Modelling the formation and evolution of inflowing and outflowing gaseous disks around stars

Abstract

The aim of this project is the theoretical and observational study of accretion and decretion disks formed around stars, both of which are governed by the same physics (gravity, radiation transfer, radiative pressure).The main object will be the outflowing disks observed in Be stars, which are the only stars in the main sequence to display such structures. Recent progress in the literature allowed the establishment of the mechanism that make these disks grow: given a source of matter (the star) and appropriate angular momentum, material can be progressively lifted to higher orbits by means of viscous diffusion. Current models assume that the material is ejected by the star from its equator in axial symmetry. This is clearly too strong an assumption, and fails to reproduce the variability observed in Be stars: Be stars are highly variable, and part of this variability is certainly associated with asymmetric mass ejections. We propose to examine the problem of how to form a decretion disk from discrete mass ejection events. For this, we will use hydrodynamic and radiative transfer codes developed by the supervisor and his collaborators. With this study we expect to explain qualitatively (and, hopefully, quantitatively) the observed variability of Be stars, and thus impose important constraints on the nature of these ejection events (ejected mass, duration and frequency of the outbursts).The aim of the second part of the project is the creation of a self-consistent magnetohydrodynamic model for accretion disks, which will simultaneously compute the dynamical, thermal and chemical structure of the disk. The simultaneous calculation of magnetohydrodynamics with the chemical evolution and radiative transfer is not common in astrophysics (due to its complexity), but it is necessary in order to transmit feedback from/to magnetohydrodynamic variables that are interconnected with chemistry, e.g. temperature, ionization and electron fraction, drift speeds. Hence, the model can be a first step towards unifying magnetohydrodynamics with the appropriate thermal and radiative physics. It will be based on a code for magnetohydrodynamic molecular disk winds developed by the applicant in her PhD, which ignores this feedback. The radiative transfer code developed by the supervisor is to be implemented into the final model, in order to produce observational predictions.We believe that the proposed research project is able to stimulate the scientific interest in many astrophysical fields: in (a) protostars, pre-main sequence stars, compact stars and other astronomical bodies where accretion disks are observed, and (b) Be stars temporarily surrounded by decretion disks. Furthermore, the findings of this project might be particularly useful for future research involving (c) stochastic processes on the surface of stars, and (d) any attempt for parallel calculations of magnetohydrodynamics and astrochemistry.