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Issue A&A Volume 370, Number 3, May II 2001 Page(s) 1044 - 1055 Section Diffuse matter in space DOI 10.1051/0004-6361:20010323

A&A 370, 1044-1055 (2001)
DOI: 10.1051/0004-6361:20010323

The effect of the initial elemental abundance on gas-grain chemical models

O. M. Shalabiea<sup>1, 2

1</sup>  Astronomy Program, SEES, Seoul National University, Seoul 151-742, Korea
²  Department of Astronomy, Cairo University, Cairo, Egypt

(Received 27 October 2000 / Accepted 16 February 2001)

Abstract
For any chemical modeling, it is important to recognize that the adopted set of initial elemental abundances is a crucial parameter. The effect of initial abundance variation has been investigated. Using the most recent observations and theoretical grain models, we have set some constraints upon the set of the initial elemental abundances. Both gas-phase and gas-grain chemical models are used in this study. At early-time stages less than 1 Myr, there is little difference between results with different initial [C]/[O] ratios. This holds for gas-phase and gas-grain models. At a later evolutionary time or in the steady state, the result of the gas-grain model shows little or no dependence on the initial [C]/[O] ratios. By contrast, at late or steady-state times, the abundances of chemical species using gas-phase models are very sensitive to any variation of the initial [C]/[O] ratios. Sulfur depletion is needed for both gas-phase and gas-grain models to reproduce the observed sulfur-bearing molecules. Our main conclusion is that the gas-grain interaction processes such as accretion, surface reactions, and desorption minimize the vital role of the initial set of elemental abundance in gas-grain chemical models.

Key words: molecular processes -- ISM: abundances, clouds, molecules -- Dust

© ESO 2001

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