Friday, March 1, 2013

1302.7284 (Sourav Chatterjee et al.)

Stellar Collisions and Blue Straggler Stars in Dense Globular Clusters    [PDF]

Sourav Chatterjee, Frederic A. Rasio, Alison Sills, Evert Glebbeek
Blue straggler stars (BSS) are abundantly observed in all Galactic globular clusters (GGC) where data exist. However, observations alone cannot reveal the relative importance of various formation channels or the typical formation times for this well studied population of non-standard stars. Using a state of the art H\'enon-type Monte Carlo code that includes all relevant physical processes, and observationally motivated initial conditions in large ranges in density, concentration, binary fraction, and mass, we create a little over 100 models with properties typical of the observed GGCs. These models can be directly compared with the observed GGCs and BSSs can be easily identified. We identify the BSSs in our models and determine their formation channels and times of creation. We find that for central densities above 10^3 Msun/pc^3 the dominant formation channel is stellar collisions. The majority of these collisions happen via binary-mediated strong scattering encounters. As a result a strong correlation between the total number of BSSs and the binary fraction in a cluster can be observed. We find that the number of BSSs in the core show weak correlation with the collision rate Gamma. Close inspection of our models reveals that the observed lack of correlation between the number of BSSs and Gamma for the observed GGCs may be because the estimate of Gamma using observed cluster properties is not a reliable indicator of the true Gamma. Our models indicate that the BSSs observed today may have formed many Gyrs ago. Denser clusters have higher contributions from collisions which typically create higher mass BSSs compared to those created via mass-transfer in a binary. As a result, we find that the BSSs in denser clusters typically have formed more recently than those in sparser clusters.
View original: http://arxiv.org/abs/1302.7284

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