Comoving stars in Gaia DR1: An abundance of very wide separation co-moving pairs

TL;DR

This paper presents a catalog of over 13,000 high-confidence comoving star pairs identified in Gaia DR1, revealing many very wide separation pairs and new stellar groups, which can inform galactic dynamics and stellar models.

Contribution

The study introduces a probabilistic method to identify comoving pairs in Gaia DR1, discovering numerous wide separation pairs and new stellar groups not previously known.

Findings

Identified 13,085 comoving star pairs in Gaia DR1.

Found many pairs with separations greater than 1 parsec.

Discovered several new comoving stellar groups.

Abstract

The primary sample of the {\it Gaia} Data Release 1 is the Tycho-Gaia Astrometric Solution (TGAS): $\approx$ 2 million Tycho-2 sources with improved parallaxes and proper motions relative to the initial catalog. This increased astrometric precision presents an opportunity to find new binary stars and moving groups. We search for high-confidence comoving pairs of stars in TGAS by identifying pairs of stars consistent with having the same 3D velocity using a marginalized likelihood ratio test to discriminate candidate comoving pairs from the field population. Although we perform some visualizations using (bias- corrected) inverse parallax as a point estimate of distance, the likelihood ratio is computed with a probabilistic model that includes the covariances of parallax and proper motions and marginalizes the (unknown) true distances and 3D velocities of the stars. We find 13,085 comoving star pairs among 10,606 unique stars with separations as large as 10 pc (our search limit). Some of these pairs form larger groups through mutual comoving neighbors: many of these pair networks correspond to known open clusters and OB associations, but we also report the discovery of several new comoving groups. Most surprisingly, we find a large number of very wide ($>1$ pc) separation comoving star pairs, the number of which increases with increasing separation and cannot be explained purely by false-positive contamination. Our key result is a catalog of high-confidence comoving pairs of stars in TGAS. We discuss the utility of this catalog for making dynamical inferences about the Galaxy, testing stellar atmosphere models, and validating chemical abundance measurements.