Anisotropic neutron star crust, solar system mountains, and gravitational waves

Abstract

"Mountains" or non-axisymmetric deformations of rotating neutron stars (NS) efficiently radiate gravitational waves (GW). We consider analogies between NS mountains and surface features of solar system bodies. Both NS and moons such as Europa or Enceladus have thin crusts over deep oceans while Mercury has a thin crust over a large metallic core. Thin sheets may wrinkle in universal ways. Europa has linear features, Enceladus has "Tiger" stripes, and Mercury has lobate scarps. NS may have analogous features. The innermost inner core of the Earth is anisotropic with a shear modulus that depends on direction. If NS crust material is also anisotropic this will produce an ellipticity, when the crust is stressed, that grows with spin frequency. This yields a braking index (log derivative of spin down rate assuming only GW spin down) very different from $n=5$ and could explain the maximum spin observed for neutron stars and a possible minimum ellipticity of millisecond pulsars.

8 pages, 2 figures, Added finite element simulations that confirm results, PRD in press

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Cited by in corpus (6)

• Properties of the Object HESS J1731-347 as a Twin Compact Star
• Results from an Einstein@Home search for continuous gravitational waves from Cassiopeia A and Vela Jr. using LIGO O2 data
• Continuous gravitational waves from thermal mountains on accreting neutron stars: effect of the nuclear pasta phase
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• Sco X-1 as a continuous gravitational waves source: modelling the secular evolution using MESA
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