Tidal truncation of accretion disks
Lead Research Organisation:
University of Cambridge
Department Name: Applied Maths and Theoretical Physics
Abstract
The prevalence and consequences of convection perpendicular to the plane of accretion disks have been discussed for several decades. Recent simulations combining convection and the magnetorotational instability have given fresh impetus to the debate, as the interplay of the two processes can enhance angular momentum transport, at least in the optically thick outburst stage of dwarf novae. In this thesis we seek to isolate and understand the most generic features of disk convection, and so undertake its study in both hydrodynamical and magnetohydrodynamical models. In the final part of this thesis (which is independent of the first two) we investigate the stress-pressure relationship in disks. The stresses accompanying MRI turbulence are related to the pressure in the disk, and have been shown to increase and decrease with the pressure. We examine the time lag associated with this dependence and discuss its implications for thermal instability.
Organisations
People |
ORCID iD |
| Roman Rafikov (Primary Supervisor) | |
| Loren Held (Student) |
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| ST/N503976/1 | 01/10/2015 | 31/03/2021 | |||
| 1781736 | Studentship | ST/N503976/1 | 01/10/2016 | 31/03/2020 | Loren Held |
| Description | Overview: This thesis (and associated publications) focused on fluid instabilities and turbulence in accretion disks (astronomical objects consisting of a disk of gas and dust around stars or black holes). Disks are common in nature, forming, for example, in environments as diverse as around young stars (protoplanetary disks) or when two stars closely orbit one another. The diverse environments within which disks form in nature means they can account for many important physical phenomena, such as the origin of planets, and how the heaviest elements are created. My work used numerical simulations, together with a combination of pen-and-paper mathematics. Instabilities and turbulence in disks can help explain the mechanism of accretion and how disks emit light that we can observe. Key discoveries: During my PhD I studied convection (the bulk motion of fluid due to a thermal gradient) in accretion disks. Using numerical simulations, together with (semi-)analytical methods, I discovered that convection in disks can drive outward angular momentum transport, and that it is not self-sustaining, thus helping to resolve two long-standing problems in the field (Jankovic+2021). I then generalized this work to study the interplay between convection and the magnetorotational instability (MRI), an instability found in disks threaded by magnetic fields. I found that the two instabilities interact in non-trivial ways, including MRI/convection cycles in which one or the other instability periodically dominates the flow; during the MRI-dominated phases the transport is significantly enhanced, which helped interpret recent numerical simulations of Dwarf Novae in outburst that claim that convection together with MRI can significantly enhance accretion. My work on convection in disks has been published in two papers in a leading journal (Held and Latter 2018, 2021). I also studied relationship between stress and pressure in MRI turbulence and its consequences for thermal instability. The stresses accompanying MRI turbulence are related to the pressure in the disk, and have been shown to increase and decrease with the pressure. I designed numerical simulations to examine the relationship between these two quantities and found that the stress lagged behind the pressure by around 5 orbits, a novel result which might have implications for thermal instability and variability in certain sub-states of X-ray binaries. This work was published in a leading journal (Held and Latter 2021). Results / publications: The work funded by STFC culminated in a Doctorate in Applied Mathematics and Theoretical Physics awarded by the University of Cambridge. This work directly lead to three publications in a leading journal (Monthly Notices of the Royal Astronomical Society). I attended two summer schools during my PhD which complemented the work I was carrying out during the project. I presented my results at several national and international conferences and seminars during the course of my PhD. |
| Exploitation Route | The papers that I published during this project have already been cited multiple times in the astrophysical literature. I expect the results to be of interest to those working on dwarf novae, the magnetorotational instability, and, more broadly, to the numerical astrophysical fluid dynamics community. |
| Sectors | Other |