2002AJ....123.2788S

Using objects found in a systematic survey of the Galactic plane in the 1612 MHz masing OH line, we discuss in detail two ``sequences'' of post-AGB evolution, a red and a blue. We argue that the red and the blue groups separate by initial mass at M_i=4 M_☉, based on evolutionary sequence turnoff colors, spectral energy distributions, outflow velocities, and scale height. The higher mass (blue) objects may have earlier asymptotic giant branch (AGB) termination. The lower mass (red) objects undergo very sudden reddening for IRAS color R₂₁∼1.2; these sources must all undergo a very similar process at AGB termination. The transition color corresponds to average initial masses of ∼1.7 M_☉. The combined color 2.5logS^MSX₂₁S^IRAS₁₂/S^MSX₈S^IRAS₂₅ proves very sensitive to distinguishing lower mass, early post-AGB objects from sources still on the AGB and also to distinguishing more evolved post-AGB objects from star-forming regions. The high-mass blue objects are the likely precursors of bipolar planetary nebulae, whereas the low-mass red objects will evolve into elliptical planetary nebulae.