Multitechnique study of the B[e] supergiant RMC 82

Abstract

B[e] supergiants (sgB[e]) are rare objects whose evolutionary stage remains uncertain. Observationally, they display strong Balmer emission lines, infrared excess and intrinsic polarization, indicating a non-spherical circumstellar envelope. We present a study of the sgB[e] RMC 82, using new spectropolarimetric data complemented by photometry from the UV to the mid-IR. Our two-component model comprises a slow, dense equatorial wind wherein dust grains form and a fast polar wind. We applied the HDUST radiative transfer code and Bayesian statistics to infer the parameters from a grid of 3240 pre-computed models. The model accurately reproduces the SED and polarized spectrum, but struggles to match the Hα emission. Our results suggest a large mass loss rate of $6.6 \times 10^{-6}\, \mathrm{{\rm M}_{\odot }\, yr^{-1}\, sr^{-1}}$. The dense wind is confined within an opening angle of 11○. The hottest dust grains are located at 277 R* with a temperature of 870 K. The dust grains are porous, with a density of 0.051 $\rm {g\, cm^{-3}}$. The central star was found to be significantly hotter than previous estimates (Teff = 27 000 K). By comparing models with different components, we find that gas reprocesses a significant amount of UV radiation, shielding the dust. However, the dust also scatters UV photons back to the inner disk, increasing its temperature and Hα emission. We conclude that self-consistent models, that account for the gas-dust interplay in the envelope, are essential for studying sgB[e] and similar objects.