Two early-type POST AGB stars at high galactic latitudes.

Abstract

Two high galactic latitude B-type stars, PHL 1580 and PHL 174, originally identified as distant early-type objects from low-resolution spectroscopy and photometry are re-analyzed using high-resolution spectra. The results of LTE and non-LTE model atmosphere analyses reveal general metal underabundances relative to solar values of typically 0.6 and 1.0 dex for PHL 1580 and PHL 174, respectively. For both stars carbon is significantly more underabundant. The anomalous compositions are compatible with those of other high galactic latitude post-AGB objects, while the derived atmospheric parameters are coincident with post-AGB evolutionary tracks. Thus it appears that PHL 1580 and PHL 174 are low-mass post-AGB objects at an evolutionary stage intermediate between those of the A-, F- and G-type objects and planetary nebulae previously identified. The severe carbon deficiency may be evidence that both objects left the AGB before the products of the CNO bi-cycle were mixed to the stellar surface.