V474 Car: A Rare Halo RS CVn Binary in Retrograde Galactic Orbit

Abstract

We report the discovery that the star V474 Car is an extremely active, high velocity halo RS CVn system. The star was originally identified as a possible pre-main sequence star in Carina, given its enhanced stellar activity, rapid rotation (10.3 days), enhanced Li, and absolute magnitude that places it above the main sequence. However, its extreme radial velocity (264 km s$^{-1}$) suggested that this system was unlike any previously known pre-MS system. Our detailed spectroscopic analysis of echelle spectra taken with the CTIO 4-m finds that V474 Car is both a spectroscopic binary with orbital period similar to the photometric rotation period, and metal poor ([Fe/H] $\simeq -$0.99). The star's Galactic orbit is extremely eccentric (e $\simeq$ 0.93) with perigalacticon of only $\sim$0.3 kpc of the Galactic center - and its eccentricity and smallness of its perigalacticon are only surpassed by $\sim$0.05%, of local F/G-type field stars. The observed characteristics are consistent with V474 Car being a high velocity, metal poor, tidally-locked chromospherically active binary (CAB), i.e.\ a halo RS CVn binary, and one of only a few such specimens known.