Twins Among the Low Mass Spectroscopic Binaries

TL;DR

This study analyzes the occurrence of nearly equal-mass binary star systems, called twins, among spectroscopic binaries with spectral types F or later, finding they are most common in short-period systems and estimating their overall frequency.

Contribution

It provides the first estimate of twin frequency in spectroscopic binaries and discusses the possible formation mechanisms leading to twin systems.

Findings

Twins are most common in binaries with periods less than 43 days.

The estimated frequency of twins in spectroscopic binaries is about 3%.

Twins do not differ significantly from other binaries in period, mass, or eccentricity distributions.

Abstract

We report an analysis of twins of spectral types F or later in the 9th Catalog of Spectroscopic Binaries (SB9). Twins, the components of binaries with mass ratio within 2% of 1.0, are found among the binaries with primaries of F and G spectral type. They are most prominent among the binaries with periods less than 43 days, a cutoff first identified by Lucy. Within the subsample of binaries with P<43 days, the twins do not differ from the other binaries in their distributions of periods (median P~7d), masses, or orbital eccentricities. Combining the mass ratio distribution in the SB9 in the mass range 0.6 to 0.85 Msun with that measured by Mazeh et al. for binaries in the Carney-Latham high proper motion survey, we estimate that the frequency of twins in a large sample of spectroscopic binaries is about 3%. Current theoretical understanding indicates that accretion of high specific angular momentum material by a protobinary tends to equalize its masses. We speculate that the excess of twins is produced in those star forming regions where the accretion processes were able to proceed to completion for a minority of protobinaries. This predicts that the components of a young twin may appear to differ in age and that, in a sample of spectroscopic binaries in a star formation region, the twins are, on average, older than the binaries with mass ratios much smaller than 1.