Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006newar..50...19y&link_type=abstract
New Astronomy Reviews, Volume 50, Issue 1-3, p. 19-23.
Astronomy and Astrophysics
Astronomy
21
Scientific paper
We present the results from three-dimensional radiation-hydrodynamics calculations of the formation and evolution of HII regions around the first stars. We carry out ray-tracing simulations of ionizing radiation transfer by embedding a population III star at the center of the primordial gas cloud formed in a small dark halo. In the case with a very massive (˜200 M&sun;) star, ionization front makes a transition from D-type to R-type within the lifetime of the central star, and a large HII region with diameter ˜1 kpc is formed. The photo-ionized and photo-heated halo gas is evacuated at a velocity of ˜30 km/s, but falls back to the halo when the halo has grown to ˜108 M&sun; in about 100 Myrs. We follow the chemical evolution of the photo-ionized gas in the fossil HII region. In our new non-equilibrium chemistry solver, we include deuterium chemistry and cooling by HD molecules. The fractionation of HD/H2 occurs when the gas temperature becomes as low as 100 K, and the HD fraction reaches 10-5 in the case with no background UV radiation. The gas then cools by HD line cooling down to a few tens Kelvin. Interestingly, at high redshift (z > 10), the minimum gas temperature is floored by that of the Cosmic Microwave Background Radiation. The characteristic mass of the collapsing gas clump is found to be ˜10 M&sun;, in an interesting contrast to a typical clump mass of ˜1000 M&sun; of the first primordial gas clouds.
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