University of Wisconsin

Space Astronomy Laboratory




Objects: 2109, 2133, 2132, 2188, 2115, and 2122
UW Astronomer: Karen Bjorkman

Less than 1% of the objects in the solar neighborhood are stars that we call supergiants. As their name implies, these objects are very large and massive and have temperatures which range from 3000 - 30,000K (our sun has a temperature of about 6000K). Because the class contains such diverse objects, we have separated them into three primary categories: hot supergiants, cool supergiants, and peculiar hot stars, B e stars, which have more abundant flux in the red region of the spectrum than expected. Asymmetries in the atmospheres of these stars give rise to polarization produced by different scattering processes. In the case of hot supergiants, we expect that scattering by electrons in temporary "plumes" close to the surface of the star will cause the observed polarization. Our program contains three different targets in this class: P Cygni, 9 Sagittae, and GX Velorum. Our cool supergiant program consists essentially of one target, namely, Delta Canis Majoris. In this case of the these stars, polarization is attributed to atomic and dust scattering off inhomogeneities in the stellar atmosphere. The B e stars (HD45677) have possibly two different mechanisms contributing to polarization. The first is electron scattering in material ejected from the poles; the second is dust scattering in a flattened disk formed by ejection of a slow wind from the equator. Since interstellar polarization is expected to be considerably smaller in the UV than in the visible region of the spectrum, we expect the analysis of these data will be simplified and smaller effects will be detectable.