Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009a%26a...508..783l&link_type=abstract
Astronomy and Astrophysics, Volume 508, Issue 2, 2009, pp.783-786
Astronomy and Astrophysics
Astrophysics
3
Astrochemistry, Ism: Molecules
Scientific paper
Context: Agreement between observed and calculated abundances of some molecules in dark clouds is improved when the model gas retains some memory of the assumed initial conditions. Aims: We assess the relevance of commonly-assumed intitial conditions giving rise to so-called early-time chemistry in models of dark cloud chemistry. Methods: Initial conditions for models of dark cloud chemistry are compared with conditions known to exist in the H2-bearing diffuse gas whose evolution they are intended to represent. Results: The initial conditions assumed for dark cloud chemistry differ in at least five important regards from those of H2-bearing diffuse gas. Models typically assume: absolute purity of H2; absence of polycyclic aromatic hydrocarbons; two times smaller gas-phase abundance of C, N, and O; near-absence of sulfur and some other easily-ionized metals; total absence of the molecules which accompany the presence of H2 in diffuse gas, for instance, OH, CH, C2H, HCO^+. Conclusions: Initial conditions commonly assumed for dark cloud chemical models should be reconsidered and mechanisms for the chemical evolution from diffuse to dark gas should be included in model calculations.
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