Anyone out there? Galactic halo post-AGB stars

Abstract

We present results of a survey of post-asymptotic giant branch stars (post-AGBs) at high galactic latitude. To date, few post-AGB stars are known throughout the Galaxy and the number of known members of the older populations like the galactic halo is even smaller. This study looks at the number of post-AGB stars which are produced using different synthetic population methods and compare the results with observations. The resulting synthetic populations are compared to observational results from a complete and studied subsample from the photographic Palomar-Green (PG) survey (with high resolution spectroscopic follow-up for post-AGB candidates) and the SDSS spectroscopic database. The results show only two candidate post-AGB stars in a complete subsample of the PG survey spanning 4200 square degrees and one in the SDSS database. We discuss and explore any observational biases which may cause the result. If found to be truely representative of the halo population, one can expect the majority of Population II stars to fail to ascend the AGB and evolve through other evolutionary channels such as the extended horizontal branch.