Winds from massive stars: implications for the afterglows of gamma-ray bursts

Recent observations suggest that long-duration gamma -ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core- collapse explosions. The pre-explosive ambient medium provides a natural te st for the most likely progenitors of GRBs. Those stars that shed their env elopes most readily have short jet crossing times and are more likely to pr oduce a GRB. We construct a simple computational scheme to explore the expe cted contribution of the presupernova. ejecta of single Wolf-Rayet (WR) sta rs to the circumstellar environment. Using detailed stellar tracks for the evolution of massive stars, we discuss the effects that the initial main-se quence mass, metallicity, rotation and membership in a binary system have o n the ambient medium. We extend the theory of GRB afterglows in winds to co nsider the effect of the relativistic fireball propagating through the)VR e jecta. Specific predictions are made for the interaction of the relativisti c blast wave with the density bumps that arise when the progenitor star rap idly loses a large fraction of its initial mass or when the ejected wind in teracts with the external medium and decelerates. A re-brightening of the a fterglow with a spectrum redder than the typical synchrotron spectrum (as s een in GRB 970508, GRB 980326 and GRB 000911) is predicted. We also calcula te the luminosity of the reflected echo that arises when circumstellar mate rial Compton-scatters the prompt radiation, and examine the spectral signat ures expected from the interaction of the GRB afterglow with the ejected me dium.