Hot Stars Mass-loss studied with Spectro-Polarimetric Interferometry (SPIN)

We present a prospective work undertaken on Spectro-Polarimetric INterferometry (SPIN). Our theoretical studies suggest that SPIN is a powerful tool for studying the mass loss from early type stars where strong Thomson scattering is present. Based on Monte Carlo simulations, we computed the expected SPIN signal for numerous hot star spectral types covering a broad range of geometries and optical depths. The SPIN technique is based on the detection and comparison of the fringe characteristics (complex visibility) between two perpendicular directions of polarization. The most obvious advantage is its ability to determine the polarization distribution in spherical winds for which no detection of polarization is achievable by classical techniques. In particular, we demonstrate that the SPIN technique is very sensitive to the $\beta$ parameter from the so-called " $\beta$ velocity law" for optically thin winds. Moreover, the location where the bulk of polarization is generated can be defined accurately. The required sensitivity for studying main sequence OB star winds is still very demanding (inferior to 0.5%), but the signal expected from denser winds or extended atmospheres is well within the capabilities of existing interferometers. The visibility curves obtained in two perpendicular polarizations for LBVs or WR stars can differ by more than 15%, and their corresponding limb-darkened radii obtained by the fit of these curves by more than 35%. The signal expected from the extended circumstellar environment of Be stars and B[ e] appears also to be easy to detect, relaxing the required instrumental accuracy to 1%. For these spectral types, the SPIN technique provide a good tool to extract the highly polarized and spatially confined envelope contribution from the bright star emission. It must be pointed out that the astrophysical environments investigated here offer a large panel of SPIN observing conditions in terms of geometry and polarization degree. The behavior of the SPIN observables can be transposed, at least qualitatively, to other astronomical objects for which important local polarization is foreseen.