A search for hot post-AGB stars in the IRAS Point Source Catalog.

Abstract

In this paper a first step is made to search for hot post-AGB stars in the IRAS Point Source Catalog. In order to find objects that evolved off the AGB a longer time ago than post-AGB objects discussed in the literature, objects that were not detected at 12 μm by IRAS were selected, The selection yielded 15 objects, of which 12 have spectral type B, and would appear to be efficient in finding hot objects. However, this result does not necessarily mean that these are old evolved objects. Some stars are associated with dense galactic cirrus and are probably normal massive supergiants, and a number of stars are good post-AGB candidates because of their galactic latitudes and their cool, extended infrared emission. The possibility remains that these objects are normal supergiants heating the local interstellar medium. Finally, three remaining objects appear to be genuine post-AGB (candidate) stars, as is attested by their spectral and photometric properties. The main conclusion derived from modelling their spectral energy distributions is that the objects evolved off the AGB about 1000 years ago, significantly longer than what is found for other post-AGB stars. Emission lines have appeared in the spectrum of one of these objects, SAO 243756, over the last 20 years. SAO 243756 bears close resemblance to other objects that have been proposed to have entered the Planetary Nebula phase recently. Several aspects of the problem of selecting post-AGB stars in general are discussed.