Stellar population of the superbubble N206 in the LMC
I. Analysis of the Of-type stars

Varsha Ramachandran, R. Hainich, W.-R. Hamann, L. M. Oskinova, T. Shenar, A. A. C. Sander, H. Todt, J. S. Gallagher

Massive stars are the key agents of feedback. Consequently, quantitative analysis of massive stars are required to understand how the feedback of these objects shapes/ creates the large scale structures of the ISM. The giant HII region N206 in the Large Magellanic Cloud contains an OB association that powers a X-ray superbubble, serving as an ideal laboratory in this context. We obtained optical spectra with the muti-object spectrograph FLAMES at the ESO-VLT. When possible, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. Detailed spectral classifications are presented for our sample Of-type stars. For the quantitative spectroscopic analysis we use the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical parameters and nitrogen abundances of our sample stars are determined by fitting synthetic spectra to the observations. The stellar and wind parameters of nine Of-type stars are used to construct wind momentum,luminosity relationship. We find that our sample follows a relation close to the theoretical prediction, assuming clumped winds. The most massive star in the N206 association is an Of supergiant which has a very high mass-loss rate. Two objects in our sample reveal composite spectra, showing that the Of primaries have companions of late O subtype. All stars in our sample have an evolutionary age less than 4 million years, with the O2-type star being the youngest. All these stars show a systematic discrepancy between evolutionary and spectroscopic masses. All stars in our sample are nitrogen enriched. Nitrogen enrichment shows a clear correlation with increasing projected rotational velocities. The mechanical energy input from the Of stars alone is comparable to the energy stored in the N206 superbubble as measured from the observed X-ray and H alpha emission.

Preprint (varsha-LMC-Of.pdf, 2.6MB)

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