Relativistic ocean r-modes during type-I X-ray bursts

Accreting neutron stars (NS) can exhibit high frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. These frequencies can be offset from the NS spin frequency by several Hz (where known independently) and can drift by 1–3 Hz. One plausible explanation is that a wave is present in the bursting ocean, the rotating frame frequency of which is the offset. The frequency of the wave should decrease (in the rotating frame) as the burst cools hence explaining the drift. A strong candidate is a buoyant r-mode. To date, models that calculated the frequency of this mode taking into account the radial structure neglected relativistic effects and predicted rotating frame frequencies of ∼4 Hz and frequency drifts of >5 Hz; too large to be consistent with observations. We present a calculation that includes frame-dragging and gravitational redshift that reduces the rotating frame frequency by up to 30percent30percent and frequency drift by up to 20percent20percent⁠. Updating previous models for the ocean cooling in the aftermath of the burst to a model more representative of detailed calculations of thermonuclear X-ray bursts reduces the frequency of the mode still further. This model, combined with relativistic effects, can reduce the rotating frequency of the mode to ∼2 Hz and frequency drift to ∼2 Hz, which is closer to the observed values

Onset of low Prandtl number thermal convection in thin spherical shells

This study considers the onset of stress-free Boussinesq thermal convection in rotating spherical shells with an aspect ratio η = ri/ro = 0.9 (ri and ro being the inner and outer radius), Prandtl numbers Pr ∈ [10−4,10−1], and Taylor numbers Ta ∈ [104,1012]. We are particularly interested in the form of the convective cell pattern that develops, and in its time scales, since this may have observational consequences. For a fixed Ta 3 × 109, the unicellular polar modes become also preferred at moderate Pr ∼ 10−2 because two new transition curves between EA and AP/SP and between AP/SP and SC modes are born at a triple-point bifurcation. The dependence on Pr and Ta of the transitions is studied to estimate the types of modes, and their critical parameters, preferred at different stellar regimes

Thermal convection in rotating spherical shells: Temperature-dependent internal heat generation using the example of triple-α burning in neutron stars

We present an extensive study of Boussinesq thermal convection including a temperature-dependent internal heating source, based on numerical three-dimensional simulations. The temperature dependence mimics triple-α nuclear reactions and the fluid geometry is a rotating spherical shell. These are key ingredients for the study of convective accreting neutron star oceans. A dimensionless parameter Ran, measuring the relevance of nuclear heating, is defined. We explore how flow characteristics change with increasing Ran and give an astrophysical motivation. The onset of convection is investigated with respect to this parameter and periodic, quasiperiodic, chaotic flows with coherent structures, and fully turbulent flows are exhibited as Ran is varied. Several regime transitions are identified and compared with previous results on differentially heated convection. Finally, we explore (tentatively) the potential applicability of our results to the evolution of thermonuclear bursts in accreting neutron star oceans