Binary Formation in Star-Forming Clouds with Various Metallicities

Cloud evolution for various metallicities is investigated by three-dimensional nested grid simulations, in which the initial ratio of rotational to gravitational energy of the host cloud \beta_0 (=10^-1 - 10^-6) and cloud metallicity Z (=0 - Z_\odot) are parameters. Starting from a central number density of n = 10^4 cm^-3, cloud evolution for 48 models is calculated until the protostar is formed (n \simeq 10^23 cm^-3) or fragmentation occurs. The fragmentation condition depends both on the initial rotational energy and cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation tends to be suppressed in clouds with higher metallicity. Fragmentation occurs when \beta_0 > 10^-3 in clouds with solar metallicity, while fragmentation occurs when \beta_0 > 10^-5 in the primordial gas cloud. Clouds with lower metallicity have larger probability of fragmentation, which indicates that the binary frequency is a decreasing function of cloud metallicity. Thus, the binary frequency at the early universe (or lower metallicity environment) is higher than at present day (or higher metallicity environment). In addition, binary stars born from low-metallicity clouds have shorter orbital periods than those from high-metallicity clouds. These trends are explained in terms of the thermal history of the collapsing cloud.Comment: 11 pages, 2 figures, Submitted to ApJL, For high resolution figures see http://astro3.sci.hokudai.ac.jp/~machida/binary-metal.pd

Formation Process of the Circumstellar Disk: Long-term Simulations in the Main Accretion Phase of Star Formation

The formation and evolution of the circumstellar disk in unmagnetized molecular clouds is investigated using three-dimensional hydrodynamic simulations from the prestellar core until the end of the main accretion phase. In collapsing clouds, the first (adiabatic) core with a size of ~10AU forms prior to the formation of the protostar. At its formation, the first core has a thick disk-like structure, and is mainly supported by the thermal pressure. After the protostar formation, it decreases the thickness gradually, and becomes supported by the centrifugal force. We found that the first core is a precursor of the circumstellar disk. This indicates that the circumstellar disk is formed before the protostar formation with a size of ~10AU, which means that no protoplanetary disk smaller than <10AU exists. Reflecting the thermodynamics of the collapsing gas, at the protostar formation epoch, the circumstellar disk has a mass of ~0.01-0.1 solar mass, while the protostar has a mass of ~10^-3 solar mass. Thus, just after the protostar formation, the circumstellar disk is about 10-100 times more massive than the protostar. Even in the main accretion phase that lasts for ~10^5yr, the circumstellar disk mass dominates the protostellar mass. Such a massive disk is unstable to gravitational instability, and tends to show fragmentation. Our calculations indicate that the planet or brown-dwarf mass object may form in the circumstellar disk in the main accretion phase. In addition, the mass accretion rate onto the protostar shows strong time variability that is caused by the perturbation of proto-planets and/or the spiral arms in the circumstellar disk. Such variability provides a useful signature for detecting the planet-sized companion in the circumstellar disk around very young protostars.Comment: 32 pages, 11 figures, Submitted to ApJ. For high resolution figures see http://www2-tap.scphys.kyoto-u.ac.jp/~machidam/astro-ph/CircumstellarDisk.pd