Theoretical Modeling of GHRS Observations of the Of/WN-Type Star R136a5

Abstract

We present a theoretical analysis of the Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) spectrum of R136a5, an O3fWN star in the R136a cluster in 30 Doradus. Using non-LTE extended and expanding model atmospheres, we find a surprisingly high mass-loss rate dot-M = 1.8 +/- 0.5 x 10^-5 solar mass/yr and an effective temperature T_eff = 42.5 +/- 2.5 kK. With the observed visual magnitude, this implies a radius R_* = 17 +/- 2 solar radius and a solar luminosity L = 8.5 +/- 1 x 10⁵ solar luminosity. We discuss possible sources of the uncertainties in the derived stellar parameters and conclude that the effective temperature may be underestimated; however, if so, the value of the mass-loss rate would not be affected.