Two-dimensional Monte Carlo radiative transfer modelling of the disc-shaped secondary of Epsilon Aurigae

Abstract

We present two-dimensional Monte Carlo radiative transfer models for the disc of the eclipsing binary Epsilon Aurigae by fitting its spectral energy distribution (SED) from optical to far-infrared (IR) wavelengths. We also report new observations of Epsilon Aurigae made by AKARI in its five mid- and far-IR photometric bands and which were used to construct our SED. The disc is optically thick and has a flared disc geometry containing gas and dust with a gas to dust mass ratio of 100. We have taken the primary of the binary to be an F0Iae-type post-asymptotic giant branch (post-AGB) star and the disc is heated by a B5V hot star with a temperature of 15 000 K at the centre of the disc. We take the radius of the disc to be 3.8 au for our models as constrained from the IR interferometric imaging observations of the eclipsing disc. Our models imply that the disc contains grains which are much bigger than the interstellar medium (ISM) grains (grain sizes 10 to 100 μm). The grain chemistry of the disc is carbonaceous and our models show that silicate and ISM dust chemistry do not reproduce the slope of the observed SED in the mid-IR to far-IR regions. This implies that the formation of the disc-shaped secondary in the Epsilon Aurigae system could be the result of accretion of matter and/or mass transfer from the primary which is now an F0Iae post-AGB star. It is not a protoplanetary disc. The disc is seen nearly edge-on with an inclination angle larger than 85°. We propose from our radiative transfer modelling that the disc is not solid and has a void of 2 au radius at the centre within which no grains are present making the region nearly transparent. The disc is not massive; its mass is derived to be less than 0.005 M⊙.