Postdoctoral Researcher
Department of Physics and Astronomy
University of Leicester, Leicester

Here are short summaries of my most recent work. A full list of publications can be found here.

Warping a protoplanetary disc with a planet on an inclined orbit

Recent observations of several protoplanetary discs have found evidence of departures from flat, circular motion in the inner regions of the disc. One possible explanation for these observations is a disc warp, which could be induced by a planet on a misaligned orbit. We present three-dimensional numerical simulations of the tidal interaction between a protoplanetary disc and a misaligned planet. For low planet masses we show that our simulations accurately model the evolution of inclined planet orbit (up to moderate inclinations). For a planet massive enough to carve a gap, the disc is separated into two components and the gas interior and exterior to the planet orbit evolve separately, forming an inner and outer disc. Due to the inclination of the planet, a warp develops across the planet orbit such that there is a relative tilt and twist between these discs. We show that when other parameters are held constant, the relative inclination that develops between the inner and outer disc depends on the outer radius of the total disc modelled. For a given disc mass, our results suggest that the observational relevance of the warp depends more strongly on the mass of the planet rather than the inclination of the orbit.

On the Papaloizou-Pringle instability in tidal disruption events

We demonstrate that the compact, thick disc formed in a tidal disruption event may be unstable to non-axisymmetric perturbations in the form of the Papaloizou-Pringle instability. We show this can lead to rapid redistribution of angular momentum that can be parametrized in terms of an effective Shakura-Sunyaev α parameter. For remnants that have initially very weak magnetic fields, this may be responsible for driving mass accretion prior to the onset of the magnetorotational instability. We thus identify a method by which the torus formed in tidal disruption event may be significantly accreted before the magnetorotational instability is established.

Apsidal precession, disc breaking and viscosity in warped discs

We demonstrate the importance of general relativistic apsidal precession in warped black hole accretion discs by comparing three-dimensional smoothed particle hydrodynamic simulations in which this effect is first neglected, and then included. If apsidal precession is neglected, we confirm the results of an earlier magnetohydrodynamic simulation which made this assumption, showing that at least in this case the α viscosity model produces very similar results to those of simulations where angular momentum transport is due to the magnetorotational instability. Including apsidal precession significantly changes the predicted disc evolution. For moderately inclined discs thick enough that tilt is transported by bending waves, we find a disc tilt which is non-zero at the inner disc edge and oscillates with radius, consistent with published analytic results. For larger inclinations, we find disc breaking.

On the Bardeen-Petterson Effect

We investigate the effect of black hole spin on warped or misaligned accretion discs - in particular (i) whether or not the inner disc edge aligns with the black hole spin and (ii) whether the disc can maintain a smooth transition between an aligned inner disc and a misaligned outer disc, known as the Bardeen-Petterson effect. We employ high-resolution 3D smoothed particle hydrodynamics simulations of α-discs subject to Lense-Thirring precession, focusing on the bending wave regime where the disc viscosity is smaller than the aspect ratio α ≲ H/R. We first address the controversy in the literature regarding possible steady-state oscillations of the tilt close to the black hole. We successfully recover such oscillations in 3D at both small and moderate inclinations (≲15°), provided both Lense-Thirring and Einstein precession are present, sufficient resolution is employed, and provided the disc is not so thick so as to simply accrete misaligned. Secondly, we find that discs inclined by more than a few degrees in general steepen and break rather than maintain a smooth transition, again in contrast to previous findings, but only once the disc scaleheight is adequately resolved. Finally, we find that when the disc plane is misaligned to the black hole spin by a large angle, the disc `tears' into discrete rings which precess effectively independently and cause rapid accretion, consistent with previous findings in the diffusive regime (α ≳ H/R). Thus, misalignment between the disc and the spin axis of the black hole provides a robust mechanism for growing black holes quickly, regardless of whether the disc is thick or thin.

Copyright © Rebecca Nealon 2018