Tidal Novae in Compact Binary White Dwarfs

TL;DR

This paper explores how tidal heating in compact binary white dwarfs can lead to runaway nuclear fusion events called tidal novae, occurring before the stars merge, with implications for understanding their evolution.

Contribution

It introduces a model of tidal heat deposition in white dwarf envelopes and predicts the occurrence of tidal novae in certain binary systems.

Findings

Tidal heating can trigger runaway fusion in the hydrogen layer.

Tidal novae may occur in all CO WD binaries with 5-20 min orbital periods.

Tidal novae precede mergers by 10^5-10^6 years.

Abstract

Compact binary white dwarfs (WDs) undergoing orbital decay due to gravitational radiation can experience significant tidal heating prior to merger. In these WDs, the dominant tidal effect involves the excitation of outgoing gravity waves in the inner stellar envelope and the dissipation of these waves in the outer envelope. As the binary orbit decays, the WDs are synchronized from outside in (with the envelope synchronized first, followed by the core). We examine the deposition of tidal heat in the envelope of a Carbon-Oxygen WD and study how such tidal heating affects the structure and evolution of the WD. We show that significant tidal heating can occur in the star's degenerate hydrogen layer. This layer heats up faster than it cools, triggering runaway nuclear fusion. Such "tidal novae" may occur in all WD binaries containing a CO WD, at orbital periods between 5 min and 20 min, and precede the final merger by 10^5-10^6 years.