The Primordial Binary Population - I: A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2

TL;DR

This study uses near-infrared adaptive optics to identify close stellar companions to A-type stars in Sco OB2, revealing new companions, a specific mass ratio distribution, and implications for binary formation theories.

Contribution

First adaptive optics survey to characterize close companions to Sco OB2 A-type stars, providing new data on companion frequency and mass distribution.

Findings

Identified 74 candidate companions, 41 of which are new.

Mass ratio distribution follows f(q) = q^-0.33, excluding random pairing.

No close companions with Ks between 12 and 14, suggesting a lower mass limit.

Abstract

We present the results of a near-infrared adaptive optics survey with the aim to detect close companions to Hipparcos members in the three subgroups of the nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type stars in the Ks band, and a subset also in the J and H band. We find 151 stellar components other than the target stars. A brightness criterion is used to separate these components into 77 background stars and 74 candidate physical companion stars. Out of these 74 candidate companions, 41 have not been reported before (14 in US; 13 in UCL; 14 in LCC). Companion star masses range from 0.1 to 3 Msun. The mass ratio distribution follows f(q) = q^-0.33, which excludes random pairing. No close (rho < 3.75'') companion stars or background stars are found in the magnitude range 12 < Ks < 14. The lack of stars with these properties cannot be explained by low-number statistics, and may imply a lower limit on the companion mass of ~ 0.1 Msun. Close stellar components with Ks > 14 are observed. If these components are very low-mass companion stars, a gap in the companion mass distribution might be present. The small number of close low-mass companion stars could support the embryo-ejection formation scenario for brown dwarfs. Our findings are compared with and complementary to visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an overall companion star fraction of 0.52 in this association. This paper is the first step toward our goal to derive the primordial binary population in Sco OB2.