Theoretical astrophysics and plasma physics at RPC

Abstract:

We review the observations of extrasolar planets, ongoing developments in theories of planet formation, orbital migration and the evolution of multiplanet systems. © 2006 IOP Publishing Ltd.

Abstract:

Recent HI observations reveal that the discs of spiral galaxies are surrounded by extended gaseous haloes. This extra-planar gas reaches large distances (several kpc) from the disc and shows peculiar kinematics (low rotation and inflow). We have modelled the extra-planar gas as a continuous flow of material from the disc of a spiral galaxy into its halo region. The output of our models are pseudo-data cubes that can be directly compared to the HI data. We have applied these models to two spiral galaxies (NGC891 and NGC2403) known to have a substantial amount of extra-planar gas. Our models are able to reproduce accurately the vertical distribution of extra-planar gas for an energy input corresponding to a small fraction (<4%) of the energy released by supernovae. However they fail in two important aspects: 1) they do not reproduce the right gradient in rotation velocity; 2) they predict a general outflow of the extra-planar gas, contrary to what is observed. We show that neither of these difficulties can be removed if clouds are ionized and invisible at 21cm as they leave the disc but become visible at some point on their orbits. We speculate that these failures indicate the need for accreted material from the IGM that could provide the low angular momentum and inflow required.

Abstract:

Using 2D magnetohydrodynamic (MHD) numerical simulations performed with two different finite-difference Eulerian codes, we analyse the effect that a toroidal magnetic field has on low-mass planet migration in non-turbulent protoplanetary discs. The presence of the magnetic field modifies the waves that can propagate in the disc. In agreement with a recent linear analysis, we find that two magnetic resonances develop on both sides of the planet orbit, which contribute to a significant global torque. In order to measure the torque exerted by the disc on the planet, we perform simulations in which the latter is either fixed on a circular orbit or allowed to migrate. For a 5-M⊕ planet, when the ratio β between the square of the sound speed and that of the Alfven speed at the location of the planet is equal to 2, we find inward migration when the magnetic field Bφ is uniform in the disc, reduced migration when Bφ decreases as r-1 and outward migration when Bφ decreases as r-2. These results are in agreement with predictions from the linear analysis. Taken as a whole, our results confirm that even a subthermal stable field can stop inward migration of an earth-like planet. © 2005 RAS.

Abstract:

The study of gas outside the plane of disc galaxies is crucial to understanding the circulation of material within a galaxy and between galaxies and the intergalactic environment. We present new HI observations of the edge-on galaxy NGC891, which show an extended halo component lagging behind the disc in rotation. We compare these results for NGC891 with other detections of gaseous haloes. Finally, we present a dynamical model for the formation of extra-planar gas.