166 research outputs found
Blue Straggler Formation in Clusters
Blue stragglers are thought to be formed from the merger or coalescence of
two stars, but the details of their formation in clusters has been difficult to
disentangle. We discuss the two main formation mechanisms for blue stragglers
(stellar collisions or mass transfer in a binary system). We then look at the
additional complications caused by the stars living in the dynamically active
environment of a star cluster. We review the recent observational and
theoretical work which addresses the question "which mechanism dominates?" and
conclude that the most likely answer is that both mechanisms are at work,
although with different importance in different environments and at different
times in the cluster lifetime. We finish with a short discussion of some
avenues for future workComment: 8 pages, 3 figures, to appear in the proceedings of the conference
"Binary Star Evolution: Mass Loss, Accretion, and Mergers" at Mykonos,
Greece, June 22-25, 201
Stellar Encounters with Multiple Star Systems and the Blue Straggler Problem
We present a technique to identify the most probable dynamical formation
scenario for an observed binary or triple system containing one or more merger
products or, alternatively, to rule out the possibility of a dynamical origin.
Our method relies on an analytic prescription for energy conservation during
stellar encounters. With this, observations of the multiple star system
containing the merger product(s) can be used to work backwards in order to
constrain the initial orbital energies of any single, binary or triple systems
that went into the encounter. The initial semi-major axes of the orbits provide
an estimate for the collisional cross section and therefore the time-scale for
the encounter to occur in its host cluster. We have applied our analytic
prescription to observed binary and triple systems containing blue stragglers,
in particular the triple system S1082 in M67 and the period distribution of the
blue straggler binaries in NGC 188. We have shown that both S1082 and most of
the blue straggler binaries in NGC 188 could have a dynamical origin, and that
encounters involving triples are a significant contributor to blue straggler
populations in old open clusters. In general, our results suggest that
encounters involving triples could make up a significant fraction of those
dynamical interactions that result in stellar mergers, in particular encounters
that produce multiple star systems containing one or more blue stragglers.Comment: 8 pages, 2 figures; to be published in the proceedings of the Binary
Star Evolution Conference held in Mykonos, Greece from June 22-25, 201
Evolving Molecular Cloud Structure and the Column Density Probability Distribution Function
The structure of molecular clouds can be characterized with the probability
distribution function (PDF) of the mass surface density. In particular, the
properties of the distribution can reveal the nature of the turbulence and star
formation present inside the molecular cloud. In this paper, we explore how
these structural characteristics evolve with time and also how they relate to
various cloud properties as measured from a sample of synthetic column density
maps of molecular clouds. We find that, as a cloud evolves, the peak of its
column density PDF will shift to surface densities below the observational
threshold for detection, resulting in an underlying lognormal distribution
which has been effectively lost at late times. Our results explain why certain
observations of actively star-forming, dynamically older clouds, such as the
Orion molecular cloud, do not appear to have any evidence of a lognormal
distribution in their column density PDFs. We also study the evolution of the
slope and deviation point of the power-law tails for our sample of simulated
clouds and show that both properties trend towards constant values, thus
linking the column density structure of the molecular cloud to the surface
density threshold for star formation.Comment: 10 pages, 9 figures, Accepted for publication by MNRA
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