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Crust-core coupling and r-mode damping in neutron stars: a toy model

Levin, Yuri and Ushomirsky, Greg (2001) Crust-core coupling and r-mode damping in neutron stars: a toy model. Monthly Notices of the Royal Astronomical Society, 324 (4). pp. 917-922. ISSN 0035-8711. doi:10.1046/j.1365-8711.2001.04323.x.

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r-modes in neutron stars with crusts are damped by viscous friction at the crust–core boundary. The magnitude of this damping, evaluated by Bildsten & Ushomirsky (BU) under the assumption of a perfectly rigid crust, sets the maximum spin frequency for neutron stars spun up by accretion in low-mass X-ray binaries (LMXBs). In this paper we explore the mechanical coupling between the core r-modes and the elastic crust, using a toy model of a constant-density neutron star having a crust with a constant shear modulus. We find that, at spin frequencies in excess of ≈50 Hz, the r-modes strongly penetrate the crust. This reduces the relative motion (slippage) between the crust and the core compared with the rigid-crust limit. We therefore revise down, by as much as a factor of 10^2–10^3, the damping rate computed by BU, significantly reducing the maximal possible spin frequency of neutron stars with solid crusts. The dependence of the crust–core slippage on the spin frequency is complicated, and is very sensitive to the physical thickness of the crust. If the crust is sufficiently thick, the curve of the critical spin frequency for the onset of the r-mode instability becomes multivalued for some temperatures; this is related to avoided crossings between the r-mode and higher-order torsional modes in the crust. The critical frequencies are comparable to the observed spins of neutron stars in LMXBs and millisecond pulsars.

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Additional Information:© 2001 RAS. Accepted 2001 January 10. Received 2000 December 14; in original form 2000 June 7. Published: 01 July 2001. We thank Lars Bildsten for comments that helped improve this manuscript, and Eliot Quataert and Re’em Sari for numerous discussions. YL thanks the Institute for Advanced Study, where part of the research for this work was done, for hospitality. YL is supported by the Theoretical Astrophysics Center at UC Berkeley, and GU is supported by NSF Grant AST-9618537 and NASA grant NGC5-7034.
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University of California, BerkeleyUNSPECIFIED
Subject Keywords:dense matter – gravitation – stars: neutron – stars: oscillations – stars: rotation
Issue or Number:4
Record Number:CaltechAUTHORS:20180404-090052677
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Official Citation:Yuri Levin, Greg Ushomirsky; Crust–core coupling and r-mode damping in neutron stars: a toy model, Monthly Notices of the Royal Astronomical Society, Volume 324, Issue 4, 1 July 2001, Pages 917–922,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85590
Deposited By: Tony Diaz
Deposited On:04 Apr 2018 16:58
Last Modified:15 Nov 2021 20:30

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