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The Astrophysical Implications of Dust Formation During The Eruptions of Hot, Massive Stars

Dust formation in the winds of hot stars is inextricably linked to the classic eruptive state of luminous blue variables (LBVs) because it requires very high mass loss rates, Mdot>10^(-2.5) Msun/year, for grains to grow and for the non-dust optical depth of the wind to shield the dust formation region from the true stellar photosphere. Thus, dusty shells around hot stars trace the history of "great" eruptions, and the statistics of such shells in the Galaxy indicate that these eruptions are likely the dominant mass loss mechanism for evolved, M>40Msun stars. Dust formation at such high Mdot also explains why very large grains (amax>1 micron) are frequently found in these shells, since amax \propto Mdot. The statistics of these shells (numbers, ages, masses, and grain properties such as amax) provide an archaeological record of this mass loss process. In particular, the velocities v, transient durations (where known) and ejected masses M of the Galactic shells and the supernova "impostors" proposed as their extragalactic counterparts are very different. While much of the difference is a selection effect created by shell lifetimes ~1/(v sqrt(M)), more complete Galactic and extragalactic surveys are needed to demonstrate that the two phenomena share a common origin given that their observed properties are essentially disjoint. If even small fractions (1%) of SNe show interactions with such dense shells of ejecta, as is currently believed, then the driving mechanism of the eruptions must be associated with the very final phases of stellar evolution, suggestive of some underlying nuclear burning instability.