Does Deuterium Enable the Formation of Primordial Brown Dwarfs?

We investigate thermal and dynamical evolution of a primordial gas cloud with an updated deuterium chemistry. We consider a fragment of a postshock-cooled sheet that is expected to form by collapse of a massive cloud (≳10⁸ M_☉) and by blast waves due to supernova explosions. At first we investigate molecule formation in a primordial shock. We show that almost all deuterium can be converted to HD within the age of the universe at the collapsed redshift in the case of a cloud that has a virial temperature of ~10⁶ K and collapses at z > 1. When the postshock sheet fragments owing to gravitational instability, the fractional H₂ and HD abundances become ~10^-2 and ~10^-5, respectively, which are 10³-10⁴ times higher than the result of molecule formation in the expanding universe after recombination. To study the subsequent evolution of a fragment, we performed one-dimensional simulations of a spherical/cylindrical cloud, of which initial conditions (e.g., fractional abundances of chemical composition, temperature) are derived from the result of the shock. It is found that, in case of a cylindrical collapse, the cooling by HD molecules keeps the temperature of the cloud less than 100 K and the cloud evolves almost isothermally. When the cloud becomes optically thick to the HD line emission (~10¹⁰ cm^-3) and the gravitational fragmentation of the cylindrical cloud becomes effective, the Jeans mass becomes comparable to 0.1 M_☉. This series of processes enables the formation of primordial low-mass stars, and possibly brown dwarfs, in primordial gas clouds.