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Mind Your Ps and Qs: The Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars

Article

Overview

Authors

  • Moe, Maxwell and Di Stefano, Rosanne

Abstract

  • 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 = {M}{comp}/M 1. 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 ? 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) ? 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) 1,q,P,e)\ne f({M}1)f(q)f(P)f(e) to the corrected distributions, which can be incorporated into binary population synthesis studies.

Published In

Publication Date

  • 2017

Identity

Digital Object Identifier (doi)

Additional Document Info

Start Page

  • 15

Volume

  • 230