Polarimetric detection of nonradial oscillation modes in the beta Cephei star beta Crucis

TL;DR

This study detects polarization variations caused by nonradial pulsation modes in beta Crucis, confirming long-standing predictions and demonstrating the utility of polarimetry in asteroseismology for mode identification and stellar modeling.

Contribution

The paper introduces the first detection of polarization variations due to nonradial modes in beta Cephei stars and combines multiple observational methods for detailed mode identification.

Findings

Identified the dominant nonradial mode as l=3, m=-3.

Determined the stellar axis position angle as 25 or 205 degrees.

Derived the star's inclination and asteroseismic age, revealing its mass and core properties.

Abstract

Here we report the detection of polarization variations due to nonradial modes in the beta Cephei star beta Crucis. In so doing we confirm 40-year-old predictions of pulsation-induced polarization variability and its utility in asteroseismology for mode identification. In an approach suited to other beta Cep stars, we combine polarimetry with space-based photometry and archival spectroscopy to identify the dominant nonradial mode in polarimetry, f2, as l = 3, m = -3 (in the m-convention of Dziembowski) and determine the stellar axis position angle as 25 (or 205) +/- 8 deg. The rotation axis inclination to the line of sight was derived as approx. 46 deg. from combined polarimetry and spectroscopy, facilitating identification of additional modes and allowing for asteroseismic modelling. This reveals a star of 14.5 +/- 0.5 Solar masses and a convective core containing approx. 28% of its mass -- making beta Crucis the most massive star with an asteroseismic age.