The nature of R 136a, the superluminous central object of the 30 Doradus nebula.

Abstract

The authors present an analysis of all their IUE ultraviolet data obtained so far for R136a, the central object of the 30 Doradus nebula. They present new average UV spectra with much higher signal-to-noise ratios than those discussed in an earlier paper. New information about the problems due to the presence of nearby stars in the crowded field is presented. There have been no large changes in the UV continuum or line spectrum of R136a over the 4 years of data span. Using new UV reddening data for 30 Doradus obtained by Fitzpatrick and Savage, the dereddened energy distribution of R136a from 0.12 to 20 μm is presented. The authors reevaluate all existing data for this object and give equal balance to tests of the star cluster hypothesis and the supermassive star hypothesis. They conclude that the supermassive star hypothesis is more probable and the following approximate parameters are derived: L ≈ 6×10⁷ L_sun;, T ≈ 75000K, R ≈ 50 R_sun;, M ≈ 2100 M_sun;, M≈ 5×10^-4 M_sun; yr^-1. A careful discussion of theoretical difficulties associated with the formation and vibrational stability of such an object is given. Possible solutions to the vibrational stability problem are outlined.