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The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearlycontinuous monitoring of its early optical afterglow decay, as well ashigh-quality optical spectra that determined a redshift of z3=2.328 for itshost galaxy, an active starburst galaxy with strong Lyman-alpha emission andseveral absorption lines. Spectral observations show multiple absorbers atz3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/srespectively relative to the host galaxy Lyman-alpha emission. We argue thatthese correspond to a fragmented shell nebula that has been radiativelyaccelerated by the gamma-ray burst (GRB) afterglow at a distance greater than0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratiosindicate that the nebula is overabundant in carbon and silicon. The high levelof carbon and silicon is consistent with a swept-up shell nebula graduallyenriched by a WCL progenitor wind over the lifetime of the nebula prior to theGRB onset. The detection of statistically significant fluctuations and colorchanges about the jet-like optical decay further supports this interpretationsince fluctuations must be present at some level due to inhomogeneities in aclumpy stellar wind medium or if the progenitor has undergone massive ejectionprior to the GRB onset. This evidence suggests that the mass-loss process in aWolf-Rayet star might lead naturally to an iron-core collapse with sufficientangular momentum that could serve as a suitable GRB progenitor.Comment: Replaced with version accepted by ApJ; 40 pages, 9 figure

GRB 021004: A Possible Shell Nebula around a Wolf‐Rayet Star Gamma‐Ray Burst Progenitor