Astrophysics Formal Seminars

Formal seminars take place every Monday during full term. For a complete list of talks please visited the talk.cam website.

Magnetic fields and the red giant dipole dichotomy problem

Stars are self-gravitating fluids in which many types of waves can propagate. Constructive interference of these waves produces global modes of oscillation, the associated brightness fluctuations of which can be detected and Fourier analysed to yield information about the stellar properties. Observations have revealed that a significant fraction of red giant stars have lower than expected amplitudes in their dipole modes, implying larger damping rates of these modes than for the rest of the population (the “dipole dichotomy” problem). Indirect evidence points towards the role of a deeply-buried magnetic field, but a detailed understanding of how gravity waves interact with strong fields is thus far lacking.

In this talk, I will present the results of recent work attacking the problem through a variety of linear, nonlinear, analytical and numerical approaches. We uncover a rich variety of physical processes that emerge when the field strength exceeds a certain critical threshold. In particular, we find that resonant interactions between gravity waves and Alfvén waves can lead to a singularity that produces efficient damping through viscous dissipation. Predicted damping rates are on par with those associated with convection, and lie in the range required to explain observations.

Convection Affects Magnetic Turbulence in White Dwarf Accretion Disks

I examine the accretion disks which power outbursts in two types of white dwarf binary systems: dwarf novae (DNe) and AM C Vns. Accretion disks in these systems are thermally unstable, causing some of the observed variations. The source of “normal outbursts” in these systems ultimately originates from ionization transitions (H for DNe and He for AM C Vns). These ionization transitions cause significant temperature dependence in opacities and equation of states, culminating in the occurrence of convection within these accretion disks. Local stratified shearing-box simulations were used to show that this convection has a significant impact on the turbulence and dynamos generated by the magnetorotational instability (MRI). Most notably, convection enhances the stress to pressure ratio, often denoted by alpha. These results were then incorporated into the disk instability model to generate the first theoretical lightcurves for dwarf novae outbursts which incorporate MRI physics.

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Large scale vortices and zonal flows in spherical rotating convection

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Radiative Transfer in Exoplanetary Atmospheres

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