Exploring wind-driving dust species in cool luminous giants. III. Wind models for M-type AGB stars: dynamic and photometric properties

Context. Stellar winds observed in asymptotic giant branch (AGB) stars are usually attributed to a combination of stellar pulsations and radiation pressure on dust. Shock waves triggered by pulsations propagate through the atmosphere, compressing the gas and lifting it to cooler regions which creates favourable conditions for grain growth. If sufficient radiative acceleration is exerted on the newly formed grains through absorption or scattering of stellar photons, an outflow can be triggered. Strong candidates for wind-driving dust species in M-type AGB stars are magnesium silicates (Mg₂SiO₄ and MgSiO₃). Such grains can form close to the stellar surface, they consist of abundant materials and, if they grow to sizes comparable to the wavelength of the stellar flux maximum, they experience strong acceleration by photon scattering.
Aims: The purpose of this study is to investigate if photon scattering on Mg₂SiO₄ grains can produce realistic outflows for a wide range of stellar parameters in M-type AGB stars.
Methods: We use a frequency-dependent radiation-hydrodynamics code with a detailed description for the growth of Mg₂SiO₄ grains to calculate the first extensive set of time-dependent wind models for M-type AGB stars. This set includes 139 solar-mass models, with three different luminosities (5000 L_⊙, 7000 L_⊙, and 10 000 L_⊙) and effective temperatures ranging from 2600 K to 3200 K. The resulting wind properties, visual and near-IR photometry and mid-IR spectra are compared with observations.
Results: We show that the models can produce outflows for a wide range of stellar parameters. We also demonstrate that they reproduce observed mass-loss rates and wind velocities, as well as visual and near-IR photometry. However, the current models do not show the characteristic silicate features at 10 and 18 μm as a result of the cool temperature of Mg₂SiO₄ grains in the wind. Including a small amount of Fe in the grains further out in the circumstellar envelope will increase the grain temperature and result in pronounced silicate features, without significantly affecting the photometry in the visual and near-IR wavelength regions.
Conclusions: Outflows driven by photon scattering on Mg₂SiO₄ grains are a viable wind scenario for M-type AGB stars, given the success of the current models in reproducing observed mass-loss rates, wind velocities, and photometry. Both synthetic and observed photometry suggest that the dusty envelopes of M-type AGB stars are quite transparent at visual and near-IR wavelengths, otherwise the variations in visual flux would not be dominated by molecular features.