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Abstract: Studies of the formation of metal-free Population III stars usually focusprimarily on the role played by H2 cooling, on account of its large chemicalabundance relative to other possible molecular or ionic coolants. However,while H2 is generally the most important coolant at low gas densities, it isnot an effective coolant at high gas densities, owing to the low criticaldensity at which it reaches local thermodynamic equilibrium LTE and to thelarge opacities that develop in its emission lines. It is therefore possiblethat emission from other chemical species may play an important role in coolinghigh density primordial gas. A particularly interesting candidate is the H3+molecular ion. This ion has an LTE cooling rate that is roughly a billion timeslarger than that of H2, and unlike other primordial molecular ions such as H2+or HeH+, it is not easily removed from the gas by collisions with H or H2. Itis already known to be an important coolant in at least one astrophysicalcontext - the upper atmospheres of gas giants - but its role in the coolingof primordial gas has received little previous study. In this paper, weinvestigate the potential importance of H3+ cooling in primordial gas using anewly-developed H3+ cooling function and the most detailed model of primordialchemistry published to date. We show that although H3+ is, in mostcircumstances, the third most important coolant in dense primordial gas afterH2 and HD, it is nevertheless unimportant, as it contributes no more than afew percent of the total cooling. We also show that in gas irradiated by asufficiently strong flux of cosmic rays or X-rays, H3+ can become the dominantcoolant in the gas, although the size of the flux required renders thisscenario unlikely to occur.

Author: S. C. O. Glover, D. W. Savin


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