Talk: What sets the maximum spin rate of neutron stars?

Speaker: Ira Wasserman (Cornell)

  •  J1614-2230: Newly discovered NS with a well-constrained mass of 2 Msun.  This places constraints on the equation of state of NSs.
  • The core material is largely unknown.  It’s agreed that electrons are replaced by muons, but not much else.  The 2 Msun NS might not require such exotic materials, such as quark matter.
  • Fastest NS is 716 Hz.  Breakup is ~2000 Hz.  Accretion implies spin-up.
  • (Cook, Shapiro, Teukolsky 1994) Starting at 1.4 Msun, the NS accretes and spins up, but all of the final rotation rates are on the order of 1000 Hz, much higher than the observed rates.  These calculations and not to date have included magnetic fields.  Usual pulsars start with a rotation rate of 10 ms, e.g. Crab, and then slowdown subsequently in the absence of accretion.
  • Grav. radiation (Wagoner 1984; Bildsten 1998): Spindown by GR.
  • {dJ}_{GR}/dt approx -G Omega^5 (epsilon I)^2 c^{-5}
  • If you have some balance between spin-up and spin down, then epsilon approx 10^{-7}.  However, what’s the source of the quadrapole Q?  R-modes through the CFS instability (Chandrasekhar 1970).
  • R-modes can damp through a shearing boundary layer, i.e. core-crust boundary and its shear modulus, at low temperature.  At high temperature, bulk viscosity can damp the R-modes.
  • Spin-up problem: During accretion, the surface temperature goes to 3e8 K.  If you balance the GR growth rate and BL shear, one approaches a limit of ~300 Hz. How to explain the higher spin rates?  bulk viscosity? (Ira says probably not.)
  • Parametric instabilities can cause the crust to heat up through dissipation through daughter modes.  Then it will either spin-up or down as it tries to equalize heating and cooling.

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