Astrophys. J., Suppl. Ser., 230, 15-15 (2017/June-0)
Mind your Ps and Qs: the interrelation between period (P) and mass-ratio (q) distributions of binary stars.
MOE M. and DI STEFANO R.
Abstract (from CDS):
We compile observations of early-type binaries identified via spectroscopy, eclipses, long-baseline interferometry, adaptive optics, common proper motion, etc. Each observational technique is sensitive to companions across a narrow parameter space of orbital periods P and mass ratios q = Mcomp/M1. After combining the samples from the various surveys and correcting for their respective selection effects, we find that the properties of companions to O-type and B-type main-sequence (MS) stars differ among three regimes. First, at short orbital periods P <= 20 days (separations a <= 0.4 au), the binaries have small eccentricities e <= 0.4, favor modest mass ratios <q>~0.5, and exhibit a small excess of twins q > 0.95. Second, the companion frequency peaks at intermediate periods log P (days) ≃ 3.5 (a ≃ 10 au), where the binaries have mass ratios weighted toward small values q ≃ 0.2-0.3 and follow a Maxwellian "thermal" eccentricity distribution. Finally, companions with long orbital periods log P (days) ≃ 5.5-7.5 (a ≃ 200-5000 au) are outer tertiary components in hierarchical triples and have a mass ratio distribution across q ≃ 0.1-1.0 that is nearly consistent with random pairings drawn from the initial mass function. We discuss these companion distributions and properties in the context of binary-star formation and evolution. We also reanalyze the binary statistics of solar-type MS primaries, taking into account that 30% ± 10% of single-lined spectroscopic binaries likely contain white dwarf companions instead of low-mass stellar secondaries. The mean frequency of stellar companions with q > 0.1 and log P (days) < 8.0 per primary increases from 0.50 ± 0.04 for solar-type MS primaries to 2.1 ± 0.3 for O-type MS primaries. We fit joint probability density functions f(M1,q,P,e)≠f(M1)f(q)f(P)f(e) to the corrected distributions, which can be incorporated into binary population synthesis studies.
© 2017. The American Astronomical Society. All rights reserved.
binaries: close - binaries: general - stars: evolution - stars: formation - stars: massive - stars: statistics - stars: statistics
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