Looking on the Dark Side: A Search for the Secondary Spectrum of ɛ Aurigae
Abstract
The enigmatic long-period eclipsing binary system ɛ Aurigae, which recently eclipsed in 2009--2011, has been the subject of a concerted observing campaign by both amateur and professional astronomers. It has been well-observed by both ground-based photometric and spectroscopic campaigns, interferometric arrays and space observatories. New, and much improved, spectroscopic orbits are now available, and interferometric observations, especially by the CHARA array, have yielded accurate angular diameters and useful constraints on the relative size of the system. All of these observations detected only the bright F-type primary star; the companion remains dark at all wavelengths except in the mid-infrared (where the cool, dusty disk around the companion radiates) and in the far ultraviolet (where photons from the hot star embedded in the disk scatter and escape). No sign of the companion has ever been seen at optical wavelengths. However, the profiles of F star spectral lines show evidence of additional absorption near and during eclipse, presumably from disk material around the companion that partially obscures the line of sight to the F star. Photons are not easily destroyed in a low density circumstellar environment but are instead usually just absorbed and re-emitted, a scattering process that merely redirects the path of the photon. Therefore, the additional absorption by the companion's disk near eclipse should result in additional emission in that line around the orbit. Although this emission from the companion would be distributed isotropically in space and therefore be weak, the possibility remains that it might be detected because of its distinctive radial velocity signature. To examine this, ~300 spectra of ɛ Aurigae obtained over the past 20 years at the Dominion Astrophysical Observatory in Canada, and Ondrejov Observatory in the Czech Republic, were analyzed. In the end, no optical spectral signature of the companion was found, but the resulting dataset provides some important insights into the nature of the F supergiant's extended - and highly variable - atmosphere.
- Publication:
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Giants of Eclipse
- Pub Date:
- July 2013
- Bibcode:
- 2013giec.conf10306B