GHRS spectroscopy of individual stars in R136a

The installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) Instrument on the Hubble Space Telescope makes it possible to observe stars in very crowded regions with high spatial and spectral purity. To demonstrate this capability, we have used the Goddard High Resolution Spectrograph (GHRS) to obtain spectra of two stars in the dense center of the 30 Doradus ionizing cluster: R136a5, and its nearest neighbor, R136a2, only 0″.17 away. R136a5 is shown to be an O3f/WN star, while R136a2 is a WN4-W star. From both WFPC photometry and GHRS, spectroscopy we estimate the following properties of R136a5: T_eff = 42,500 K, R = 16.4 R_⊙, L_bol = 8 × 10⁵ L_⊙, and M ≈ 50 M_⊙ - all indicating that, despite its spectral type, R136a5 is a massive, main-sequence O star. An astonishing aspect of these spectra is the high rate of mass loss in R136a5, as indicated by the strength of He H II λ1640 emission. The observed mass-loss rate, Ṁ = 1.8 × 10^-5 M_⊙ yr^-1, is an order of magnitude higher than is assumed by current stellar evolutionary models. We argue that this high rate of mass loss will alter drastically the evolutionary path of R136a5. If so, evolutionary models for massive stars require substantial revision.