A rare O3-type dwarf revealed in the wing of the Small Magellanic Cloud

C. J. Evans1, R. Hainich2, L. M. Oskinova2, J. S. Gallagher III3, Y.-H. Chu4, R. A. Gruendl4, W.-R. Hamann2, V. Henault-Brunet5, H. Todt2

1 Royal Observatory Edinburgh, UK
2 Institut für Physik und Astronomie, Universität Potsdam, Germany
3 Department of Astronomy, University of Wisconsin-Madison, USA
4 Astronomy Department, University of Illinois, USA
5 Institute for Astronomy, University of Edinburgh, UK

Sk 183 is the visually-brightest star in the N90 nebula, a young star-forming region in the Wing of the Small Magellanic Cloud (SMC). We present new optical spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the basis of its nitrogen spectrum, the star also displays broadened He I absorption which suggests a later type. We propose that Sk 183 has a composite spectrum and that it is similar to another star in the SMC, MPG 324. This brings the number of rare O2- and O3-type stars known in the whole of the SMC to a mere three. We estimate physical parameters for Sk 183 from analysis of its spectrum. For a single-star model, we estimate an effective temperature of 46 +/-2 kK, a low mass-loss rate of ~10-7 Msun/yr, and a spectroscopic mass of 46+9-8 Msun (for an adopted distance modulus of 18.7 mag to the young population in the SMC Wing). An illustrative binary model requires a slightly hotter temperature (~47.5 kK) for the primary component. In either scenario, Sk 183 is the earliest-type star known in N90 and will therefore be the dominant source of hydrogen-ionising photons. This suggests it is the primary influence on the star formation along the inner edge of the nebula.

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