A systematic investigation of the mass loss mechanism in dust forming long--period variable stars

J.M. Winters, T. Le Bertre, K.S. Jeong, Ch. Helling, E. Sedlmayr

Astronomy & Astrophysics, 361, 641-659 (2000)

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Abstract:

In order to investigate the relations between the mass loss from pulsating red giants and quantities which can be obtained from observations, we have explored the behavior of theoretical models which treat the time-dependent hydrodynamics of circumstellar outflows, including a detailed treatment of the dust formation process. This approach, while ignoring effects such as a possible nonsphericity of the stellar atmospheres which are difficult to assess, accounts correctly for factors such as the grain formation and destruction which are crucial to the mass-loss mechanism.

We built a grid of sim 150 models covering a wide range of physical situations. This grid allows us to characterize the effects of different parameters, such as the stellar luminosity and temperature, the period and the amplitude of the pulsation, and the C/O element abundance ratio, on the behavior of AGB winds and on the rates of mass loss.

We find two regimes for the stellar outflows. The first one (A) is characterized by stable winds with a layered structure of the circumstellar dust shell, outflow velocities in excess of 5 kms-1, and a large rate of mass loss. These outflows are dominated by radiation pressure on dust. For these models we find good correlations between near-infrared colors and the mass loss rates. In the second regime (B), the winds are slow and do not present a layered structure. The outflows displaying the second behavior come, e.g., from red giants with low luminosity, high temperature, or short period. For them there is no correlation between color and mass loss rate. The mass loss rates are low and never exceed 3*10-7 Msunyr-1 Radiation pressure on dust plays only a minor role in this regime.

We have explored the effect of different parameters on the behavior of the stellar winds. We find that, in general, all other parameters been kept identical, there is a narrow range of values for each parameter within which the models abruptly change from B to A, and that once a model is stabilized in the A mode the changes in the values of each parameter have only a smooth effect on the wind characteristics.

Keywords:

Hydrodynamics -- Stars: circumstellar matter -- Stars: carbon -- Stars: mass--loss -- Stars: AGB and post-AGB -- Stars: winds, outflows