Astrophysics Formal Seminars

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

Shock substructure in partially-ionised plasma

The partially-ionised nature of the lower solar atmosphere introduces new and exciting complexities to shock solutions. Here we study numerically the slow-mode shock triggered via a magnetic discontinuity, mimicking the slow-mode shocks that can form as a result of magnetic reconnection. In single-fluid ideal MHD , the slow-mode shock occurs as a discontinuous jump in parameters. However, in the two-fluid partially-ionised plasma, the shock occupies a finite width due to the coupling and decoupling of plasma and neutral species across the shock. It is found that this finite width region allows for shock substructure that can affect the overall dynamics of the system. In particular, we find that an intermediate shock can exist where the plasma velocity transitions from super to sub Alfvenic velocities. A key feature of this type of shock is that the magnetic field is reversed across the interface. We present numerical results analysing the formation and evolution of intermediate shocks as substructure within slow-mode shocks.

Dynamics of Inclined Disks Around Eccentric Orbit Binaries

Abstract not available

Polarized H-alpha Emission from Supernova Remnant Shock Waves Efficiently Accelerating Cosmic Rays

It is frequently stated that over 99.999% of the visible universe is in the plasma “state”. It is much less frequently commented that the overwhelming majority of this plasma is collisionless, in that particle mean free paths against Coulomb collisions are much longer than typical plasma dimensions. The theory of shock waves propagating in such plasmas poses special problems, relying on plasma turbulence to compress and heat the postshock gas, and also to scatter and accelerate cosmic rays as a necessary part of the shock dissipation.

We develop spectropolarimetry of H alpha as a diagnostic of such phenomena. Neutral hydrogen in the interstellar medium impacted by a collisionless shock “sees” an anisotropic distribution of scattering electrons and ions. In such circumstances line emission excited by these scattering particles will in general be polarized, and such polarization can be used to make inferences about collisionless plasma processes at the shock. Following an initial prediction that such polarization should exist (Laming 1990), and recent observational validation in the NW limb of SN 1006 (Sparks et al. 2015), we revisit the calculations with updated atomic data as a diagnostic of shock energy loss to cosmic rays (Shimoda et al. 2018). Such energy loss has the effect of increasing the mild “collimation” of the shocked flow, increasing the degree of polarization expected. We make comparisons with existing observations of SN 1006 and for “knot g” in Tycho’s supernova remnant, attempting to infer the shock energy losses to cosmic rays in each case.

Title to be confirmed

Abstract not available

Resonant relaxation of stars around a supermassive black hole

In the vicinity of a supermassive black hole, stars move on nearly Keplerian orbits. Yet, because of the enclosed stellar mass and general relativity, the potential slightly deviates from the Keplerian one, which causes the stellar orbits to precess. Similarly, as a result of the finite number of stars, the mutual gravitational torques between pairs of stars also drive a rapid reorientation of the stars’ orbital orientation, much faster than the standard two-body relaxation driven by local scatterings. Overall, the combination of these two effects leads to a stochastic evolution of stellar orbital angular momentum vectors, through a process named ``resonant relaxation’’. Owing to recent developments in the diffusion theory of long-range interacting systems, I will show how one can fully describe such dynamics, in particular scalar resonant relaxation (relaxation of the norm of the angular momentum) and vector resonant relaxation (relaxation of the direction of the angular momentum vector). I will also highlight some astrophysical applications of these new methods, for example to understand the inefficiency of resonant relaxation to induce stellar tidal disruptions.