CRA seminar (Dr. Gen Chiaki)

Title : Chemo–thermal evolution of metal-poor star-forming gas clouds

Abstract: The gravitational collapse of metal–poor gas clouds and star formation are investigated. Recently, the long-lived and metal–poor stars have been observed in the Galactic halo. Yet, the origin of these stars is still unknown. We perform three-dimensional hydrodynamics simulations to see the critical condition for which cloud fragmentation occurs and low-mass stars are formed for four clouds with various metallicities. We first consider accretion of gas-phase metal molecules onto grains as well as the all  relevant chemical reactions. The metal and dust amounts and dust size distribution are consistently calculated in the models of Population III supernova, which mainly supply grains in the early Universe. Also, radiative cooling by chemical species such as metal ions, atoms, and molecules is included.  The mass resolution becomes from hundreds of solar masses to tens of Earth masses when the first protostellar core appears in the collapse center. We find that, several decades after the first protostar formation, fragmentation does not occur in the vast majority of clouds even with metallicity higher than 10^{-4} Zsun (solar metallicity), where dust cooling is effective. With 10^{-5} Zsun, two of four clouds  undergo enough dust cooling. In one of these two clouds, however, rapid gas heating along with H_2  molecular formation prevents cloud elongation, which is the precursor of fragmentation. With 10^{-4} Zsun,  H_2 formation heating is important for all clouds, but OH/H2O molecular cooling enhances cloud elongation  in one of four cloud. If the cloud elongation is sufficiently promoted before dust cooling is important, the  cloud becomes unstable again to fragment. We also find that he different heating/cooling processes become important for different clouds because the overall thermal evolution of these clouds is different. The collapse timescale of these clouds is important to determine the collapse timescale.