103/cc to 105/cc
several species including HCO(+), HCN, and CN show gas-phase abundance
reductions of over an order of magnitude.
The CO: H2O ratio in the grain mantles for
our standard model is on the order of 10:1, in reasonable agreement with
observations of nonpolar CO ice features in rho Ophiuchus and Serpens.
We have also
examined the interdependence of CO depletion with the space density of
molecular hydrogen and binding energy to the grain surface.
We find that the
observed depletion of CO in Taurus in inconsistent with CO bonding in an H2O rich
mantle, in agreement with observations.
We suggest that if interstellar
grains consist of an outer layer of CO ice, then the binding energies for many
species to the grain mantle may be lower than commonly used, and a significant
portion of molecular material may be maintained in the gas phase.
KEYWORDS: carbon monoxide, chemical evolution, cosmic rays, cyano compounds, depletion, desorption, interstellar chemistry, interstellar gas, molecular clouds, ophiuchi clouds, water, chemical analysis, molecular physics
CODE: bergin95