The Formation of Population III Binaries from Cosmological Initial Conditions

Previous high resolution cosmological simulations predict the first stars to appear in the early universe to be very massive and to form in isolation. Here we discuss a cosmological simulation in which the central 50 solar mass clump breaks up into two cores, having a mass ratio of two to one, with one fragment collapsing to densities of 10^{-8} g/cc. The second fragment, at a distance of 800 astronomical units, is also optically thick to its own cooling radiation from molecular hydrogen lines, but is still able to cool via collision-induced emission. The two dense peaks will continue to accrete from the surrounding cold gas reservoir over a period of 10^5 years and will likely form a binary star system.Comment: Accepted by Science, first published online on July 9, 2009 in Science Express. 16 pages, 4 figures, includes supporting online materia

Dwarf Galaxies with Ionizing Radiation Feedback. I: Escape of Ionizing Photons

We describe a new method for simulating ionizing radiation and supernova feedback in the analogues of low-redshift galactic disks. In this method, which we call star-forming molecular cloud (SFMC) particles, we use a ray-tracing technique to solve the radiative transfer equation for ultraviolet photons emitted by thousands of distinct particles on the fly. Joined with high numerical resolution of 3.8 pc, the realistic description of stellar feedback helps to self-regulate star formation. This new feedback scheme also enables us to study the escape of ionizing photons from star-forming clumps and from a galaxy, and to examine the evolving environment of star-forming gas clumps. By simulating a galactic disk in a halo of 2.3e11 Msun, we find that the average escape fraction from all radiating sources on the spiral arms (excluding the central 2.5 kpc) fluctuates between 0.08% and 5.9% during a ~20 Myr period with a mean value of 1.1%. The flux of escaped photons from these sources is not strongly beamed, but manifests a large opening angle of more than 60 degree from the galactic pole. Further, we investigate the escape fraction per SFMC particle, f_esc(i), and how it evolves as the particle ages. We discover that the average escape fraction f_esc is dominated by a small number of SFMC particles with high f_esc(i). On average, the escape fraction from a SFMC particle rises from 0.27% at its birth to 2.1% at the end of a particle lifetime, 6 Myrs. This is because SFMC particles drift away from the dense gas clumps in which they were born, and because the gas around the star-forming clumps is dispersed by ionizing radiation and supernova feedback. The framework established in this study brings deeper insight into the physics of photon escape fraction from an individual star-forming clump, and from a galactic disk.Comment: 15 pages, 12 figures, Accepted for publication in the Astrophysical Journal, Image resolution reduced, High-resolution version of this article is available at http://www.jihoonkim.org/index/research.html#sfm

Dwarf Galaxies with Ionizing Radiation Feedback. II: Spatially-resolved Star Formation Relation

We investigate the spatially-resolved star formation relation using a galactic disk formed in a comprehensive high-resolution (3.8 pc) simulation. Our new implementation of stellar feedback includes ionizing radiation as well as supernova explosions, and we handle ionizing radiation by solving the radiative transfer equation rather than by a subgrid model. Photoheating by stellar radiation stabilizes gas against Jeans fragmentation, reducing the star formation rate. Because we have self-consistently calculated the location of ionized gas, we are able to make spatially-resolved mock observations of star formation tracers, such as H-alpha emission. We can also observe how stellar feedback manifests itself in the correlation between ionized and molecular gas. Applying our techniques to the disk in a galactic halo of 2.3e11 Msun, we find that the correlation between star formation rate density (estimated from mock H-alpha emission) and molecular hydrogen density shows large scatter, especially at high resolutions of <~ 75 pc that are comparable to the size of giant molecular clouds (GMCs). This is because an aperture of GMC size captures only particular stages of GMC evolution, and because H-alpha traces hot gas around star-forming regions and is displaced from the molecular hydrogen peaks themselves. By examining the evolving environment around star clusters, we speculate that the breakdown of the traditional star formation laws of the Kennicutt-Schmidt type at small scales is further aided by a combination of stars drifting from their birthplaces, and molecular clouds being dispersed via stellar feedback.Comment: 16 pages, 15 figures, Accepted for publication in the Astrophysical Journal, Image resolution greatly reduced, High-resolution version of this article is available at http://www.jihoonkim.org/index/research.html#sfm