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Observations imply that long \gamma-ray bursts (GRBs) are originated fromexplosions of massive stars, therefore they may occur in the molecular cloudswhere their progenitors were born. We show here that the prompt optical-UVemission from GRBs may be delayed due to the dust extinction, which can wellexplain the observed optical delayed onset and fast rise in GRB 080319B. Thedensity and the size of the molecular cloud around GRB 080319B are roughlyconstrained to be \sim10^3cm^{-3} and \sim 8pc, respectively. We alsoinvestigate the other GRBs with prompt optical-UV data, and find similar valuesof the densities and sizes of the local molecular clouds. The futureobservations of prompt optical-UV emission from GRBs in subsecond timescale,e.g., by UFFO-Pathfinder and SVOM-GWAC, will provide more evidence and probesof the local GRB environments.

Delayed Onset and Fast Rise of Prompt Optical-UV Emission from Gamma-Ray Bursts in Molecular Clouds