Spectroscopic and photometric monitoring of southern post asymptotic giant branch stars

Abstract

A programme of photometric and spectroscopic monitoring has been undertaken in order to investigate the characteristics of suspected post asymptotic giant branch (post-AGB) stars (AI CMi, HD 179821, SAO 209008, HD 70379, HD 95767, HD 172481, HD 168625 and CPD-59 6723). Long time baseline BV RI photometry was obtained at the Mount John University Observatory (MJUO) over intervals of 1032 and 1463 days. This was complemented with contemporaneous high-resolution spectroscopy also acquired at MJUO. As a group, post-AGB stars are predicted to have certain broad properties. Analyses of the light curves show that none of the observed stars have stable light and radial velocity curves. Most showed the small amplitude (∆V ~ 0.2 mag), semi-regular light variations expected for objects with high luminosity to mass ratios. Semi-regular radial velocity variations of between 10-20 km s-1 were seen in most of the stars. Examination of the sodium D lines at high resolution showed many components, between 4 and 7 per star. These are due to a combination of interstellar, circumstellar and photospheric lines. The interstellar lines were used to place lower limits on the distances to the stars. None of the stars showed a clearly identifiable photospheric component. To account for the profiles a range of models were developed, with a combination of photospheric and circum stellar absorptions, in conjunction with chromospheric emission fitting. Radial velocity measurements revealed generally large mean radial velocities consistent with the stars belonging to the Galactic thick disk population. HD 172481 has a mean radial velocity of ~ -83 km s-1, indicating likely Galactic halo membership. Hα profiles all showed varying degrees of emission. The hotter stars exhibited P Cygni profiles. For cooler stars the profiles were a combination of large photospheric absorption with a smaller self-absorbed emission superimposed. This emission is caused by mass loss on the order of 10-7 Mסּ yr-1 and arises in a region within a few stellar radii of the star. The profiles are also variable due to relative motion between the three underlying components. All but one of the stars observed showed signs of cool, 100-400 K, circumstellar material. The amount of extinction provided by this material is expected to be small, on the order of a few tenths of a magnitude. Only HD 172481 showed clear indications of circumstellar extinction. The masses of these shells were estimated from the 60 µm fluxes acquired by the IRAS satellite. With the loose classification criteria for post-AGB stars, we have also observed some quite individual behaviour in a number of our objects. AI CMi showed light and spectrum variations similar to the RV Tauri class of variables. HD 70379 showed a variable amplitude due to a combination of two simultaneous radial oscillations. HD 172481 has a cool Mira type companion which causes the I-band light curve to vary with a consistent large amplitude (∆V ~ 0.55 mag) but with a variable amplitude in the BVR-bands. HD 95767 proved to be a binary star with a cool companion. During the time of our observations, initial low amplitude variations due to pulsations of the primary, were disrupted by the periastron passage of the companion. This also caused emission lines to be seen in the spectrum.