The Runaway White Dwarf LP400-22 Has a Companion

TL;DR

This paper reports the discovery of a binary system involving an extremely low mass white dwarf and an unseen companion, likely another white dwarf or neutron star, with an unusual galactic orbit suggesting a complex dynamical history.

Contribution

It provides the first detection of a radial velocity companion to LP400-22 and discusses its unique galactic orbit, proposing possible formation scenarios.

Findings

Radial velocity variations indicate a companion with >0.37 solar masses.

The system has a highly unusual galactic orbit with high velocity.

The companion is likely another white dwarf or neutron star, not a main sequence star.

Abstract

We report the detection of a radial velocity companion to the extremely low mass white dwarf LP400-22. The radial velocity of the white dwarf shows variations with a semi-amplitude of 119 km/s and a 0.98776 day period, which implies a companion mass of M > 0.37 Msun. The optical photometry rules out a main sequence companion. Thus the invisible companion is another white dwarf or a neutron star. Using proper motion measurements and the radial velocity of the binary system, we find that it has an unusual Galactic orbit. LP400-22 is moving away from the Galactic center with a velocity of 396 km/s, which is very difficult to explain by supernova runaway ejection mechanisms. Dynamical interactions with a massive black hole like that in the Galactic center can in principle explain its peculiar velocity, if the progenitor was a triple star system comprised of a close binary and a distant tertiary companion. Until better proper motions become available, we consider LP400-22 to be most likely a halo star with a very unusual orbit.