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Tidal Novae in Compact Binary White Dwarfs
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White dwarfs' tidal effects may create novae

Theoretical physicists at Cornell may have found a new way to explain the formation of novae, stars that suddenly become very bright then quickly fade.
At the heart of the theory is a pair of old, dense stars called white dwarfs, orbiting each other so closely that their gravitational forces create violent tidal waves of plasma that break near the surfaces of the stars. The phenomenon is what the researchers have dubbed a tidally induced nova.

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RE: White Dwarfs Binaries
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Title: Orbital Evolution of Compact White Dwarf Binaries
Authors: David L. Kaplan, Lars Bildsten, Justin D. R. Steinfadt

The new-found prevalence of extremely low mass (ELM, Mhe<0.2 solar mass) helium white dwarfs (WDs) in tight binaries with more massive WDs has raised our interest in understanding the nature of their mass transfer. Possessing small (Menv~1e-3 solar masses) but thick hydrogen envelopes, these objects have larger radii than cold WDs and so initiate mass transfer of H-rich material at orbital periods of 6-10 minutes. Building on the original work of D'Antona et al., we confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich matter and highlight that the inspiraling direct-impact double WD binary HM Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion under mass loss, thus enabling a larger range of donor masses that can stably transfer matter and become a He mass transferring AM CVn binary. Even once in the long-lived AM CVn mass transferring stage, these He WDs have larger radii due to their higher entropy from the prolonged H burning stage.

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Binary White Dwarfs
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Taking advantage of a little-known feature of the Sloan Digital Sky Survey, a team of astronomers led by Carles Badenes of the University of Pittsburgh has helped to clarify the origins of an important type of exploding star - using nothing but a few thousand small, faint stars in our own cosmic backyard.
The astronomical fireworks the team studied are so-called "Type Ia supernovae," exploding stars so incredibly
bright that we can see them even in the most distant galaxies. In a paper published today on the arXiv preprint server, Badenes and colleagues compared the number of these supernovae
in distant galaxies to the number of binary white dwarfs in our galaxy. The rate is similar, suggesting that merging white dwarfs are indeed a reasonable explanation for these giant explosions.

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RE: White Dwarfs Binaries
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Title: The Merger Rate of Binary White Dwarfs in the Galactic Disk
Authors: Carles Badenes, Dan Maoz

We use multi-epoch spectroscopy of about 4000 white dwarfs in the Sloan Digital Sky Survey to constrain the properties of the Galactic population of binary white dwarf systems and calculate their merger rate. With a Monte Carlo code, we model the distribution of DRVmax, the maximum radial velocity shift between exposures of the same star, as a function of the binary fraction within 0.05 AU, fbin, and the power-law index in the separation distribution at the end of the common envelope phase, alpha. Although there is some degeneracy between fbin and alpha, the data constrain the combination of these parameters, which determines a white dwarf merger rate per unit stellar mass of 1.4(+3.4,-1.0)e-13 /yr/Msun (1-sigma limits). This is remarkably similar to the measured rate of Type Ia supernovae per unit stellar mass in Milky-Way-like Sbc galaxies. The rate of super-Chandrasekhar mergers is only 1.0(+1.6,-0.6)e-14 /yr/Msun. We conclude that there are not enough close binary white dwarf systems to reproduce the observed Type Ia SN rate in the 'classic' double degenerate super-Chandrasekhar scenario. On the other hand, if sub-Chandrasekhar mergers can lead to Type Ia SNe, as recently suggested by some studies, they could make a major contribution to the overall Type Ia SN rate.

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Title: White Dwarfs in Ultrashort Binary Systems
Authors: GianLuca Israel, Simone Dall'Osso (INAF - Osservatorio Astronomico di Roma, Italy)

This contribution summarises, as of early 2008, the observational and theoretical understanding of the physics, and emission properties of two Double Degenerate Binaries with the shortest orbital period known to date. In particular, the Unipolar Inductor Model and its coupling to GW emission have been introduced to explain a number of puzzling features that these two sources have in common and that are thought difficult to reconcile with models of mass transfer in such systems. Emphasis was put on the relevant new physical features that characterise the model. The role of spin-orbit coupling through the Lorentz torque and the role of GW emission in keeping the electric interaction active at all times has been thoroughly discussed in all their implications. Application of the model to both HM Cnc and V407 Vul accounts in a natural way for their main observational properties. Constraints on physical parameters are derived in order for the model to work, and can be verified by future observations.

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