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

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 amplitudev ,p eriodP ) and chemical (theC=O abundance ratio) input parameters, an improved approximative mass-loss formula has been derived which depends only on the stellar parameters (eective temperature Te, 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 _ M (in M=yr) on Te :l og _ M =8 :86 1:95logM=M 6:81logT=K+2:47logL=L. 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 dierence for the models in the lower mass range of consideration (Mi =1 : 0t o 1:3M), we now nd more realistic, larger superwind mass-loss rates for larger stellar masses: i.e., _ M between0: 4a nd 1:0 10 4 M/yr for Mi between 1.85 and 2.65 M, removing between 0.6 and 1.2 M, respectively, during the nal 30 000 yrs on the tip-AGB.