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D. E. Reichart (Caltech)
Optical afterglows have been identified for less than half of all well-localized gamma-ray burst afterglows. I argue, primarily by ruling out the non-ad hoc alternatives, that optically dark gamma-ray burst afterglows, or dark bursts for short, are dark because they are extinguished in the source frame. I show, with the aid of a simple dust sublimation/fragmentation model, that if this is indeed that case, the dark bursts originate in large (> 10 pc), massive (> 105 Msun) clouds. Furthermore, I show that such clouds are gravitationally unstable, and probably have been collapsing and fragmenting. Consequently, the dark bursts probably have origins in the large-scale, fragmentation-driven star formation of collapsing, massive clouds, whereas visible bursts probably have origins in the smaller-scale star-formation of gravitationally stable clouds.