Evidence for a Mass-Transfer Origin for the Long-Period Blue Straggler Binaries in the Old Open Cluster NGC 188

Type Conference Paper
Names Robert D. Mathieu, A. M. Geller
Proceedings Title Bulletin of the American Astronomical Society
Conference Name American Astronomical Society, AAS Meeting #218, #217.07
Volume 43
Pages 21707
Date May 1, 2011
URL http://adsabs.harvard.edu/abs/2011AAS...21821707M
Library Catalog NASA ADS
Abstract The discovery of many blue straggler binaries with long orbital periods (of order 1000 days) in the old open cluster NGC 188 has focused attention on three theoretical models for blue straggler formation: i) mass transfer in binary stars, ii) stellar collisions during dynamical encounters of multiple star systems, or iii) mergers of inner binaries in primordial triples driven by the Kozai mechanism. A critical discriminant between these formation channels are the secondary stars of blue straggler binaries. The merger and collision scenarios predict main-sequence secondary stars, while the mass-transfer scenario predicts white dwarf secondary stars. We present the observed secondary-mass distribution for the long-period NGC 188 blue stragglers. The distribution is peaked around a mean value of 0.55 solar masses with a standard deviation of 0.2 solar masses. A Monte Carlo analysis rules out the hypothesis that this distribution is populated by main-sequence companions chosen randomly from a standard initial mass function (at > 99% confidence level)or a flat mass-ratio distribution (at the 95.5% confidence level), suggesting that many of the companions are likely stellar remnants, such as white dwarfs. Comparisons to a sophisticated N-body open cluster model reveal that the observed distributions of companion mass and eccentricity for the long-period NGC 188 blue straggler binaries are not consistent with an origin in collisions, as collision products are predicted to have significantly more massive companions and higher eccentricities than are observed. These findings are consistent with a mass-transfer origin for most, and possibly all, of the long-period blue straggler binaries in NGC 188. We gratefully acknowledge funding from the National Science Foundation under grant AST-0908082, and our collaborators of the WIYN Open Cluster Study.
UW-Madison Astronomy Home