Variable Stars

029.08

The Secret Lives of Cepheids: Discovery of Strong FUV Emissions in the Classical Cepheids Polaris and beta Dor

1/7/2007 9:20:00 AM

1/7/2007 6:30:00 PM

07. Binary Stars, Variable Stars

Scott G. Engle¹, E. F. Guinan¹, N. R. Evans^2
1Villanova Univ., ²Harvard-Smithsonian CfA.

We report on the surprising recent discovery of strong FUV emissions in two bright, nearby Classical Cepheids from analyses of FUSE archival observations. Polaris (V = +1.98, F7 Ib-II, P = 3.97-d; d ~ 132-pc) and beta Dor (V = +3.77, F6 Ia; P = 9.84-d; d ~ 350-pc) are currently the only two Cepheids to have been observed with FUSE, and beta Dor is the only one to have multiple spectra. The FUSE wavelength region is ideal for such a study since it is uncontaminated by the continua of these F supergiants. Both Cepheids show strong C III (977, 1176) and O VI (1032, 1038) emissions, indicative of 50,000 - 500,000 K plasma, well above the photospheric temperatures. Also, 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 X-ray observation of Polaris, in which an X-ray detection (log Lx = 28.8 ergs/sec) was found. Further FUV/X-ray observations have been proposed with FUSE/XMM 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 will be discussed, along with their impact on understanding the structure, heating, dynamics and possible magnetic activity of Cepheid (and related Hybrid Star) atmosphere.
We gratefully acknowledge support for this project from NASA grant Chandra-GO6-7011A and NSF grant AST05-07542.