June 2012: How to ignite White Dwarf tidal novae
Graduate Student, Jim Fuller, talks about his recent paper:
Compact binary white dwarf systems are frequently formed after a common envelope phase of binary stellar evolution in which the orbit shrinks to periods of hours or less. The orbit then decays due to gravitational radiation until the white dwarfs merge or begin stable mass transfer, producing a plethora of possible outcomes (e.g., AM CVn systems, single sdB stars, R Cor Bor stars, magnetic white dwarfs, massive neutron stars, and type Ia supernovae). However, the outcome of the mass transfer process depends on the tidal synchronization and heating processes that occur within the white dwarfs as they spiral towards each other.
In this paper, we investigate these pre-merger tidal processes in detail, estimating both the magnitude and location of tidal heating within the white dwarfs. We examine the excitation and propagation of gravity (buoyancy) waves that produce tidal heating and synchronization. We have found that the gravity waves become large in amplitude in the outer layers of the white dwarfs, causing them to break and dissipate, just as ocean waves break as they approach the shore. As the waves break, they deposit their angular momentum and energy, heating up the outer layers of the white dwarfs. Using a stellar evolution program, we calculate the effect of the tidal heating on the white dwarf structure and luminosity.
We find that tidal heating is very important at orbital periods less than about thirty minutes, causing the white dwarfs to become significantly hotter and brighter. Depending on the exact location of the tidal heating, the base of the hydrogen atmosphere of the white dwarf may heat up enough to reignite thermonuclear fusion. The ignition of fusion sparks a thermonuclear runaway, causing the hydrogen atmosphere to erupt in an outburst similar to a classical nova. If we are lucky enough to see them, these bright tidal novae will reveal the locations of elusive compact white dwarf binaries in our galaxy and beyond.
Figure caption: Surface temperature of a tidally heated M=0.6M_sun white dwarf with a M=0.3M_sun companion, as a function of orbital period for different angular momentum transport times t_coup. Longer coupling times correspond to deeper heat deposition. Stars mark the occurrence of a tidal nova. The asterisk marks the location of the more massive white dwarf in the observed system SDSS J065133+284423.