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Red supergiants (RSGs) are a He-burning phase in the evolution of moderatelyhigh mass stars (10-25 solar masses). The evolution of these stars,particularly at low metallicities, is still poorly understood. The latest-typeRSGs in the Magellanic Clouds are cooler than the current evolutionary tracksallow, occupying the region to the right of the Hayashi limit where stars areno longer in hydrodynamic equilibrium. We have discovered four Cloud RSGs inthis region that display remarkably similar unusual behavior. All of them showconsiderable variations in their V magnitudes and effective temperatures (andspectral types). Two of these stars, HV 11423 and [M2002] SMC 055188, have beenobserved in an M4.5 I state, considerably later and cooler than any othersupergiant in the SMC. These stars suffer dramatic physical changes ontimescales of months - when they are at their warmest, they are also brighter,more luminous, and show an increased amount of extinction. This variableextinction is characteristic of the effects of circumstellar dust, and can beconnected with sporadic dust production from these stars in their coolerstates. We suggest that these unusual properties are indicative of an unstable(and short-lived) evolutionary phase not previously associated with RSGs, andconsider the implications such behavior could have for our understanding of thelatest stages of massive star evolution in low-metallicity environments.

The Coolest Stars in the Clouds: Unusual Red Supergiants in the Magellanic Clouds