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Cool star mass loss processes are discussed relative to impact on evolution of stars and stellar populations, emphasizing loss that affects fates of stars with initial masses of 1-9 solar masses. One epoch of mass loss, the episode that concludes their evolution up the asymptotic giant branch, is important for these stars. Observational and theoretical bases for various mass loss formulae in use are also reviewed and compared.

Observations tend to select mass loss rates near the critical rate, Ṁcrit = M/L. There are two reasons for this. In some situations, such as near the tip of the AGB, the mass loss rate is very sensitive to stellar parameters. In this case, stars with Ṁ ≪ Ṁcrit have dust-free, hard-to-measure mass loss rates while stars with Ṁ ≫ Ṁcrit do not survive very long and thus make up a small fraction of any sample. Selection effects dominate the fitting of empirical formulae; observations of mass loss rates tell us more about which stars are losing mass than about how a star loses mass. In other situations, such as for some of the stars along the RGB, a steady state situation occurs where the loss of mass leads to a decrease in mass loss rate while the evolutionary changes lead to an increase; the result is a steady state with Ṁ = Ṁcrit. To determine the envelope mass and composition at the end of a phase of intensive mass loss requires stellar evolution models capable of responding on a time scale ~ tKH and thus, a new generation of stellar modeling codes. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

We present numerical hydrodynamical modelling of the effects of a giant planet or brown-dwarf companion orbiting within the extended atmosphere and wind formation zone of an approximately solar-mass Mira variable star. The large-scale, time-dependent accretion flows within the radially oscillating and outflowing circumstellar gas around Miras are related to Bondi–Hoyle–Lyttleton flows, but have not, to our knowledge, been previously modelled. The new models presented in this paper illustrate the changes in accretion and wake dynamics as the companion mass is varied over a range from 10 to 50 Jupiter masses , and generalize the results of the single model we presented in an earlier paper. The character of the accretion on to the companion changes greatly as the companion mass is increased. At the lowest companion masses considered here, a low continuous rate of mass accretion is punctuated by large, nearly periodic bursts of accretion. When the companion mass is large, the mass accretion has both a continuous part and a rapidly varying, nearly stochastic part. Surprisingly, the angular momentum of the accreted gas shows an opposite trend with mass, varying nearly periodically at large companion masses and stochastically at low masses. These trends can be understood as the result of the interplay between the shocks and radial oscillations in the circumstellar gas and the wake flow behind the companion. Boundary conditions also affect the character of the accretion. The equation of state, however, is found to have little effect, at least for gamma-law gases, with gamma in the range from 1 to 5/3. Models with accretion bursts may produce observable optical brightenings, and may affect SiO maser emission, as we suggested in previous papers. Interruptions of continuous accretion, or shadowing effects, could give rise to bursts of dimming in the optical. Such dimming effects are likely to be correlated with bursts and optical flashes, helping to explain some rather mysterious Hipparcos observations. [ABSTRACT FROM AUTHOR]

It has long been clear that most, if not all, of the mass loss experienced by stars from 0.8 to 8 solar masses occurs near the tip of the AGB and/or the RGB. Evolutionary studies have incorporated empirical mass loss laws but theoretical models suggest quite different dependence of mass loss rate on stellar parameters. We are combining evolutionary model calculations with ISUEVO with mass loss modeling using the Bowen code in a systematic study of final stages of stellar evolution. We mapped the RGB (without steady mass loss) to the “Death Zone” as a function of mixing length, mass, and metallicity. We compared these results with observation data from Origlia. We are investigating a possible mass loss mechanism through companions as a complement to mass loss through pulsation. By the end of the project we expect to provide a reliable prescription for AGB mass loss. [ABSTRACT FROM PUBLISHER]