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Astron. Astrophys. 334, 1047-1055 (1998)

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Bistability in large chemical networks: a global view

H.-H. Lee 1, E. Roueff 2, G. Pineau des Forêts 2, O.M. Shalabiea 1, 3, R. Terzieva 4 and Eric Herbst 1, 5

1 Department of Physics, The Ohio State University, Columbus, OH 43210, USA
2 DAEC, Observatoire de Paris, F-92195 Meudon Principal Cedex, France
3 Department of Astronomy, Faculty of Science, Cairo University, Egypt
4 Department of Chemistry and Chemical Physics Program, The Ohio State University, Columbus, OH 43210, USA
5 Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA

Received 22 August 1997 / Accepted 29 January 1998

Abstract

The nature of bistability in large gas-phase chemical networks of dense interstellar clouds at 10 K is examined. The dependence of bistability on the parameter [FORMULA], the cosmic ray ionization rate divided by the total hydrogen density, for a wide range of elemental depletions is investigated in detail. We confirm that bistability can exist at steady-state for a range of [FORMULA], but we also confirm that the range of bistability is very dependent on elemental depletions, and also dependent on which network is utilized. In particular, bistability is a more salient feature in the new neutral-neutral model than it is in the new standard model. With the former model, we find that for some gas-phase elemental abundances, the bistability range is non-existent while for others the bistability range includes gas densities as high as 1 105 cm-3 assuming a standard value for [FORMULA]. When all of our new neutral-neutral model results are plotted on one diagram with the fractional electron abundance as ordinate and the parameter [FORMULA] as abscissa, it is found that bistability is confined to a vertical band which is narrower at small [FORMULA] (high densities). Above and below the band lie the so-called "high metal" and "low metal" single-phase results. The intermediate electron abundances at which bistability occurs are best obtained by relatively high abundances of the element sulfur because this element, unlike real metals, is a "soft" ionizer; i.e., its abundance is not totally in ionized forms. We present newly-obtained steady-state results for a variety of molecules in both the HIP (high ionization phase) and LIP (low ionization phase) solutions for a bistable model at a rather high density near 3 104 cm-3 with a standard cosmic ray ionization rate. Both the steady-state results as well as a variety of early-time results are compared with observations in TMC-1 and L134N.

Key words: ISM: clouds – ISM: abundances – ISM: molecules – molecular processes

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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