Polarization due to rotational distortion in the bright star Regulus

Abstract

Polarization in stars was first predicted by Chandrasekhar 1 who calculated a substantial linear polarization at the stellar limb for a pure electron-scattering atmosphere. This polarization will average to zero when integrated over a spherical star but could be detected if the symmetry is broken, for example by the eclipse of a binary companion. Nearly 50 years ago, Harrington and Collins 2 modeled another way of breaking the symmetry and producing net polarization -the distortion of a rapidly rotating hot star. Here we report the first detection of this effect. Observations of the linear polarization of Regulus, with two different high-precision polarimeters, range from +42 parts-per-million (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 + .− . % of its critical angular velocity for breakup, and has an inclination greater than 76.5 degrees. The rotation axis of the star is at a position angle of 79.5±0.7 degrees. The conclusions are independent of, but in good agreement with, the results of previously published interferometric observations of Regulus 3 . The accurate measurement of rotation in early-type stars is important for understanding their stellar environments 4 , and course of their evolution 5 . Discovery Project grants DP140100121 and DP160103231.We wish to acknowledge Daniela Opitz, Jacob Sturges and the staff at the AAT for their assistance in making the HIPPI observations. We thank Robert Spurr of RT Solutions for providing the VLIDORT software. We would also like to thank the anonymous referees for their thoughtful feedback.

The rotation of α Oph investigated using polarimetry

“…The modelling approach described here is essentially the same as used for our study of Regulus (Cotton et al 2017a), but is presented here with a little more detail.…”

“…Fig. 2 of Cotton et al 2017a). There is a distinctive change in sign of polarization in the hotter models as described by Harrington (2017).…”

“…However, subsequent studies using more complete stellar-atmosphere models showed that the expected polarization was small at visible wavelengths (Collins 1970;Sonneborn 1982). It was not until the development of polarimeters that could measure linear polarization to parts-per-million (ppm) levels of precision (Hough et al E-mail: j.bailey@unsw.edu.au 2006; Bailey et al 2015) that the effect was detected (Bailey et al 2010) and confirmed by multiwavelength polarimetry and detailed modelling of the bright star Regulus (α Leo; Cotton et al 2017a).…”

“…1 we show the modelled polarization for rapidly rotating (ω = 0.95) stars of different polar temperatures. The models were calculated using the methods described in Cotton et al (2017a) and in section 3 of this paper. The polarization wavelength dependence seen in these models is a complex combination of several effects.…”

“…In this paper we report polarimetric observations of α Oph in section 2. We compare the data with models using a similar approach to that adopted in Cotton et al (2017a) in section 3, and discuss the results in section 4.…”