A new L-dwarf member of the moderately metal-poor triple system HD 221356

TL;DR

This paper reports the discovery of a new L1+/-1 spectral type co-moving companion in the metal-poor HD 221356 system, providing insights into low-mass stellar and substellar objects near the hydrogen burning limit.

Contribution

The discovery of a fourth component in the HD 221356 system, including its spectral classification and mass estimation, is a novel addition to known multiple star systems.

Findings

New co-moving L1+/-1 companion identified

Companion's mass estimated near the stellar-substellar boundary

System's low metallicity affects companion's colors

Abstract

We report on the discovery of a fourth component in the HD 221356 star system, previously known to be formed by an F8V, slightly metal-poor primary ([Fe/H]=-0.26), and a distant M8V+L3V pair. In our ongoing common proper motion search based on VISTA Hemisphere Survey (VHS) and 2MASS catalogues, we have detected a faint (J=13.76+/-0.04 mag) co-moving companion of the F8 star located at angular separation of 12.13+/-0.18 arcsec (position angle of 221.8+/-1.7), corresponding to a projected distance of ~312 AU at 26 pc. Near-infrared spectroscopy of the new companion, covering the 1.5-2.4 micron wavelength range with a resolving power of R~600, indicates an L1+/-1 spectral type. Using evolutionary models the mass of the new companion is estimated at ~0.08 solar masses, which places the object close to the stellar-substellar borderline. This multiple system provides an interesting example of objects with masses slightly above and below the hydrogen burning mass limit. The low mass companions of HD 221356 have slightly bluer colours than field dwarfs with similar spectral type, which is likely a consequence of the sub-solar metallicity of the system.