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N. L. King (STScI), A. A. Cole (U Mass), A. Nota (STScI)
The Andromeda galaxy still holds surprises: observational evidence suggests that it harbors many more massive evolved ``transition'' stars than we currently count. These transition stars are the stages of evolution between O and B main sequence types and the Wolf-Rayet stars. Finding these stars would address the following questions:
Is the lifetime of the Luminous Blue Variable (LBV) longer than current predictions?
What is the nature of massive stars in M31? (For example, if a large fraction of these stars found in older areas or the field there is a strong argument for a binary nature or runaway histories.)
We discuss the implications of (1) their numbers relative to their progenitors, represented by the local unevolved O and stars, and (2) their location with respect to other massive star indicators. We know from the NW half of M31 that these stars are often far from HII regions and young OB associations.
We use two methods to find these massive evolved stars. LBV-type stars and their relatives can be identified by their emission line properties and location in near-infrared color-color plots. For this search and study, we use a combination of KPNO 0.9-m H\alpha and [SII] images and the 2MASS survey to identify the luminous transition-type supergiants in M31. The infrared data has the advantage of seeing deeper into dusty areas. Additionally, we use WFPC2 archival broadband optical images to examine blending effects and constrain the ages and initial masses of a number these stars from their neighbors.