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

Abstract

A series of 353 red electronic spectra (from three observatories, mostly from 6300 to 6700 A) obtained between 1994 and 2010, and of 171 UBV photometric observations (from two observatories) of the 2010 eclipse, were analyzed in an effort to better understand ε Aur, the well-known, but still enigmatic eclipsing binary with the longest known orbital period (∼27 yrs). The main results follow. (1) We attempted to recover a spectrum of the companion by disentangling the observed spectra of the ε 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 Hα line profiles over the past 16 years, with particular emphasis on the 2009–2011 eclipse period, by subtracting a mean out-of-eclipse Hα 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 Hα “shell” absorptions seen projected against the F star. Significantly, the Hα 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^d.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. Although variability on similar timescales is also seen in the spectrum and in photometry out of eclipse, we were unable to find a coherent periodicity in these data. Nevertheless, these results appear to rule out regular stellar pulsations as the cause of this variability.