Theoretical astrophysics and plasma physics at RPC

Abstract:

The possibility is explored that accretion on an intermediate mass black hole contributes to the ionisation of the interstellar medium of the Compact Blue Dwarf galaxy MRK996. Chandra observations set tight upper limits (99.7 per cent confidence level) in both the X-ray luminosity of the posited AGN, Lx(2-10keV)<3e40erg/s, and the black hole mass, <1e4/\lambda Msolar, where \lambda, is the Eddington ratio. The X-ray luminosity upper limit is insufficient to explain the high ionisation line [OIV]25.89\mu m, which is observed in the mid-infrared spectrum of the MRK996 and is proposed as evidence for AGN activity. This indicates that shocks associated with supernovae explosions and winds of young stars must be responsible for this line. It is also found that the properties of the diffuse X-ray emission of MRK996 are consistent with this scenario, thereby providing direct evidence for shocks that heat the galaxy's interstellar medium and contribute to its ionisation.

Abstract:

We study the eccentricity distribution of a thick disc sample of stars observed in the Radial Velocity Experiment (RAVE) and compare it to that expected in four simulations of thick disc formation in the literature (accretion of satellites, heating of a primordial thin disc during a merger, radial migration, and gas-rich mergers), as compiled by Sales et al. (2009). We find that the distribution of our sample is peaked at low eccentricities and falls off smoothly and rather steeply to high eccentricities. This distribution is fairly robust to changes in distances, thin disc contamination, and the particular thick disc sample used. Our results are inconsistent with what is expected for the pure accretion simulation, since we find that the dynamics of local thick disc stars implies that the majority must have formed "in situ". Of the remaining models explored, the eccentricity distribution of our stars appears to be most consistent with the gas-rich merger case.

Abstract:

We report on the discovery of new members of nearby young moving groups, exploiting the full power of combining the RAVE survey with several stellar age diagnostic methods and follow-up high-resolution optical spectroscopy. The results include the identification of one new and five likely members of the beta Pictoris moving group, ranging from spectral types F9 to M4 with the majority being M dwarfs, one K7 likely member of the epsilon Cha group and two stars in the Tuc-Hor association. Based on the positive identifications we foreshadow a great potential of the RAVE database in progressing toward a full census of young moving groups in the solar neighbourhood.

Abstract:

We analyze the linear stability of a dilute, hot plasma, taking into account the effects of stratification and anisotropic thermal conduction. The work is motivated by attempts to understand the dynamics of the intracluster medium in galaxy clusters. We show that magnetic field configurations that nominally stabilize either the heat-flux driven buoyancy instability (associated with a positive thermal gradient) or the magnetothermal instability (negative thermal gradient) can lead to previously unrecognized g-mode overstabilities. The driving source of the overstability is either radiative cooling (positive temperature gradient) or the heat flux itself (negative temperature gradient). While the implications of these overstabilities have yet to be explored, we speculate that the cold fronts observed in many relaxed galaxy clusters may be related to their nonlinear evolution. © 2010. The American Astronomical Society. All rights reserved.