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