403,270 research outputs found
The role of stellar collisions for the formation of massive stars
We use direct N-body simulations of gas embedded star clusters to study the
importance of stellar collisions for the formation and mass accretion history
of high-mass stars. Our clusters start in virial equilibrium as a mix of gas
and proto-stars. Proto-stars then accrete matter using different mass accretion
rates and the amount of gas is reduced in the same way as the mass of stars
increases. During the simulations we check for stellar collisions and we
investigate the role of these collisions for the build-up of high-mass stars
and the formation of runaway stars. We find that a significant number of
collisions only occur in clusters with initial half-mass radii r_h < 0.1 pc.
After emerging from their parental gas clouds, such clusters end up too compact
compared to observed young, massive open clusters. In addition, collisions lead
mainly to the formation of a single runaway star instead of the formation of
many high mass stars with a broad mass spectrum. We therefore conclude that
massive stars form mainly by gas accretion, with stellar collisions only
playing a minor role if any at all. Collisions of stars in the pre-main
sequence phase might however contribute to the formation of the most massive
stars in the densest star clusters and possibly to the formation of
intermediate-mass black holes with masses up to a few 100 Msun.Comment: 10 pages, 8 figures, MNRAS in pres
The hierarchical formation of a stellar cluster
Recent surveys of star forming regions have shown that most stars, and
probably all massive stars, are born in dense stellar clusters. The mechanism
by which a molecular cloud fragments to form several hundred to thousands of
individual stars has remained elusive. Here, we use a numerical simulation to
follow the fragmentation of a turbulent molecular cloud and the subsequent
formation and early evolution of a stellar cluster containing more than 400
stars. We show that the stellar cluster forms through the hierarchical
fragmentation of a turbulent molecular cloud. This leads to the formation of
many small subclusters which interact and merge to form the final stellar
cluster. The hierarchical nature of the cluster formation has serious
implications in terms of the properties of the new-born stars. The higher
number-density of stars in subclusters, compared to a more uniform distribution
arising from a monolithic formation, results in closer and more frequent
dynamical interactions. Such close interactions can truncate circumstellar
discs, harden existing binaries, and potentially liberate a population of
planets. We estimate that at least one-third of all stars, and most massive
stars, suffer such disruptive interactions.Comment: 6 pages, 4 figures, accepted for publication in MNRAS. Version
including hi-res colour postscript figure available at
http://star-www.st-and.ac.uk/~sgv/ps/clufhier.ps.g
The Effect of Dark Matter on the First Stars: A New Phase of Stellar Evolution
Dark matter (DM) in protostellar halos can dramatically alter the current
theoretical framework for the formation of the first stars. Heat from
supersymmetric DM annihilation can overwhelm any cooling mechanism,
consequently impeding the star formation process and possibly leading to a new
stellar phase. The first stars to form in the universe may be ``dark stars'';
i.e., giant (larger than 1 AU) hydrogen-helium stars powered by DM annihilation
instead of nuclear fusion. Possibilities for detecting dark stars are
discussed.Comment: 3 pages, 2 figures, Proceedings for First Stars 2007 Conference in
Santa Fe, NM, July 200
What Sets the Initial Rotation Rates of Massive Stars?
The physical mechanisms that set the initial rotation rates in massive stars
are a crucial unknown in current star formation theory. Observations of young,
massive stars provide evidence that they form in a similar fashion to their
low-mass counterparts. The magnetic coupling between a star and its accretion
disk may be sufficient to spin down low-mass pre-main sequence (PMS) stars to
well below breakup at the end stage of their formation when the accretion rate
is low. However, we show that these magnetic torques are insufficient to spin
down massive PMS stars due to their short formation times and high accretion
rates. We develop a model for the angular momentum evolution of stars over a
wide range in mass, considering both magnetic and gravitational torques. We
find that magnetic torques are unable to spin down either low or high mass
stars during the main accretion phase, and that massive stars cannot be spun
down significantly by magnetic torques during the end stage of their formation
either. Spin-down occurs only if massive stars' disk lifetimes are
substantially longer or their magnetic fields are much stronger than current
observations suggest.Comment: 12 pages, 10 figures, Accepted for publication in Ap
Triggered Star Formation by Massive Stars
We present our diagnosis of the role that massive stars play in the formation
of low- and intermediate-mass stars in OB associations (the Lambda Ori region,
Ori OB1, and Lac OB1 associations). We find that the classical T Tauri stars
and Herbig Ae/Be stars tend to line up between luminous O stars and
bright-rimmed or comet-shaped clouds; the closer to a cloud the progressively
younger they are. Our positional and chronological study lends support to the
validity of the radiation-driven implosion mechanism, where the Lyman continuum
photons from a luminous O star create expanding ionization fronts to evaporate
and compress nearby clouds into bright-rimmed or comet-shaped clouds. Implosive
pressure then causes dense clumps to collapse, prompting the formation of
low-mass stars on the cloud surface (i.e., the bright rim) and
intermediate-mass stars somewhat deeper in the cloud. These stars are a
signpost of current star formation; no young stars are seen leading the
ionization fronts further into the cloud. Young stars in bright-rimmed or
comet-shaped clouds are likely to have been formed by triggering, which would
result in an age spread of several megayears between the member stars or star
groups formed in the sequence.Comment: 2007, ApJ, 657, 88
Star formation environments and the distribution of binary separations
We have carried out K-band speckle observations of a sample of 114 X-ray
selected weak-line T Tauri stars in the nearby Scorpius-Centaurus OB
association. We find that for binary T Tauri stars closely associated to the
early type stars in Upper Scorpius, the youngest subgroup of the OB
association, the peak in the distribution of binary separations is at 90 A.U.
For binary T Tauri stars located in the direction of an older subgroup, but not
closely associated to early type stars, the peak in the distribution is at 215
A.U. A Kolmogorov-Smirnov test indicates that the two binary populations do not
result from the same distibution at a significance level of 98%. Apparently,
the same physical conditions which facilitate the formation of massive stars
also facilitate the formation of closer binaries among low-mass stars, whereas
physical conditions unfavorable for the formation of massive stars lead to the
formation of wider binaries among low-mass stars. The outcome of the binary
formation process might be related to the internal turbulence and the angular
momentum of molecular cloud cores, magnetic field, the initial temperature
within a cloud, or - most likely - a combination of all of these. We conclude
that the distribution of binary separations is not a universal quantity, and
that the broad distribution of binary separations observed among main-sequence
stars can be explained by a superposition of more peaked binary distributions
resulting from various star forming environments. The overall binary frequency
among pre-main-sequence stars in individual star forming regions is not
necessarily higher than among main-sequence stars.Comment: 7 pages, Latex, 4 Postscript figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/pubs/pubs.html ; accepted for
publication in ApJ Letter
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