IUE observations of the exciting stars of giant HII regions in M 33 :supermassive stars ?

Abstract

International Ultraviolet Explorer ultraviolet spectrometry is presented for seven bright starlike objects located in large H II regions in M33. Six of these have UV spectra very similar to R136a, the alleged supermassive star in the center of the 30 Dor nebula in the LMC. The UV line spectra suggest temperatures of 40,000-45,000 K. These temperatures, in combination with the observed UV flux, demonstrate that these objects produce essentially all the ionizing photons necessary to power their H II regions. Bolometric magnitudes are conservatively estimated at -12 to -14. Whatever its nature, R136a is shown to be the dominant source of ionization for 30 Dor. The possibility that such objects are supermassive stars is discussed. It is also found that the ultraviolet extinction curve in M33 resembles that of the LMC rather than the Galaxy.