George H. Bowen

From wind to superwind : the evolution of mass-loss rates for Mira models

Dynamical atmosphere models were calculated for a large grid of variables with Mira-like properties satisfying the Iben radius-luminosity-mass relationship for evolving AGB stars. Their masses ranged from 0.7 to 2.4 M ○. , and their periods from 150 to 800 days. All were fundamental-mode pulsators, had solar metallicity, and included effects of dust. As a natural consequence of evolutionary changes in stellar parameters, the mass-loss rate increases as an approximately exponential function of time

The self-regulated winds of long-period variable stars

Numerical models of the dynamically extended atmospheres of long-period variable or Mira stars have shown that their winds have a very simple, power-law structure when averaged over the pulsation cycle. This structure is stable and robust despite the pulsational wave disturbances, and appears to be strongly self-regulated. Observational studies support these conclusions. The numerical models also show that dust-free winds are nearly adiabatic, with little heating or cooling. However, the classical, steady, adiabatic wind solution to the hydrodynamic equations fails to account for an extensive region of nearly constant outflow velocity. An important process or group of processes is missing from this solution. Since gas parcels moving out in the wind are periodically overrun by pulsational waves, we investigate analytic solutions which include the effects of wave pressure, heating and the resulting entropy changes. In the case of dust-free winds we find that only a modest amount of wave pressure is needed in an analytic model for a steady, constant-velocity, locally adiabatic outflow. Wave pressure is represented with a term like that in the Reynolds turbulence equation for the mean velocity. The waves damp relatively quickly with radius, as a result of the work they do in driving the mean flow. Although the pressure from individual waves is modest, the waves are likely the primary agent of the self-regulation of the dust-free winds. In dusty Miras, the numerical models show that the radiation pressure on grains and the subsequent momentum transfer to the gas play the dominant roles in driving the wind, and that wave pressure is not very important. In the models of the dusty wind, the gas variables also adopt a power-law dependence on radius. Heating is required at all radii to maintain this flow, and grain heating and heat transfer to the gas are significant. Both hydrodynamic and gas/grain thermal feedbacks can transform the flow towards particular self-regulated forms.

A Model for the Variability of SiO Masers in Mira Variables

Mass loss from M-Miras makes a major contribution to the injection of heavy elements into the interstellar medium and is a key stage in stellar evolution in which a late-type star embarks on its route towards the formation of a white dwarf. Recent VLBI images of SiO maser emission find that SiO masers lie in an incomplete ring around the host M-Mira (Diamond et al. 1994), stellar radius from the photosphere in the case of TX Cam, i.e. in the zone where mass loss is initiated. SiO masers show complicated and time-varying structure. With the new VLBI data and new hydrodynamic pulsation models and maser models, it is now possible to use SiO maser emission to examine critically our understanding of the mass loss mechanism in late-type stars.