The linear polarisation of southern bright stars measured at the parts-per-million level

TL;DR

This study measures the linear polarisation of 50 bright southern stars with high sensitivity, revealing intrinsic polarisation in many B stars and late giants, and highlighting differences from northern star observations due to interstellar medium variations.

Contribution

First detailed measurement of southern bright stars' polarisation at parts-per-million sensitivity, identifying intrinsic polarisation mechanisms in a larger star sample than previous surveys.

Findings

Higher polarisation levels than northern stars, due to interstellar dust and stellar populations.

Intrinsic polarisation detected in over twenty stars, especially B-type and late giants.

Circumstellar discs in Be stars align with polarisation position angles.

Abstract

We report observations of the linear polarisation of a sample of 50 nearby southern bright stars measured to a median sensitivity of $\sim$4.4 $\times 10^{-6}$. We find larger polarisations and more highly polarised 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 polarised stars, as well as using a wavelength more sensitive to intrinsic polarisation in late-type giants. Significant polarisation 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 twenty. The four most highly polarised stars in the sample are the four classical Be stars ($\alpha$ Eri, $\alpha$ Col, $\eta$ Cen and $\alpha$ 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 polarisation in most B stars in the sample; amongst these are a number of close binaries and an unusual binary debris disk system. However these circumstances do not account for the high polarisations of all the B stars in the sample and other polarigenic mechanisms are explored. Intrinsic polarisation 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 disk systems, the majority of A to K type stars show polarisation levels consistent with interstellar polarisation.