The Secret XUV Lives of Cepheids: FUV/X-ray Observations of Polaris and beta Dor

TL;DR

This study reveals unexpected high-energy FUV and X-ray emissions in Cepheids Polaris and beta Dor, showing variability and suggesting possible magnetic activity, shocks, or winds, which challenges previous understanding of these stars.

Contribution

First detection of strong FUV emissions and variability in Cepheids, combined with X-ray observations, indicating high-temperature plasma and dynamic processes not previously observed.

Findings

Strong FUV emissions detected in Polaris and beta Dor

X-ray emissions observed in both stars

Beta Dor shows FUV variability correlated with pulsation period

Abstract

We report on the surprising recent discovery of strong FUV emissions in two bright, nearby Classical Cepheids from analyses of FUSE archival observations and one of our own approved observations just prior to the failure of the satellite. Polaris and beta Dor are currently the only two Cepheids to have been observed with FUSE, and beta Dor is the only one to have multiple spectra. Both Cepheids show strong C III (977A, 1176A) and O VI (1032A, 1038A) emissions, indicative of 50,000-500,000 K plasma, well above the photospheric temperatures of the stars. More remarkably, beta Dor displays variability in the FUV emission strengths which appears to be correlated to its 9.84-d pulsation period. This phenomenon has never before been observed in Cepheids. The FUV studies are presented along with our recent Chandra/XMM X-ray observations of Polaris and beta Dor, in which X-ray detections were found for both stars (as well as for the prototype Classical Cepheid, delta Cep). Further X-ray observations have been proposed to unambiguously determine the origin and nature of the observed high energy emissions from the targets, possibly arising from warm winds, shocks, or pulsationally induced magnetic activity. The initial results of this study are discussed.