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 \epsilon 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 Å and 5348.326 Å and Fe I line at 5110.435 Å which originated from the disk shaped secondary , H \alpha and the shell components of the Na D _ { 1 } and D _ { 2 } 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 two higher than the density in the leading edge .
Using a geometrical model , in which a homogeneous , cylindrical Keplarian disk eclipses the F0Ia primary star and the shell absorption lines originate from the gaseous atmosphere around an opaque disk , 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 disk is 5.4 M _ { \sun } and the mass of the primary is 2.5 M _ { \sun } and a disk size of 8.9 AU .
The low-mass of the primary , with enhanced s -process elements found by Sadakane et al .
( 2010 ) , supports that it is a post-AGB F supergiant .
For the high-mass binary model , the modelled radial velocity curve deviates significantly from the observations .