Spectropolarimetric Variability and Co-Rotating Structure in HD 92207

TL;DR

This study presents low-resolution spectropolarimetric observations of the variable A0 supergiant HD 92207, revealing polarization variability linked to wind structures and suggesting complex circumstellar environments.

Contribution

First detailed spectropolarimetric analysis of HD 92207 over multiple epochs, linking polarization variability to wind structures and proposing a spiral wind density model.

Findings

Polarization varies over the star's rotational phase.

Halpha polarization differs from continuum polarization.

Wind-related variability is observed in Halpha line.

Abstract

We report on low resolution (R~3000) spectropolarimetry of the A0 supergiant star HD 92207. This star is well-known for significant spectral variability. The source was observed on seven different nights spanning approximately 3 months in time. With a rotation period of approximately 1 year, our data covers approximately a quarter of the star's rotational phase. Variability in the continuum polarization level is observed over this period of time. The polarization across the Halpha line on any given night is typically different from the degree and position angle of the polarization in the continuum. Interestingly, Hbeta is not in emission and does not show polarimetric variability. We associate the changes at Halpha as arising in the wind, which is in accord with the observed changes in the profile shape and equivalent width of Halpha along with the polarimetric variability. For the continuum polarization, we explore a spiral shaped wind density enhancement in the equatorial plane of the star, in keeping with the suggestion of Kaufer etal (1997). Variable polarization signatures across Halpha are too complex to be explained by this simple model and will require a more intensive polarimetric follow-up study to interpret properly.