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On the relevance of the r-mode instability for accreting neutron stars and white dwarfs

On the relevance of the r-mode instability for accreting neutron stars and white dwarfs
On the relevance of the r-mode instability for accreting neutron stars and white dwarfs
We present a case study for the relevance of the r-mode instability for accreting compact stars. Our estimates are based on approximations that facilitate "back of the envelope" calculations. We discuss two different cases. (1) For recycled millisecond pulsars, we argue that the r-mode instability may be active at rotation periods longer than the Kepler period (which provides the dynamical limit on rotation) as long as the core temperature is larger than about 2×105 K. Our estimates suggest that the instability may have played a role in the evolution of the fastest spinning pulsars and that it may be presently active in the recently discovered 2.49 ms X-ray pulsar, SAX J1808.4-3658, as well as the rapidly spinning neutron stars observed in low-mass X-ray binaries (LMXBs). This provides a new explanation for the remarkably similar rotation periods inferred from kilohertz, quasi-periodic oscillations in the LMXBs. The possibility that the rotation of recycled pulsars may be gravitational-radiationlimited is interesting, because the gravitational waves from a neutron star rotating at the instability limit may well be detectable with the new generation of interferometric detectors. (2) We also consider white dwarfs and find that the r-mode instability may possibly be active in short-period white dwarfs. Our order-of-magnitude estimates (for a white dwarf of M=Mand R=0.01 R composed of C12) show that the instability could be operating for rotational periods shorter than P2733 s. This number is in interesting agreement with the observed periods (greater than 28 s) of the rapidly spinning DQ Herculis stars. However, we find that the instability grows too slowly to affect the rotation of these stars significantly.
accretion, accretion disks - pulsars, general - stars, neutron - stars, oscillations - white dwarfs
0004-637X
307-314
Andersson, Nils
2dd6d1ee-cefd-478a-b1ac-e6feedafe304
Kokkotas, Kostas D.
0b8bb98a-a65f-434b-855b-ec5b488b4a96
Stergioulas, Nikolaos
7ff499f3-91e1-497f-bee6-4d8b5ca581da
Andersson, Nils
2dd6d1ee-cefd-478a-b1ac-e6feedafe304
Kokkotas, Kostas D.
0b8bb98a-a65f-434b-855b-ec5b488b4a96
Stergioulas, Nikolaos
7ff499f3-91e1-497f-bee6-4d8b5ca581da

Andersson, Nils, Kokkotas, Kostas D. and Stergioulas, Nikolaos (1999) On the relevance of the r-mode instability for accreting neutron stars and white dwarfs. The Astrophysical Journal, 516 (1), 307-314. (doi:10.1086/307082).

Record type: Article

Abstract

We present a case study for the relevance of the r-mode instability for accreting compact stars. Our estimates are based on approximations that facilitate "back of the envelope" calculations. We discuss two different cases. (1) For recycled millisecond pulsars, we argue that the r-mode instability may be active at rotation periods longer than the Kepler period (which provides the dynamical limit on rotation) as long as the core temperature is larger than about 2×105 K. Our estimates suggest that the instability may have played a role in the evolution of the fastest spinning pulsars and that it may be presently active in the recently discovered 2.49 ms X-ray pulsar, SAX J1808.4-3658, as well as the rapidly spinning neutron stars observed in low-mass X-ray binaries (LMXBs). This provides a new explanation for the remarkably similar rotation periods inferred from kilohertz, quasi-periodic oscillations in the LMXBs. The possibility that the rotation of recycled pulsars may be gravitational-radiationlimited is interesting, because the gravitational waves from a neutron star rotating at the instability limit may well be detectable with the new generation of interferometric detectors. (2) We also consider white dwarfs and find that the r-mode instability may possibly be active in short-period white dwarfs. Our order-of-magnitude estimates (for a white dwarf of M=Mand R=0.01 R composed of C12) show that the instability could be operating for rotational periods shorter than P2733 s. This number is in interesting agreement with the observed periods (greater than 28 s) of the rapidly spinning DQ Herculis stars. However, we find that the instability grows too slowly to affect the rotation of these stars significantly.

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More information

Published date: 1999
Keywords: accretion, accretion disks - pulsars, general - stars, neutron - stars, oscillations - white dwarfs

Identifiers

Local EPrints ID: 29430
URI: http://eprints.soton.ac.uk/id/eprint/29430
ISSN: 0004-637X
PURE UUID: 67c16001-df6b-46e2-b2c5-ab8384aec251
ORCID for Nils Andersson: ORCID iD orcid.org/0000-0001-8550-3843

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Date deposited: 20 Dec 2006
Last modified: 29 Oct 2019 01:59

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Author: Nils Andersson ORCID iD
Author: Kostas D. Kokkotas
Author: Nikolaos Stergioulas

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