Spectral variability of the binary HR 4049

The C I, Na I D, and Hα lines of the post-AGB binary HR 4049 have been studied. Na I D variability results from a photospheric absorption component ([Na/H]=-1.6 ± 0.2) which follows the velocity of the primary and a stationary, nonphotospheric component. An emission component is attributed to the circumbinary disc, and an absorption component to massloss from the system with a velocity of 5.3 ± 0.5 km s-1. The Hα profile varies with the orbital period. The two strong shell type emission peaks are identified as from one single broad emission feature with an absorption centered around -7.5 km s-1. The intensity variations are largely attributed to a differential amount of reddening towards the Hα emitting region and the stellar continuum. The radial velocities suggest that the Hα emission moves in phase with the primary, but with a slightly lower velocity amplitude. From this we infer that the Hα emission comes from outside the orbit of the primary, but still gravitationally bound to the primary. Hα also shows a weak emission feature at -21.3 ± 3.5 km s-1, which originates from the circumbinary disc and a weak absorption feature at -7.5 ± 1.6 km s-1 due to absorption by the circumbinary disc. We propose two competing models that could account for the observed velocity and intensity variations of the Hα profile. Model I: light from the primary reflects on a localized spot near the inner radius of the circumbinary disc which is closest to the primary. Model II: Hα emission originates in the outer layers of the extended atmosphere of the primary due to activity. These activities are locked to the position of the primary in its orbit. We discuss the similarities of variability and shape of the Hα emission of HR 4049 with those of early type T-Tauri stars (e.g SU Aur)