arXiv subjects

Jim Pringle

Publications and source records attributed to Jim Pringle.

5 recordsLinked to original sources

arXiv2026

On the isotropy of viscosity in accretion discs

Accretion discs are fundamental to many astrophysical systems, providing the conversion of gravitational potential energy into radiation that we can observe. In many systems there is evidence that discs are warped; from spatially-resolved observations of protoplanetary discs, to the features of lightcurves and line profiles from discs around supermassive black holes in galaxy centres. The dynamics of warped discs is largely controlled by the physical nature of the internal disc viscosity. While typically disc viscosity is hydromagnetic in origin, simulations of magnetized discs cannot match observed rates of angular momentum transport in planar discs and thus cannot be used to determine the ratio of the torques responsible for driving accretion to those responsible for evolving the disc warp. The analytic work of Ogilvie is the most comprehensive model for warped disc evolution, but makes assumptions that need to be tested. In particular, it assumes that the disc viscosity is Navier-Stokes, and therefore small-scale and isotropic. Here we attempt to test this model using the long periods of X-ray binaries that are due to precession of the disc. These systems have well-constrained estimates of the component of viscosity responsible for driving accretion, and by looking at systems with and without evidence for disc misalignment and precession we can constrain the component of viscosity responsible for flattening the disc. We conclude that the observational constraints suggest that the Ogilvie model provides an adequate description of the disc evolution, but that there are indications that the internal disc viscosity might be marginally non-isotropic.

astro-ph.HE↗

arXiv2020

Be star discs: powered by a non-zero central torque

Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disc. Current disc models, referred to as "decretion discs", make use of the zero torque inner boundary condition typically applied to accretion discs, with the 'decretion' modelled by adding mass to the disc at a radius of about two per cent larger than the inner disc boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disc are implausibly large. What is required is that the disc has not only a source of mass but also a continuing source of angular momentum. We argue that the disc evolution may be more physically modelled by application of the non-zero torque inner boundary condition of Nixon & Pringle (2020), which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disc cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disc material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disc. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disc inner regions to drive the observed disc evolution.

astro-ph.HE↗

arXiv2016

A self-consistent model for the evolution of the gas produced in the debris disc of $β$ Pictoris

This paper presents a self-consistent model for the evolution of gas produced in the debris disc of $β$ Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy, which is coupled to a dynamical viscous $α$ model. We present new gas observations of $β$ Pic, of C I observed with APEX and O I observed with Herschel, and show that these along with published C II and CO observations can all be explained with this new model. Our model requires a viscosity $α$ > 0.1, similar to that found in sufficiently ionised discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium. This new model can be tested from its predictions for high resolution ALMA observations of C I. We also constrain the water content of the planetesimals in $β$ Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.

astro-ph.EP↗

arXiv2011

The Relationship between Accretion Disc Age and Stellar Age and its Consequences for Proto-Stellar Discs

We show that for young stars which are still accreting and for which measurements of stellar age, disc mass and accretion rate are available, nominal disc age (Disc Age = Disc Mass / Accretion Rate) is approximately equal to the stellar age, at least within the considerable observational scatter. We then consider theoretical models of proto-stellar discs through analytic and numerical models. A variety of viscosity prescriptions including empirical power laws, magnetohydrodynamic turbulence and gravitational instability were considered within models describing the disc phenomena of dead zones, photoevaporation and planet formation. These models are generally poor fits to the observational data, showing values of 'Disc Age' which are too high by factors of 3 - 10. We then ask whether a systematic error in the measurement of one of the observational quantities might provide a reasonable explanation for this discrepancy. We show that for the observed systems only disc mass shows a systematic dependence on the value of 'Disc Age / Stellar Age' and we note that a systematic underestimate of the value of disc mass by a factor of around 3 - 5, would account for the discrepancy between theory and observations.

astro-ph.SR↗

arXiv2011

The final parsec problem: aligning a binary with an external accretion disc

We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in galaxy merger events involving supermassive black holes, and in the formation of stellar--mass binaries in star clusters. We work out the gravitational torque between the binary and disc, and show that their angular momenta J_b, J_d stably counteralign if their initial orientation is sufficiently retrograde, specifically if the angle theta between them obeys cos(theta) < -J_d/2J_b, on a time short compared with the mass gain time of the central accretor(s). The magnitude J_b remains unchanged in this process. Counteralignment can promote the rapid merger of supermassive black hole binaries, and possibly the formation of coplanar but retrograde planets around stars in binary systems.

astro-ph.GA↗