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 amplitude Δv, period P) and chemical (the εC/εO 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 M 1(in M⊙/yr) on Teff: log M = 8.86-1.95·log M/M⊙-6.81·log T/K+2.47·log L/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 difference for the models in the lower mass range of consideration (Mi = 1.0 to ≈ 1.3 M⊙), we now find more realistic, larger superwind mass-loss rates for larger stellar masses: i.e., Ṁ between ≈ 0.4 and 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 final 30000 yrs on the tip-AGB.