A study of K i 7699 Å and related shell lines during the recent eclipse of ϵ Aurigae

Abstract

We report high-resolution (R = 30 000, 45 000 and 75 000) echelle and medium-resolution (R = 22 000 and 10 000) spectroscopic observations of the long period, eclipsing binary ϵ Aurigae during the 2009–2011 eclipse. Low-excitation shell lines, viz, the K i line at 7699 Å (with 346 data points), Cr i lines at 5345.807 and 5348.326 Å and Fe i line at 5110.435 Å which originated from the disc shaped secondary, Hα and the shell components of the Na D1 and D2 lines show significant variation in their shapes and radial velocities during the eclipse. The equivalent width curve shown by the K i line around the ingress and egress phases indicates that the gas density in the trailing edge is about a factor of 2 higher than the density in the leading edge. Using a geometrical model, in which a homogeneous, cylindrical Keplarian disc eclipses the F0Ia primary star and the shell absorption lines originate from the gaseous atmosphere around an opaque disc, we fit the equivalent width and the radial velocity curves of the K i line covering the full eclipse. A reasonably good fit can be achieved by a low-mass binary model where the mass of the central star of the disc is 5.4 M⊙ and the mass of the primary is 2.5 M⊙ and a disc size of 8.9 au. The low mass of the primary, with enhanced s-process elements found by Sadakane et al., supports that it is a post-asymptotic giant branch F supergiant. For the high-mass binary model, the modelled radial velocity curve deviates significantly from the observations.