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We present various infrared two-color diagrams (2CDs) for AGB stars, post-AGBstars, and Planetary Nebulae (PNe) and investigate possible evolutionarytracks. We use catalogs from the available literature for the sample of 4903AGB stars (3373 O-rich; 1168 C-rich; 362 S-type), 660 post-AGB stars (326post-AGB; 334 pre-PNe), and 1510 PNe in our Galaxy. For each object in thecatalog, we cross-identify the IRAS, AKARI, MSX, and 2MASS counterparts. The IR2CDs can provide useful information about the structure and evolution of thedust envelopes as well as the central stars. To find possible evolutionarytracks from AGB stars to PNe on the 2CDs, we investigate spectral evolution ofpost-AGB stars by making simple but reasonable assumptions on the evolution ofthe central star and dust shell. We perform radiative transfer modelcalculations for the detached dust shells around evolving central stars in thepost-AGB phase. We find that the theoretical dust shell model tracks using dustopacity functions of amorphous silicate and amorphous carbon roughly coincidewith the densely populated observed points of AGB stars, post-AGB stars, andPNe on various IR 2CDs. Even though some discrepancies are inevitable, the endpoints of the theoretical post-AGB model tracks are generally converged to theregion of the observed points of PNe on most 2CDs.

Infrared Two-Color Diagrams for AGB stars, post-AGB stars, and Planetary Nebulae