The massive fast spinning white dwarf in the HD 49798/RX J0648.0-4418 binary

TL;DR

This paper reviews the properties of the unique HD 49798/RX J0648.0-4418 binary system, highlighting its massive, rapidly spinning white dwarf and discussing its evolutionary implications and uncertain final fate.

Contribution

It provides the first detailed discussion of the properties and evolutionary prospects of this rare binary system with a massive, fast-spinning white dwarf.

Findings

White dwarf mass measured at 1.2 solar masses

White dwarf spin period of 13.2 seconds

Current accretion insufficient for spin-up, suggesting contraction

Abstract

I review the properties and discuss some of the puzzling aspects of the unique binary system composed of the luminous hot subdwarf HD 49798 and a white dwarf with mass of 1.2 solar masses and spin period of 13.2 s. This is one of the few massive white dwarfs with a dynamically measured mass and the one with the shortest spin period. It emits pulsed X-rays with a very soft spectrum, powered by accretion from the tenuous stellar wind of its companion of sdO spectral type. The current level of mass accretion cannot provide enough angular momentum to explain the small, but precisely measured, spin-up rate of 72 nanoseconds per year, which is instead best interpreted as the result of the radial contraction of this young white dwarf. The higher mass transfer rate expected during the future evolutionary stages of HD 49798 will drive the white dwarf above the Chandrasekhar limit, but the final fate, a type Ia SN explosion or the collapse to a millisecond pulsar, is uncertain.