An empirical view of the extended atmosphere and inner envelope of the asymptotic giant branch star R Doradus. II. Constraining the dust properties with radiative transfer modelling

Abstract

Mass loss in oxygen-rich asymptotic giant branch (AGB) stars remains poorly understood, as the dust detected around them appears too transparent to drive winds through absorption alone. The current paradigm invokes outflows driven by photon scattering on relatively large grains ($\sim0.3\,\mu$m), but whether such grains exist in sufficient quantities remains uncertain. We test whether the dust around the oxygen-rich AGB star R~Doradus can drive its wind by combining polarimetric constraints, elemental abundance limits, and force-balance calculations. We examine Fe-free silicates (MgSiO$_3$), alumina (Al$_2$O$_3$), and Fe-bearing silicates (MgFeSiO$_4$) to evaluate whether any species can produce enough radiative pressure under realistic conditions. Using high-angular-resolution polarimetric observations from SPHERE/ZIMPOL at the VLT, we model the circumstellar dust with RADMC-3D and explore a broad parameter space in grain size, density structure, and wavelength-dependent stellar radius. For models consistent with the observations, we assess wind feasibility using updated gas-density profiles, elemental depletion constraints, and radiation-pressure thresholds. Although several dust configurations reproduce the observed scattering patterns, none generate sufficient radiative force at realistic gas-to-dust ratios, even under maximal elemental depletion. Our results for R~Doradus indicate that photon scattering on dust cannot by itself launch the wind, implying that additional mechanisms must contribute.