Proper Motion of the Faint Star near KIC 8462852 (Boyajian's Star) - Not a Binary System

TL;DR

This study measures the proper motion of a faint star near KIC 8462852, demonstrating it is not gravitationally bound, thus clarifying the nature of the nearby star and its relation to Boyajian's Star.

Contribution

The paper provides the first proper motion measurement of the faint star near KIC 8462852, conclusively showing it is not a binary companion.

Findings

The faint star moved 67 mas since 2014.

Proper motion indicates the star is not gravitationally bound.

The star's velocity exceeds the circular velocity at current separation.

Abstract

A faint star located 2 arcsec from KIC 8462852 was discovered in Keck 10 m adaptive optics imaging in the $JHK$ near-infrared (NIR) in 2014 by Boyajian et al. (2016). The closeness of the star to KIC 8462852 suggested the two could constitute a binary, which might have implications for the cause of the brightness dips seen by {\it Kepler} (Boyajian et al. (2016) and in ground-based optical studies Boyajian et al. (2018). Here, NIR imaging in 2017 using the Mimir instrument resolved the pair and enabled measuring their separation. The faint star had moved $67 \pm 7$ milliarcsec (mas) relative to KIC 8462852 since 2014. The relative proper motion of the faint star is $23.9 \pm 2.6$ mas yr$^{-1}$, for a tangential velocity of $45 \pm 5$ km s$^{-1}$ if it is at the same 390 pc distance as KIC 8462852. Circular velocity at the 750 AU current projected separation is $1.5$ km s$^{-1}$, hence the star pair cannot be bound.