An improved mass-loss description for dust-driven superwinds and tip-AGB evolution models

A. Wachter, K.-P. Schröder, J.M. Winters, T.U. Arndt, and E. Sedlmayr

Astronomy & Astrophysics, 384, 452-459 (2002)

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Abstract:

We derive an improved description of dust-driven stellar mass-loss for the cool winds of carbon-rich tip-AGB stars. We use pulsating wind models in which the mass loss is driven by radiation pressure on dust grains, for C-rich chemistry. From a larger set of these models, selected for representative dynamical (pulsational velocity amplitude Delta v, period P) and chemical (the C/O abundance ratio) input parameters, an improved approximative mass-loss formula has been derived which depends only on the stellar parameters (effective temperature Teff, luminosity L and mass M). Due to the detailed consideration of the chemistry and the physics of the dust nucleation and growth processes, there is a particularly strong dependence of the mass-loss rate dot{M} (in Msun/yr) on Teff: log dot{M} = 8.86 - 1.95 log M/ Msun - 6.81 log T/K + 2.47 log L/Lsun.
The dependence of the model mass-loss on the pulsational period has explicitly been accounted for in connection with the luminosity dependence, by applying an observed period--luminosity relation for C-rich Miras. We also apply the improved mass-loss description to our evolution models, and we revisit their tip-AGB mass-loss histories and the total masses lost, in comparison to our earlier work with a preliminary mass-loss description. While there is virtually no difference for the models in the lower mass range of consideration (Mi = 1.0 to approx 1.3 Msun), we now find more realistic, larger superwind mass-loss rates for larger stellar masses: i.e., dot{M} between approx 0.4 and 1.0 10-4Msun/yr for Mi between 1.85 and 2.65 Msun, removing between 0.6 and 1.2 Msun, respectively, during the final
30000 yrs on the tip-AGB.