Binary stars in Upper Scorpius

TL;DR

This study investigates binary star statistics in the 8-million-year-old Upper Scorpius region, revealing higher binary frequencies than in the field and notable differences in binary properties, especially at close separations.

Contribution

First comprehensive speckle-interferometric survey of binaries in Upper Scorpius, producing a catalog and analyzing binary statistics across a wide separation range.

Findings

Binary frequency is higher than in the field for certain mass ratios.

Close binaries (<100 au) dominate the excess in binary frequency.

Distribution of mass ratios varies with separation, showing more twins at smaller distances.

Abstract

To address the statistics of binary stars in the 8-Myr old Upper Scorpius star formation region, we conducted speckle-interferometric survey of 614 association members more massive than 0.4 Msun (spectral types earlier than M3V) based on the list of Luhman et al. (2018). We resolved 187 pairs, 55 of those are new discoveries. Using also the published data and the Gaia DR2, a catalog of 250 physical binaries is produced. We carefully estimated detection limits for each target and studied binary statistics in the separation range from 0.06" to 20" (9 to 2800 au), as well as clustering at larger separations. The frequency of companions with mass ratios q>0.3 in this separation range is 0.33+-0.04 and 0.35+-0.04 for early M and solar-type stars, respectively, larger by 1.62+-0.22 and 1.39+-0.18 times compared to field stars of similar masses. The excess is produced mostly by pairs closer than 100 au. At separations from 100 to 10^4 au, the separation distribution and companion fraction resemble those of solar-type stars in the field. However, unlike in the field, we see a relative deficit of equal-mass binaries at separations of ~500 au, compared to smaller and larger separations. The distribution of q depends on the separation, with a preference of larger q and a larger fraction of twins with q>0.95 at smaller separations. The binary population of Upper Scorpius differs from binaries in the field in several ways and suggests that binary statistics is not universal.