Contributed Talk - Splinter HotStars

Friday, 15 September 2023, 16:40   (H 2036)

Do stars in binaries evolve differently at low metallicity?

Daniel Pauli
Universität Potsdam

The complex evolution of binary systems in low metallicity environments is poorly understood. There are large uncertainties in mass loss rates, and the efficiency of mixing and mass transfer. So far, the observational data, which is crucial for calibrating stellar evolution models, is scarce. Especially the binary fraction of these stars is unknown! Unfortunately this knowledge is crucial to answer the profound questions on the origin of the observed Wolf-Rayet star population, the upper mass regime of the initial mass function, and the properties of post interaction massive binaries. To remedy this situation, we have embarked on the first consistent optical multi-epoch and multi-wavelength spectroscopic survey of the most massive stars in the SMC (Z < 1/5 Zsol) galaxy. We have secured the HST UV spectroscopy, as well the multi-epoch ESO VLT optical spectroscopy of the complete sample of known stars with spectral type earlier than O6. In this talk I will present the first exciting results of our analysis. Namely, we were able to identify several binary systems hinting to a binary fraction > 50%. For these binaries we were able to distinguish between pre- and post-interaction as well as contact binaries. Our analysis of the post-interaction binaries suggest something surprising: We found a core-He burning primary star in wide orbit that is spectroscopially classified as an O-type star. Furthermore, we discovered the most massive contact binary known so far. I will discuss how these observations can be explained by stellar evolutionary models by linking the observed data of different evolutionary stages (from O to WR stages) to achieve a consistent picture. I will highlight how this impacts our understanding of the Early Universe, i.e. at the point where star formation peaked.