Spectral classification and HR diagram for symbiotic stars in the galactic bulge

Abstract

A sample of 45 symbiotic stars observed in the direction of the galactic bulge are classified in spectral type using TiO $\lambda \lambda 6180, 7100 \AA$ and VO $\lambda \lambda 7865 \AA$ absorption feature indices and visual comparison. The effective temperatures obtained are used together with K data from the literature to estimate the bolometric luminosity of the giant components if located at d=8.5 kpc. The ($T_{eff}$, $L/L_{\odot}$) pairs are plotted on the HR diagram together with evolutionary tracks for metal rich intermediate mass stars. The mass of the giant components seem to be scattered in the interval ($\sim 1.5 - \sim 4 M_{\odot}$). The giant components of D-type symbiotics appear as if they were more evolved objects than the corresponding giants in S-type systems. Nevertheless, it is unlikely that they form an evolutionary sequence. The results point also to the restriction of the symbiotic phenomenon to the thermally pulsating phase of the AGB, strengthening an accelerated mass loss scenario for the latest evolutionary stages of intermediate mass stars.