2009A&A...501.1207R

The transition from the asymptotic giant branch (AGB) to the planetary nebula (PN) phase is critical in the shaping of PNe. It has been suggested that the most asymmetric PNe are the descendants of massive AGB stars. Since these AGB stars are believed to have evolved into heavily obscured post-AGB stars and PNe, the compilation of a sample of bona fide obscured post-AGB stars and PNe is important for understanding the formation of asymmetric PNe. We aim to identify and characterize a large number of post-AGB stars and PNe in the IR and to assess their degree of optical obscuration. The improved positions will enable future detailed studies to determine their true nature, whereas the optical and IR properties can be used to investigate their spectral behavior. We used 2MASS, Spitzer GLIMPSE, MSX, and IRAS data in search of the near-IR counterparts of a sample of 165 presumably obscured IRAS post-AGB and PN candidates, and DSS red images to identify the optical counterparts among the objects detected in the near-IR. The IR spectral energy distributions (SEDs) in the wavelength range from 1µm to 100µm of the sources with unambiguous near-IR counterparts were analyzed using appropriate color-color diagrams. We have identified the near-IR counterparts of 119 sources out of the 165 IRAS post-AGB and PN candidates in our sample. The improved astrometric coordinates of these sources have allowed us to find optical counterparts for 59 of them, yielding a reduced sample of 60 optically obscured post-AGB star and PN candidates. Among the 119 sources with near-IR counterparts, only 80 have unambiguous identifications in the 2MASS Point Source Catalogue. For these sources, we find that objects with and without optical counterparts, while having similar mid- and far-IR colors, are segregated in color-color diagrams that use the near-IR J band to compute one of the colors.