54 research outputs found
A Super-Alfvenic Model of Dark Clouds
Supersonic random motions are observed in dark clouds and are traditionally
interpreted as Alfven waves, but the possibility that these motions are
super-Alfvenic has not been ruled out. In this work we report the results of
numerical experiments in two opposite regimes; M_a ~ 1 and M_a >> 1, where M_a
is the initial Alfvenic Mach number --the ratio of the rms velocity to the
Alfven speed. Our results show that models with M_a >> 1 are consistent with
the observed properties of molecular clouds that we have tested --statistics of
extinction measurements, Zeeman splitting measurements of magnetic field
strength, line width versus integrated antenna temperature of molecular
emission line spectra, statistical B-n relation, and scatter in that relation--
while models with M_a ~ 1 have properties that are in conflict with the
observations. We find that both the density and the magnetic field in molecular
clouds may be very intermittent. The statistical distributions of magnetic
field and gas density are related by a power law, with an index that decreases
with time in experiments with decaying turbulence. After about one dynamical
time it stabilizes at B ~ n^{0.4}. Magnetically dominated cores form early in
the evolution, while later on the intermittency in the density field wins out,
and also cores with weak field can be generated, by mass accretion along
magnetic field lines.Comment: 10 figures, 2 tables include
The Formation of the First Stars in the Universe
In this review, I survey our current understanding of how the very first
stars in the universe formed, with a focus on three main areas of interest: the
formation of the first protogalaxies and the cooling of gas within them, the
nature and extent of fragmentation within the cool gas, and the physics -- in
particular the interplay between protostellar accretion and protostellar
feedback -- that serves to determine the final stellar mass.
In each of these areas, I have attempted to show how our thinking has
developed over recent years, aided in large part by the increasing ease with
which we can now perform detailed numerical simulations of primordial star
formation. I have also tried to indicate the areas where our understanding
remains incomplete, and to identify some of the most important unsolved
problems.Comment: 74 pages, 4 figures. Accepted for publication in Space Science
Review
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