Theoretical astrophysics and plasma physics at RPC

Abstract:

We present results from our study of the active dwarf is an element of Eri (K2 V) based on ultraviolet spectra recorded with the Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectroscopic Explorer. A combination of simple theoretical arguments and observational constraints derived from measured line fluxes are used to deduce new information about the structure of the upper transition region/corona. The area filling factor of emitting material is determined in the upper atmosphere as a function of temperature. This provides new constraints on how the magnetic field might spread out in the atmosphere of an active main sequence star. Measured emission line widths are used, together with a new semi-empirical model of the atmosphere, to place limits on the energy fluxes carried by MHD waves. These are compared with estimates of the energy input required to support the combined radiative/conductive losses in the upper atmosphere. It is shown that, in principle, waves which propagate at the Alfven speed could provide sufficient energy to heat the corona.

Abstract:

Three-dimensional hydrodynamical simulations are used to investigate the structural stability of cooling flows that are episodically heated by jets from a central AGN. The radial profile of energy deposition is controlled by (a) the power of the jets, and (b) the pre-outburst density profile. A delay in the ignition of the jets causes more powerful jets to impact on a more centrally concentrated medium. The net effect is a sufficient increase in the central concentration of energy deposition to cause the post-outburst density profile to be less centrally concentrated than that of an identical cluster in which the outburst happened earlier and was weaker. These results suggest that the density profiles of cooling flows oscillate around an attracting profile, thus explaining why cooling flows are observed to have similar density profiles. The possibility is raised that powerful FR II systems are ones in which this feedback mechanism has broken down and a runaway growth of the source parameters has occurred.

Abstract:

We have investigated the dynamics of the merging process in the minor merger hypothesis for active galactic nuclei. We find that for a satellite galaxy to be able to merge directly with the nucleus of the host galaxy (for example, to give rise to the compact dust discs which are seen in early-type active galaxies), the initial orbit of the satellite is required to be well aimed. For the case of the host galaxy being a disc galaxy, if the initial orbits of the satellites are randomly oriented with respect to the host galaxy, then the orbits of those which reach the host nuclear regions in a reasonable time are also fairly randomly oriented once they reach the nucleus. We note that this result might be able to provide an explanation of why the jet directions in the nuclei of Seyfert galaxies are apparently unrelated to the plane of the galaxy discs.

Abstract:

An axial model for a two-stage discharge with an electron-emissive electrode is further examined. Scaling laws are derived and help to understand two-stage physics. Efficiency gains are obtained when the second-stage is placed in the upstream part of the acceleration region and the first and two stage voltages are comparable. A parametric study to determine the best position and voltage of the intermediate electrode is carried out. © 2003 by The Authors.