The linear polarization of Southern bright stars measured at the parts-per-million level

Abstract

We report observations of the linear polarization of a sample of 50 nearby Southern bright stars measured to amedian sensitivity of~4.4×10-6.We find larger polarizations and more highly polarized stars than in the previous PlanetPol survey of Northern bright stars. This is attributed to a dustier interstellar medium in the mid-plane of the Galaxy, together with a population containing more B-type stars leading to more intrinsically polarized stars, as well as using a wavelength more sensitive to intrinsic polarization in late-type giants. Significant polarization had been identified for only six stars in the survey group previously, whereas we are now able to deduce intrinsic polarigenic mechanisms for more than 20. The four most highly polarized stars in the sample are the four classical Be stars (α Eri, α Col, η Cen and α Ara). For the three of these objects resolved by interferometry, the position angles are consistent with the orientation of the circumstellar disc determined. We find significant intrinsic polarization in most B stars in the sample; amongst these are a number of close binaries and an unusual binary debris disc system. However, these circumstances do not account for the high polarizations of all the B stars in the sample and other polarigenic mechanisms are explored. Intrinsic polarization is also apparent in several late-type giants which can be attributed to either close, hot circumstellar dust or bright spots in the photosphere of these stars. Aside from a handful of notable debris disc systems, the majority of A-K type stars show polarization levels consistent with interstellar polarization.