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Published March 21, 2013 | Published + Accepted Version
Journal Article Open

Dynamical tides in compact white dwarf binaries: helium core white dwarfs, tidal heating and observational signatures


Tidal dissipation in compact white dwarf (WD) binary systems significantly influences the physical conditions (such as surface temperature and rotation rate) of the WDs prior to mass transfer or merger. In these systems, the dominant tidal effects involve the excitation of gravity waves and their dissipation in the outer envelope of the star. We calculate the amplitude of tidally excited gravity waves in low-mass (0.3 M_⊙) helium (He) core WDs as a function of the tidal forcing frequency ω. Like carbon–oxygen (CO) WDs studied in our previous paper, we find that the dimensionless tidal torque F(ω) (inversely proportional to the effective tidal quality factor) depends on ω in an erratic way. On average, F(ω) scales approximately as ω^6, and is several orders of magnitude smaller for He WDs than for CO WDs. We find that tidal torques can begin to synchronize the WD rotation when the orbital period is less than about an hour, although a nearly constant asynchronization is maintained even at small periods. We examine where the tidally excited gravity waves experience non-linear breaking or resonant absorption at a critical layer, allowing us to estimate the location and magnitude of tidal heating in the WD envelope. We then incorporate tidal heating in the MESA stellar evolution code, calculating the physical conditions of the WD as a function of orbital period for different WD models. We find that tidal heating makes a significant contribution to the WD luminosity for short-period (∼10 min) systems such as SDSS J0651+2844. We also find that for WDs containing a hydrogen envelope, tidal heating can trigger runaway hydrogen shell burning, leading to a nova-like event before the onset of mass transfer.

Additional Information

© 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2012 December 11. Received 2012 December 8; in original form 2012 November 3. We thank Bill Paxton, Lars Bildsten and Eliot Quataert for useful discussion. JF acknowledges the hospitality (during the autumn of 2011) of the Kavli Institute for Theoretical Physics at UCSB (funded by the NSF through Grant 11-Astro11F-0016) where part of the work was carried out. This work has been supported in part by NSF grants AST-1008245 and AST-1211061, and NASA grants NNX12AF85G, NNX10AP19G and NNX11AL13H.

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