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Non-dissipative tidal synchronization in accreting binary white dwarf systems

Racine, Étienne and Phinney, E. Sterl and Arras, Phil (2007) Non-dissipative tidal synchronization in accreting binary white dwarf systems. Monthly Notices of the Royal Astronomical Society, 380 (1). pp. 381-398. ISSN 0035-8711. doi:10.1111/j.1365-2966.2007.12087.x.

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We study a non-dissipative hydrodynamical mechanism that can stabilize the spin of the accretor in an ultracompact double white dwarf (WD) binary. This novel synchronization mechanism relies on a non-linear coupling between tides and the uniform (or rigid) rotation mode, which spins down the background star. The essential physics of the synchronization mechanism is summarized as follows. As the compact binary coalesces due to gravitational wave emission, the largest star eventually fills its Roche lobe and accretion starts. The accretor then spins up due to infalling material and eventually reaches a spin frequency where a normal mode of the star is resonantly driven by the gravitational tidal field of the companion. If the resonating mode satisfies a set of specific criteria, which we elucidate in this paper, it exchanges angular momentum with the background star at a rate such that the spin of the accretor locks at this resonant frequency, even though accretion is ongoing. Some of the accreted angular momentum that would otherwise spin up the accretor is fed back to the orbit through this resonant tidal interaction. In this paper we solve analytically a simple dynamical system that captures the essential features of this mechanism. Our analytical study allows us to identify two candidate Rossby modes that may stabilize the spin of an accreting WD in an ultracompact binary. These two modes are the l=4, m=2 and l=5, m=3 Chandrasekhar–Friedman–Schutz (CFS) unstable hybrid r modes, which, for an incompressible equation of state, stabilize the spin of the accretor at frequency 2.6 ωorb and 1.54 ωorb, respectively, where ωorb is the binary's orbital frequency. For an n= 3/2 polytrope, the accretor's spin frequency is stabilized at 2.13 ωorb and 1.41 ωorb, respectively. Since the stabilization mechanism relies on continuously driving a mode at resonance, its lifetime is limited since eventually the mode amplitude saturates due to non-linear mode–mode coupling. Rough estimates of the lifetime of the effect lie from a few orbits to possibly millions of years. We argue that one must include this hydrodynamical stabilization effect to understand stability and survival rate of ultracompact binaries, which is relevant in predicting the Galactic WD gravitational background that LISA will observe.

Item Type:Article
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URLURL TypeDescription Paper
Phinney, E. Sterl0000-0002-9656-4032
Additional Information:© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2007 June 7. Received 2007 June 6; in original form 2006 December 4. Published Online: 3 August 2007. ÉR wishes to thank Jeandrew Brink for many useful discussions on r modes. ÉR and ESP acknowledge support from NASA ATP grant NNG04GK98G awarded to ESP.
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Subject Keywords:gravitational waves • stellar dynamics • binaries: close • novae, cataclysmic variables • white dwarfs
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Record Number:CaltechAUTHORS:RACmnras07
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ID Code:12972
Deposited By: Archive Administrator
Deposited On:13 Jan 2009 00:04
Last Modified:08 Nov 2021 22:34

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