135 research outputs found
Supersonic Gas Streams Enhance the Formation of Massive Black Holes in the Early Universe
The origin of super-massive black holes in the early universe remains poorly
understood.Gravitational collapse of a massive primordial gas cloud is a
promising initial process,but theoretical studies have difficulty growing the
black hole fast enough.We report numerical simulations of early black hole
formation starting from realistic cosmological conditions.Supersonic gas
motions left over from the Big Bang prevent early gas cloud formation until
rapid gas condensation is triggered in a proto-galactic halo. A protostar is
formed in the dense, turbulent gas cloud, and it grows by sporadic mass
accretion until it acquires 34,000 solar masses.The massive star ends its life
with a catastrophic collapse to leave a black hole -- a promising seed for the
formation of a monstrous black hole.Comment: Published in Science, combined with updated SOM, additional images
and movies are available at
http://www-utap.phys.s.u-tokyo.ac.jp/naoki.yoshida/Blackhole/0929e.htm
Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets
The population of close-in super-Earths, with gas mass fractions of up to 10%
represents a challenge for planet formation theory: how did they avoid runaway
gas accretion and collapsing to hot Jupiters despite their core masses being in
the critical range of ? Previous
three-dimensional (3D) hydrodynamical simulations indicate that atmospheres of
low-mass planets cannot be considered isolated from the protoplanetary disc,
contrary to what is assumed in 1D-evolutionary calculations. This finding is
referred to as the recycling hypothesis. In this Paper we investigate the
recycling hypothesis for super-Earth planets, accounting for realistic 3D
radiation hydrodynamics. Also, we conduct a direct comparison in terms of the
evolution of the entropy between 1D and 3D geometries. We clearly see that 3D
atmospheres maintain higher entropy: although gas in the atmosphere loses
entropy through radiative cooling, the advection of high entropy gas from the
disc into the Bondi/Hill sphere slows down Kelvin-Helmholtz contraction,
potentially arresting envelope growth at a sub-critical gas mass fraction.
Recycling, therefore, operates vigorously, in line with results by previous
studies. However, we also identify an "inner core" -- in size 25% of
the Bondi radius -- where streamlines are more circular and entropies are much
lower than in the outer atmosphere. Future studies at higher resolutions are
needed to assess whether this region can become hydrodynamically-isolated on
long time-scales.Comment: 16 pages, 12 figures, accepted for publication at MNRA
Radiation pressure feedback in the formation of massive stars
We investigate the radiation pressure feedback in the formation of massive
stars in 1, 2, and 3D radiation hydrodynamics simulations of the collapse of
massive pre-stellar cores. In contrast to previous research, we consider
frequency dependent stellar radiation feedback, resolve the dust sublimation
front in the vicinity of the forming star down to 1.27 AU, compute the
evolution for several 10^5 yrs covering the whole accretion phase of the
forming star, and perform a comprehensive survey of the parameter space. The
most fundamental result is that the formation of a massive accretion disk in
slowly rotating cores preserves a high anisotropy in the radiation field. The
thermal radiation escapes through the optically thin atmosphere, effectively
diminishing the radiation pressure feedback onto the accretion flow.
Gravitational torques in the self-gravitating disk drive a sufficiently high
accretion rate to overcome the residual radiation pressure. Simultaneously, the
radiation pressure launches an outflow in the bipolar direction, which grows in
angle with time and releases a substantial fraction of the initial core mass
from the star-disk system. Summarized, for an initial core mass of 60, 120,
240, and 480 Msol these mechanisms allow the star to grow up to 28.2, 56.5,
92.6, and at least 137.2 Msol respectively.Comment: 5 pages, 3 figures, Proceedings of the 39th Liege International
Astrophysical Colloquium: The multi-wavelength view of Hot, Massive Star
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