Optical and Ultraviolet Spectral Morphology of Luminous OB Stars in the Galaxy and the Magellanic Clouds

Abstract

Three areas of current progress relevant to the theme of this Symposium will be discussed. (1) New spectroscopic observations of the 30 Doradus central cluster, obtained independently by the author and by J. Melnick, confirm the presence of numerous very early O-type members, including several of type O3. In combination with sophisticated new direct imagery of the luminous central object R136 by A. Walker and by G. Weigelt, these results have evident implications for understanding the ionization of the supergiant H II region, as well as for the interpretation of R136 itself and of the apparently similar regions seen in more distant galaxies. In particular, no evidence remains for a supermassive object in 30 Doradus, but its central cluster is revealed as a spectacular grouping of very massive hot stars. (2) A further member of the Ofpe/WN9 category in the LMC has been identified, bringing their number to seven, with no exact spectroscopic counterparts yet known in the Galaxy. One of these objects is currently in a state of outburst and has been interpreted by O. Stahl et al. as the hottest known Hubble-Sandage variable. (3) An extensive survey of IUE high-resolution data has revealed a strong correlation between the ultraviolet stellar wind features and the optical spectral classifications for the majority of normal O stars. These results are relevant to future studies with the High Resolution Spectrograph on the Hubble Space Telescope, which may observe restricted UV wavelength ranges in faint extragalactic OB stars lacking optical data of comparable quality.

A spectroscopic survey of WNL stars in the Large Magellanic Cloud: general properties and binary status

“…Note also that the unsubtracted nebular emission leads to partially filled-up absorptions. Together with our relatively low spectral resolution, which affects narrower absorption lines more than broader emission lines, some O3If/WN6 stars, whose very weak Heii λ4686 emission lines (small EWs) clearly distinguishes them from genuine WN stars (Walborn 1986), display an artificially attenuated absorption-line spectrum, thus favoring a (diluted) WN-rather than an O-star classification. Indeed, Massey et al (2004Massey et al ( , 2005 proposed the O2If classification for some of the O3If/WN6 stars using higherresolution data and applying the criteria of Walborn et al (2002).…”

Massive stars in galaxies

“…These variations in "photo-spheric" temperature are shown as dashed lines in Figure 3. The nitrogen enrichment in their photospheres and of their ejecta suggests they are evolving to the W-R stage (Davidson, Walborn, and Gull 1982;Shore and Sanduleak 1984; Walborn 1982).…”

“…Weigelt and Baier (1985) used holographic speckle interferometry to resolve R136a into eight separate components within 1" or ~ 0.3 pc. Walborn (1986) has shown that the upper limit to the mass of its brightest component is ~ 250 3K 0 an< 3 that the entire 30 Dor cluster may contain 15 to 20 stars with original masses of My own personal favorite very-massive star is r\ Car. Eta Car is famous for its outburst in the 1840s during which it became the second brightest star in the sky.…”

New O3 giants in the Large Magellanic Cloud

“…The 03 class was first recognized among the exciting stars of the Carina nebula (Walborn 1971(Walborn , 1982. Recently, a considerable number have been identified in the central cluster of the 30 Doradus nebula in the Large Magellanic Cloud (Melnick 1985;Walborn 1986), and one in NGC 346, the largest H II region of the Small Magellanic Cloud (Walborn and Blades 1986;Niemela, Marracó, and Cabanne 1986).…”

“…The 03 class was first recognized among the exciting stars of the Carina nebula (Walborn 1971(Walborn , 1982. Recently, a considerable number have been identified in the central cluster of the 30 Doradus nebula in the Large Magellanic Cloud (Melnick 1985;Walborn 1986), and one in NGC 346, the largest H II region of the Small Magellanic Cloud (Walborn and Blades 1986;Niemela, Marracó, and Cabanne 1986). These stars have effective temperatures near 50,000 K and masses of the order of 100 SOÎq (Kudritzki and Hummer 1986); they are of significance for understanding the formation and evolution of the most massive stars as well as the ionization of giant H II regions.…”