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

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 Deltav, period P) and chemical (the epsilon(C)/epsilon(O) abundance ratio) input parameters, an improved approximative mass-loss formula has been derived which depends only on the stellar parameters (effective temperature T-eff, 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)over dot (in M-circle dot/yr) on T-eff: log (M)over dot = 8.86- 1.95.log M/M-circle dot 6.81.log T/K+2.47.log L/L-circle dot. 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 (M-i = 1.0 to approximate to 1.3 M-circle dot), we now find more realistic, larger superwind mass-loss rates for larger stellar masses: i.e., (M)over dot M between approximate to0.4 and 1.0 x 10(-4) M-circle dot/yr for M-i between 1.85 and 2.65 M-circle dot, removing between 0.6 and 1.2 M-circle dot, respectively, during the final 30 000 yrs on the tip-AGB