Did you know that although standard measurements for polarisation at the parts-per-million level in nearby bright stars have been completed for the northern hemisphere, no such study had been conducted for the south?
**In my best Top Gear presenter voice** : Until now...
The HIPPI group's latest paper (accepted to MNRAS) has done just that. 50 nearby bright stars for the southern hemisphere have had their linear polarisation measured to a (median) sensitivity of about 4.4 parts-per-million.
We found that in the southern hemisphere the stars tend to have higher polarisations than in the north. This is mostly because the Solar System sits above the mid-galactic plane (1, 2) and many of the stars we observed in the survey were also in or below the galactic plane (in the northern hemisphere PlanetPol survey, many of them were above the plane, and often closer to the Sun (3). The interstellar medium polarises light over long distances, so an abundance of the medium can mean more polarisation (4).
HIPPI's southern hemisphere survey results are in green; PlanetPol's northern hemisphere survey results in red. The positions are in galactic coordinantes, the darkness of the spot shows the distance, the size of the bars show the amount of polarisation.
Another reason was that many of our sources happen to be types of stars with intrinsic polarisation. Usually stars do not have a net polarisation as the polarisation in their limbs cancels itself out (5). If the star is prone to star-spots, surrounded by dust, or distorted in some way, it can produce polarisation from the star (or its surrounds) because either the limb is distorted/asymmetric or the surrounding material polarises the light. In our survey, some of the stars were in close binaries or were late giants with surrounding hot gas or bright spots (6).
Why is this important?
It helps future studies of polarised light by helping other astronomers (and our own group) to gage the amount of polarised light we expect (particularly in calibrating new instruments). It also tells us about the interstellar medium itself. The polarisation in the ISM is due to trends in the alignment of the particles and that is related to the structure of and magnetic fields in our local galaxy.
If you can't wait for the MNRAS version, you can checkout the pre-print here: http://arxiv.org/abs/1509.07221
1: Bahcall 1985
2: Dehnen 1998
3: Bailey 2010
4: Hiltner 1949
5: Wiktorowicz 2014
6: Cotton 2015