Rolf-Peter Kudritzki

Peculiar CNO photospheric abundances in the central star of NGC 2392

Using new, high signal-to-noise CFHT ESPaDOnS visual spectrograms, and archive IUE and FUSE UV spectrograms, together with state-of-the-art non-LTE hydrodynamical model atmospheres, we have obtained accurate He, C, N, O photospheric abundance determinations in the central stars of NGC 2392, IC 4593, and NGC 6826. We compare with the corresponding nebular abundances, taken from the literature. The central star of NGC 2392 shows high He, N, and very low C, O abundances. We propose that these peculiar abundances must have originated in a common-envelope phase of interaction with a close binary companion. If we assume that the companion is more evolved than the visible central star, this offers a way of solving the old mystery of the discrepant He II Zanstra temperature of NGC 2392.

Chemical abundances of massive stars in Local Group galaxies

The relative abundances of elements in galaxies can provide valuable information on the stellar and chemical evolution of a galaxy. While nebulae can provide abundances for a variety of light elements, stars are the only way to directly determine the abundances of iron-group and s-process and r-process elements in a galaxy. The new 8m and 10m class telescopes and high-efficiency spectrographs now make high-quality spectral observations of bright supergiants possible in dwarf galaxies in the Local Group. We have been concentrating on elemental abundances in the metal-poor dwarf irregular galaxies, NGC 6822, WLM, Sextants A, and GR8. Comparing abundance ratios to those predicted from their star formation histories, determined from color-magnitude diagrams, and comparing those ratios between these galaxies can give us new insights into the evolution of these dwarf irregular galaxies. Iron-group abundances also allow us to examine the metallicities of the stars in these galaxies directly, which affects their inferred mass loss rates and predicted stellar evolution properties.

Identification and Study of Blue Supergiants in Nearby Galaxies

For the past two years we have been using the FORS multi-object spectrograph at the VLT to investigate the blue supergiant populations in nearby spiral galaxies. For the first time we have obtained quantitative spectroscopy to a distance up to 7 Mpc, estimating abundances and wind properties of luminous A-type supergiants. We have begun to test the wind momentum-luminosity relationship as an extragalactic distance indicator beyond the Local Group. Our very preliminary results on single blue supergiants in NGC 3621 and NGC 300 make us look with optimism at the future of the project, especially when looking ahead to the new generation of multi-object spectrographs which will be soon made available at 8 meter-class telescopes.