Photometric Study of Transition Objects Which Lie Between the Asymptotic Giant Branch (AGB) and Planetary Nebulae (PNe)

Post-Asymptotic Giant Branch (post-AGB) stars are generally believed to be on their way to the planetary nebula stage. However, there has been a question about the evolutionary path for the low-mass ones. We investigate the evolution of 31 very likely and possible post-AGB candidates selected from the Torun catalogue of Galactic post-AGB sources. The sources in the sample belong to the IRAS selected sources (IRASsel), High Galactic latitude supergiants (hglsg), High Galactic latitude B-type supergiants (hglB) and Bright stars with infrared (IR) excess (IRexc) classifications in the catalogue and are found to lie between AGB and planetary nebulae (PNe). Out of the 31 selected sources, 26 were studied. We compared the IRAS color-color plots of these sources with that of van der Veen & Habing and Szczerba and found that the 26 sources are possibly post-AGB stars. Their temperature and dynamical time plots showed that 11 objects in our sample have their shells completely detached. One object was found to be on the evolutionary track of Mcore = 0.55Mo and may likely evolve to a white dwarf, by-passing the PN stage.