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Massive Star Geriatrics

In this dissertation, I present a series of observational studies investigating how mass loss from massive stars affects their evolution in the last stages of life prior to death by core-collapse. First, I carry out a mid-infrared photometric search for massive, luminous, self-obscured stars similar to the dust-obscured progenitors of the luminous outbursts in NGC 6946 (SN 2008S) and NGC 300 in 2008 at the extreme end of the AGB sequence in four nearby galaxies: M 33, NGC 300, M 81, and NGC 6946. I verified that stars analogous to the progenitors of these two transients are truly rare in all four galaxies with only ∼ 1 such object existing in any galaxy at a time. Next, I present the discovery of a previously unknown luminous star in M 33 that we dubbed “Object X” — the brightest mid-infrared star in M 33. I propose that Object X is a M ∼> 30M⊙ evolved star obscured in its own dust ejected during episodic mass loss events over at least ∼half a century. It may emerge from its current ultra-short evolutionary phase as a hotter, post red supergiant star. Finally, I present the results of a mid-infrared search in 7 nearby (∼< 4 Mpc) galaxies using Spitzer IRAC images for analogs of η Carinae, the most massive