288 research outputs found
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
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