Massive stars at low metallicity: an empirical investigation

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With the first detection of a gravitational wave on September 14, 2015, the focus of scientific interest moved to stars with a high mass. The two Black Holes which merged when emitting this gravitational wave had about 30 solar masses each, and must have formed by the collapse of stars which were initially even much more massive. It has been suggested that these stars must have been formed in a low-metallicity environment, because a low metal content leads to weaker winds; hence, at the end of the stellar evolution, a larger mass is left that collapses into the Black Hole.Massive stars are still not well understood. Most of the existing investigations of massive stars concern objects in our own Galaxy. The agreement with theoretical simulations of their evolution is not satisfactory. Major reasons for the discrepancies are presumably our poor knowledge of two important effects, the mass loss by stellar winds, and the interior mixing processes induced by stellar rotation.Our Milky Way has a relatively high metal content. Galaxies with low metallicity, as invoked as the birth place of the gravitational wave progenitors, are also found in our immediate cosmic neighborhood, namely the Small Magellanic Cloud. It is close enough that the relatively bright massive stars can be studied individually. About half of all massive stars of this dwarf galaxy belong to the relatively young cluster NGC346. This cluster thus provides an ideal laboratory to study a complete massive star population in a starburst at low metallicity.To this purpose, we have secured the spectra of the massive stars in NGC346. In the visual light, we employed the integral-field spectrograph MUSE at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). This instrument takes spectra simultaneously for each of the 90000 pixels in the field-of-view. First inspection of the data show 200-300 stellar sources. Since the full spectral analysis also requires the ultraviolet spectral range, we have additionally applied for observations with the Hubble Space Telescope, which have been approved as well. With the long-slit spectrograph STIS, major parts of the cluster will be covered in form of a mosaic.Within the project proposed here, these new and superb data shall be analyzed in due time. The scientific exploitation will address various questions of high interest in current astrophysics. We will establish in which respects this population differs from those at higher metallicity. How small is the mass loss at this low metallicity? How does the smallness of their mass loss affect the evolution of the stars? How are the stellar masses statistically distributed?Our unique new observations will also allow to investigate this young, metal-poor stellar cluster as a whole. We will model the feedback of the combined stellar radiation and winds on the diffuse matter, i.e. on the giant region that is ionized by the embedded stars.