[Astrogazer (OP)]

https://www.nature.com/articles/s41550-017-0238-6 [nofollow]

In this abstract discussing polarised light originating from the equatorial horizons of Regulus caused by its very high spin speed, it talks about:-

“ Observations of the linear polarization of Regulus, with two different high-precision polarimeters, range from +42 ppm at a wavelength of 741 nm to –22 ppm at 395 nm. The reversal from red to blue is a distinctive feature of rotation-induced polarization. Using a new set of models for the polarization of rapidly rotating stars, we find that Regulus is rotating at 96.5+0.6−0.8%
 of its critical angular velocity for break-up, and has an inclination greater than 76.5°. The rotation axis of the star is at a position angle of 79.5 ± 0.7°.”

What is this inclination angle of 76 deg, relative to what? 

And the 42 and 22 ppm, what is this small quantity referring to, the percentage of light that is polarised?

Thanks

Jim

[Halc]

What is this inclination angle of 76 deg, relative to what? 

Not sure, but one of them (spin inclination) is perhaps relative to the axis of its orbit about its immediate companion.  There is also the axis of the orbit of the pair as it orbits the other pair of the 4-star system.

The only other possibility is the orbital or spin axis relative to our line of sight from Earth here. Yes, it would have been nice if they would have specified.

And the 42 and 22 ppm, what is this small quantity referring to, the percentage of light that is polarised?

The 22 figure is negative, so it seems unlikely that it means a percentage of light, which I don't see being meaningfully negative.

[evan_au]

A draft of the paper is available from a pre-print server here:
https://arxiv.org/ftp/arxiv/papers/1804/1804.06576.pdf

One of the references in this paper is "INTRINSIC POLARIZATION OF RAPIDLY ROTATING EARLY-TYPE STARS"
It is available for free here: http://articles.adsabs.harvard.edu/pdf/1968ApJ...151.1051H

[Astrogazer (OP)]

Thank you for that link, I didn't realize that there might be free versions of the whole paper available.   I’m still a bit of trouble understanding what the datum is, the 0 degrees which as a result of the edge effects of the high speed spinning star results in a 79 deg twist.   Whilst I understand the general physics as I took physics to first year degree level, I’m having trouble with the terminology.  The authors seem to use abbreviations etc without explaining what they mean, still, I suppose that if I’m reading such a paper I’m supposed to know what the abbreviations mean and what the 0 degree datum is.