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

Abstract

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. 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${}_2$SiO${}_4$ grains. The grid consists of $\sim 4000$ models, with luminosities from $L_{\star }=890{\mathrm{L}}_{\odot }$ to $L_{\star }=40000{\mathrm{L}}_{\odot }$ and effective temperatures from 2200K to 3400K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75M${}_{\odot }$ to 3M${}_{\odot }$. 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). We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio $L_{\ast }/M_{\ast }$: increasing $L_{\ast }/M_{\ast }$ 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 $\sim 14000{\mathrm{L}}_{\odot }$, and consequently at pulsation periods longer than $\sim 800$ days. The final grain radii range from 0.25 micron to 0.6 micron. The amount of condensed Si is typically between 10% and 40%, with gas-to-dust mass ratios between 500 and 4000.

Accepted to A&A, 17 pages, 15 figures

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