A study of synthetic and observed Hα spectra of TT Hydrae

The formation and properties of accretion disks and circumstellar material in Algol-type systems are not very well understood. In order to study the underlying physics of these structures, we have calculated synthetic H alpha spectra of TT Hya, which is an Algol-type eclipsing binary with an accretion disk. Both the primary and secondary stars were considered in the calculations, as well as a disk surrounding the primary. The Roche model for the secondary star was assumed. The synthetic spectra cover all the phases, including primary eclipse, and are compared with the observed spectra. The influence of various effects and free parameters of the disk on the emerging spectrum was studied. This enabled us to put some constraints on the geometry, temperature, density, and velocity fields within the disk. Differences found between the observed and synthetic spectra unravel the existence of a gas stream, as well as a hotter disk-gas interaction region. An additional cooler circumstellar region between the C1 and C2 Roche surfaces is suggested to account for various observed effects. A new computer code called SHELLSPEC was created for this purpose and is briefly described in this work as well. It is designed to solve simple radiative transfer along the line of sight in three-dimensional moving media. The scattered light from a central object is taken into account assuming an optically thin environment. Output intensities are then integrated through the two-dimensional projection surface of a three-dimensional object. The assumptions of the code include LTE and optional known state quantities and velocity fields in three dimensions.