The Massive Post--Red Supergiants IRC +10420 and HD 179821

The transition of a massive post-main-sequence star from red supergiant to luminous blue variable or Wolf-Rayet star is a rapid and therefore rarely observed phase of evolution. We present evidence that the unusual yellow supergiants IRC +10420 and HD 179821 (= AFGL 2343) are undergoing such a transition. Polarimetric, coronagraphic near-infrared images reveal that each star is surrounded by an extended dust reflection nebula. These detections set IRC + 10420 and HD 179821 apart from 89 Her, HD 161796, and several other luminosity class I stars that are thought to be descended from low- to intermediate-mass progenitors. The nebulae surrounding IRC + 10420 and HD 179821 are very similar; each appears circularly symmetric, is detected out to ∼9" from the central star at 1.25 μm, and is unusually red for reflection nebulosity. The last result suggests that the dust grains in these envelopes are larger than is typical of grains in the dust envelopes of low- and intermediate-mass evolved stars. Under the assumption that both stars lie at their kinematic distances (∼6 kpc in each case), our images suggest that each circumstellar envelope has a dynamical lifetime of ∼5000 yr, contains ∼5 Msun of gas and dust, and was likely ejected during a prior OH/IR supergiant phase. Thus, in addition to bolstering the post-red supergiant classification of both stars, these imaging results appear to place strong constraints on the evolutionary and mass-loss histories of supernova progenitors.