The double degenerate system NLTT 11748

TL;DR

This paper reports the discovery of a close binary system involving an extremely low-mass white dwarf and a likely more massive white dwarf companion, with detailed orbital and spectral analysis indicating its halo membership.

Contribution

It provides the first detailed characterization of the NLTT 11748 system, including orbital parameters and companion mass constraints, highlighting its unique properties among low-mass white dwarfs.

Findings

Orbital period of 5.64 hours determined.

Companion mass minimum estimated at 0.75 M_sun.

System likely belongs to the Galactic halo.

Abstract

We show that the extremely low-mass white dwarf NLTT 11748 (0.17 M_sun) is in a close binary with a fainter companion. We obtained a series of radial velocity measurements of the low-mass white dwarf using the Halpha core and determined an orbital period of 5.64 hours. The velocity semi-amplitude (K=274.8 km/s) and orbital period imply that it is a degenerate star, and that the minimum mass for the companion is 0.75 M_sun (assuming a mass of 0.167 M_sun for the primary). Our analysis of Balmer line profiles shows that a 0.75 M_sun white dwarf companion does not contribute more than 2% or 5% of the flux (V-band) for helium- or hydrogen-rich surfaces, respectively. The kinematics of the system suggest that it belongs to the Galactic halo.