Other
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...432..485r&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 432, no. 2, p. 485-495
Other
5
Chemical Equilibrium, Galactic Evolution, Hydrodynamics, Intergalactic Media, Thermochemical Properties, Thermochemistry, Thermodynamic Equilibrium, Atomic Excitations, Charge Exchange, Helium, Hydrogen, Interstellar Matter, Molecular Excitation, Photodissociation, Photoionization, Recombination Reactions
Scientific paper
The thermochemical equilibrium states of pregalactic homogeneous clouds have been calculated as functions of the particle number density. Attention is mainly focused on the dependence of these equilibrium states upon five free parameters: the external flux level, its spectral shape, the mass of the cloud, the net cosmic-ray ionization rate, and the abundance by number of He nuclei. The cloud is assumed to be constituted by a gas with nine chemical species: H, H2, H(-), e(-), H(+), H2(+), He, He(+), and He(++). A total of 24 chemical reactions are considered. These reactions include processes of photoionization, photodissociation, recombination, and charge exchange. The equilibrium is calculated considering heating rates due to photodissociation and photoionization, and cooling rates due to collisional excitation of atomic and molecular species. In the range of density under consideration, there are at least two zones where the gas pressure decreases as the density increases, and where, in general, there are many thermochemical equilibrium states for the same density. Depending on the adopted values for the free parameters, self-shielding of the dissociating radiation produces a third many-valued zone between the two mentioned above. The physical relevance of the equilibrium states is discussed. In particular, the results suggest that for M approximately 109 solar mass, the onset of the gravitational collapse could be triggered by hydrogen recombination effects. It is also shown that a cloud of 107 solar mass cooling isobarically from a high-temperature, high-ionization state reaches thermochemical equilibrium in a relatively short period of time (less than 3 x 107 yr) if the pressure is below a critical value. Otherwise, a gravitationally unstable state is reached. These time-dependent calculations show that the results for the thermochemical equilibrium states help us to infer the final state toward which nonequilibrium configurations evolve.
Iba~nez Miguel H. S.
Izotov Yu. I.
Parravano Antonio
Rosenzweig Patricia
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