Theoretical astrophysics and plasma physics at RPC

Abstract:

High-quality Hipparcos data for a complete sample of nearly 12 000 main-sequence and subgiant stars, together with Padua isochrones, are used to constrain the star formation history of the solar neigbourhood and the processes that stochastically accelerate disc stars. The velocity dispersion of a coeval group of stars is found to increase with time from ∼8 km s-1 at birth as t0.33. In the fits, the slope of the initial mass function (IMF) near 1 M⊙ proves to be degenerate with the rate at which the star formation rate declines. If the slope of the IMF is to lie near Salpeter's value, -2.35, the star formation rate has to be very nearly constant. The age of the solar neighbourhood is found to be 11.2 ± 0.75 Gyr with remarkably little sensitivity to variations in the assumed metallicity distribution of old disc stars. This age is only a gigayear younger than the age of the oldest globular clusters when the same isochrones and distance scale are employed. It is compatible with current indications of the redshift of luminous galaxy formation only if there is a large cosmological constant. A younger age is formally excluded because it provides a poor fit to the number density of red stars. Since this density is subject to a significantly uncertain selection function, ages as low as 9 Gyr are plausible even though they lie outside our formal error bars.

Abstract:

In the solar-neighbourhood, older stars have larger random velocities than younger ones. It is argued that the increase in velocity dispersion with time is predominantly a gradual process rather than one induced by discrete events such as minor mergers. Ephemeral spiral arms seem to be the fundamental drivers of disk heating, although scattering by giant molecular clouds plays an important moderating role. In addition to heating the disk, spiral arms cause stars' guiding centres to diffuse radially. The speed of this diffusion is currently controversial. Data from the Hipparcos satellite has made it clear that the Galaxy is by no means in a steady state. This development enormously increases the complexity of the models required to account for the data. There are preliminary indications that we see in the local phase-space distribution the dynamical footprints of long-dissolved spiral waves.

Abstract:

We discuss the transport of angular momentum induced by tidal effects in a disk surrounding a star in a pre-main sequence binary system. We consider the effect of both density and bending waves. Although tidal effects are important for truncating protostellar disks and for determining their size, it is unlikely that tidally-induced angular momentum transport plays a dominant role in the evolution of protostellar disks. Where the disk is magnetized, transport of angular momentum is probably governed by MHD turbulence. In a non self-gravitating laminar disk, the amount of transport provided by tidal waves is probably too small to account for the lifetime of protostellar disks. In addition, tidal effects tend to be localized in the disk outer regions.

Abstract:

An energetic argument implies that a galaxy like the Milky Way is blowing a powerful wind that carries away most of the heavy elements currently synthesized and has impacted the IGM out to at least 180 kpc. Rich clusters of galaxies appear to be closed systems in which most heavy elements are ejected from galaxies. More supernovae are required than the yield of core-collapse SNe from a Salpeter IMF. X-ray observations imply that the IGM in groups and clusters as been strongly preheated. SNe probably cannot supply the required energy, which must come from AGN.