Models of gas-grain chemistry in interstellar cloud cores with a stochastic approach to surface chemistry

We present a gas-grain model of homogeneous cold cloud cores with time-independent physical conditions. In the model, the gas-phase chemistry is treated via rate equations while the diffusive granular chemistry is treated stochastically. The two phases are coupled through accretion and evaporation. A small network of surface reactions accounts for the surface production of the stable molecules water, formaldehyde, methanol, carbon dioxide, ammonia, and methane. The calculations are run for a time of 10 7 years at three different temperatures: 10 K, 15 K, and 20 K. The results are compared with those produced in a totally deterministic gas-grain model that utilizes the rate equation method for both the gas-phase and surface chemistry. The results of the different models are in agreement for the abundances of the gaseous species except for later times when the surface chemistry begins to affect the gas. The agreement for the surface species, however, is somewhat mixed. The average abundances of highly reactive surface species can be orders of magnitude larger in the stochastic-deterministic model than in the purely deterministic one. For non-reactive species, the results of the models can disagree strongly at early times, but agree to well within an order of magnitude at later times for most molecules. Strong exceptions occur for CO and H2CO at 10 K, and for CO2 at 20 K. The agreement seems to be best at a temperature of 15 K. As opposed to the use of the normal rate equation method of surface chemistry, the modified rate method is in significantly better agreement with the stochastic-deterministic approach. Comparison with observations of molecular ices in dense clouds shows mixed agreement.

[1]  O. Biham,et al.  Enhanced production of HD and D2 molecules on small dust grains in diffuse clouds , 2003, astro-ph/0311524.

[2]  E. Dartois,et al.  Detection of abundant solid methanol toward young low mass stars , 2003, astro-ph/0304476.

[3]  E. Herbst,et al.  Deuterium fractionation on interstellar grains studied with the direct master equation approach , 2003, astro-ph/0306562.

[4]  T. Millar,et al.  The importance of new rate coefficients for deuterium fractionation reactions in interstellar chemistry , 2002 .

[5]  E. Herbst,et al.  On the master equation approach to diffusive grain-surface chemistry: The H, O, CO system , 2002, astro-ph/0306545.

[6]  A. Tielens,et al.  Molecular Hydrogen Formation in the Interstellar Medium , 2002, astro-ph/0207035.

[7]  A. Kouchi,et al.  Efficient Formation of Formaldehyde and Methanol by the Addition of Hydrogen Atoms to CO in H2O-CO Ice at 10 K , 2002 .

[8]  E. Herbst,et al.  New models of interstellar gas–grain chemistry – I. Surface diffusion rates , 2002 .

[9]  P. Caselli,et al.  Deuterium fractionation on interstellar grains studied with modified rate equations and a Monte Carlo approach , 2002, astro-ph/0202368.

[10]  S. Charnley.  Stochastic Theory of Molecule Formation on Dust , 2001 .

[11]  N. Bell,et al.  A stochastic approach to grain surface chemical kinetics , 2001 .

[12]  P. Caselli,et al.  Modified rate equations revisited. A corrected treatment for diffusive reactions on grain surfaces , 2001 .

[13]  V. Pirronello,et al.  Formation of Carbon Dioxide by Surface Reactions on Ices in the Interstellar Medium , 2001 .

[14]  E. Herbst,et al.  New models of interstellar gas–grain chemistry – III. Solid CO2 , 2001 .

[15]  O. Biham,et al.  Master Equation for Hydrogen Recombination on Grain Surfaces , 2000, astro-ph/0012267.

[16]  Alexander G. G. M. Tielens,et al.  An Inventory of Interstellar Ices toward the Embedded Protostar W33A , 2000 .

[17]  O. Biham,et al.  Molecular Hydrogen Formation on Astrophysically Relevant Surfaces , 1999, astro-ph/9906071.

[18]  S. Charnley.  Stochastic Astrochemical Kinetics , 1998 .

[19]  E. Herbst,et al.  The Sensitivity of Gas-Phase Chemical Models of Interstellar Clouds to C and O Elemental Abundances and to a New Formation Mechanism for Ammonia , 1998 .

[20]  T. Prusti,et al.  Detection of Abundant CO2 Ice in the Quiescent Dark Cloud Medium toward Elias 16 , 1998 .

[21]  P. Caselli,et al.  A Proposed Modification of the Rate Equations for Reactions on Grain Surfaces , 1998 .

[22]  A. Tielens,et al.  Deuterated Methanol in the Orion Compact Ridge , 1997 .

[23]  A. Adamson,et al.  Three Micron Hydrocarbon and Methanol Absorption in Taurus , 1996 .

[24]  T. Kerr,et al.  High-Resolution Studies of Solid CO in the Taurus Dark Cloud: Characterizing the Ices in Quiescent Clouds , 1995 .

[25]  E. Herbst,et al.  The importance of classes of neutral-neutral reactions in the production of complex interstellar molecules , 1995 .

[26]  E. Herbst,et al.  New gas–grain chemical models of quiescent dense interstellar clouds: the effects of H2 tunnelling reactions and cosmic ray induced desorption , 1993 .

[27]  E. Herbst,et al.  Models of gas-grain chemistry in dense interstellar clouds with complex organic molecules , 1992 .

[28]  D. Gillespie.  A rigorous derivation of the chemical master equation , 1992 .

[29]  S. Prasad,et al.  A model for gas phase chemistry in interstellar clouds. I - The basic model, library of chemical reactions, and chemistry among C, N, and O compounds , 1980 .

[30]  D. Williams,et al.  Model for surface reactions on interstellar grains — A numerical study , 1977 .

[31]  M. Allen,et al.  The molecular composition of dense interstellar clouds , 1977 .

[32]  D. Gillespie.  A General Method for Numerically Simulating the Stochastic Time Evolution of Coupled Chemical Reactions , 1976 .

[33]  Eric Herbst,et al.  The formation and depletion of molecules in dense interstellar clouds , 1973 .

[34]  E. Dishoeck,et al.  Astrochemistry: From Molecular Clouds to Planetary Systems , 2000 .