Spectral and photometric analysis of the eclipsing binary epsilon Aurigae prior to and during the 2009-2011 eclipse

TL;DR

This study analyzes spectral and photometric data of epsilon Aurigae from 1994 to 2010, revealing the presence of an extended atmosphere around the unseen companion, complex variability during eclipse, and challenging previous hypotheses about the eclipse mechanism.

Contribution

It provides new insights into the spectral features and variability of epsilon Aurigae, especially regarding the extended atmosphere and intrinsic stellar variability during eclipse.

Findings

Disentangled the primary star's spectrum but not the companion's.

Detected an extended atmosphere around the unseen companion via H-alpha line profiles.

Identified a dominant 66.21-day variability period during eclipse, intrinsic to the F star.

Abstract

A series of 353 red electronic spectra obtained between 1994 and 2010, and of 171 UBV photometric observations of the 2010 eclipse, were analyzed in an effort to better understand the eclipsing binary eps Aur. The main results follow. (1) We attempted to recover a spectrum of the companion by disentangling the observed spectra of the eps Aur binary failed, but we were able to disentangle the spectrum of telluric lines and obtain a mean spectrum of the F-type primary star. The latter was then compared to a grid of synthetic spectra for a number of plausible values of T(eff) and log(g), but a reasonably good match was not found. However, we conclude that the observed spectrum is that of a low-gravity star. (2) We examined changes in the complex H-alpha line profiles over the past 16 years, with particular emphasis on the 2009-2011 eclipse period, by subtracting a mean out-of-eclipse H-alpha profile (appropriately shifted in radial velocity) from the observed spectra. We find that the dark disk around the unseen companion has an extended 'atmosphere' that manifests itself via blueshifted and redshifted H-alpha 'shell' absorptions seen projected against the F star. Significantly, the H-alpha shell line first appeared three years before first contact of the optical eclipse when the system was not far past maximum separation. (3) Analyses of radial velocities and central intensities of several strong, unblended spectral lines, as well as UBV photometry, demonstrated that these observables showed apparent multiperiodic variability during eclipse. The dominant period of 66.21 was common to all the observables, but with different phase shifts between these variables. This result strongly supports our earlier suggestion that the photometric variability seen during eclipse is intrinsic to the F star, and therefore, the idea of a central brightening due to a hole in the disk should be abandoned.