An extensive grid of DARWIN models for M-type AGB stars : I. Mass-loss rates and other properties of dust-driven winds

Abstract

Context. The stellar winds of asymptotic giant branch (AGB) stars are commonly attributed to radiation pressure on dust grains, formed in the wake of shock waves that arise in the stellar atmospheres. The mass loss due to these outflows is substantial, and modelling the dynamical properties of the winds is essential both for studies of individual stars and for understanding the evolution of stellar populations with low to intermediate mass.

Aims. The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters.

Methods. We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg ₂SiO ₄ grains. The grid consists of ~4000 models, with luminosities from L _⋆ = 890 L _⊙ to L _⋆ = 40 000 L _⊙ and effective temperatures from 2200 to 3400 K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75 M _⊙ to 3 M _⊙. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si).

Results. We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio L _*∕ M _*: increasing L _*∕ M _* by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than ~14 000 L _⊙, and consequently at pulsation periods longer than ~800 days. The final grain radii range from 0.25 to 0.6 μm. The amount of condensed Si is typically between 10 and 40%, with gas-to-dust mass ratios between 500 and 4000.