A Simple and Accurate Network for Hydrogen and Carbon Chemistry in the ISM

Chemistry plays an important role in the interstellar medium (ISM), regulating heating and cooling of the gas, and determining abundances of molecular species that trace gas properties in observations.

Although solving the time-dependent equations is necessary for accurate abundances and temperature in the dynamic ISM, a full chemical network is too computationally expensive to incorporate in numerical simulations. In this paper, we propose a new simplified chemical network for hydrogen and carbon chemistry in the atomic and molecular ISM. We compare our chemical network in detail with results from a full photo-dissociation region (PDR) code, and also with the Nelson & Langer (1999) (NL99) network previously adopted in the simulation literature. We show that our chemical network gives similar results to the PDR code in the equilibrium abundances of all species over a wide range of densities, temperature, and metallicities, whereas the NL99 network shows significant disagreement.

Applying our network in 1D models, we find that the CO-dominated regime delimits the coldest gas and that the corresponding temperature tracks the cosmic ray ionization rate in molecular clouds.

We provide a simple fit for the locus of CO dominated regions as a function of gas density and column. We also construct a simple model to calculate the composition of turbulent molecular clouds in slab and spherical geometry, and compare the results to observations. We find that the observed abundances of H2, CO and CH may be far from chemical equilibrium, due to dynamical effects in molecular clouds.

Munan Gong, Eve C. Ostriker, Mark G. Wolfire
(Submitted on 27 Oct 2016)

Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:1610.09023 [astro-ph.GA] (or arXiv:1610.09023v1 [astro-ph.GA] for this version)
Submission history
From: Munan Gong
[v1] Thu, 27 Oct 2016 22:11:23 GMT (4450kb,D)

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