Epsilon Sagittarii: An Extreme Rapid Rotator with a Decretion Disk

TL;DR

This paper presents high-precision polarization observations of epsilon Sagittarii, revealing it as an extremely rapid rotator with a decretion disk, likely formed through binary interaction, and discusses its unique stellar properties.

Contribution

The study provides the highest measured stellar rotation rate to date and characterizes a star with a decretion disk, offering insights into extreme stellar rotation and binary evolution.

Findings

Rotation rate ω ≥ 0.995, the highest in our galaxy.

Presence of a low-density edge-on gas disk causing polarization.

Star's properties suggest binary interaction rather than single-star evolution.

Abstract

We report high-precision multi-wavelength linear-polarization observations of the bright B9 (or A0) star $\epsilon$ Sagittarii. The polarization shows the distinctive wavelength dependence expected for a rapidly rotating star. Analysis of the polarization data reveals an angular rotation rate $\omega$ (= $\Omega/\Omega_{crit})$ of 0.995 or greater, the highest yet measured for a star in our galaxy. An additional wavelength-independent polarization component is attributed to electron scattering in a low-density edge-on gas disk that also produces the narrow absorption components seen in the spectrum. Several properties of the star (polarization due to a disk, occasional weak H$\alpha$ emission, and multiple periodicities seen in space photometry) resemble those of Be stars, but the level of activity in all cases is much lower than that of typical Be stars. The stellar properties are inconsistent with single rotating-star evolutionary tracks, indicating that it is most likely a product of binary interaction. The star is an excellent candidate for observation by interferometry, optical spectropolarimetry to detect the \"{O}hman effect, and UV polarimetry; any of which would allow its extreme rotation to be tested and its stellar properties to be refined.