Massive stars in the Galactic Center Quintuplet cluster
The presented thesis describes the observations of the Galactic center
Quintuplet cluster, the spectral analysis of the cluster Wolf-Rayet
stars of the nitrogen sequence to determine their fundamental stellar
parameters, and discusses the obtained results in a general context.
The Quintuplet cluster was discovered in one of the first infrared surveys of the Galactic center region (Okuda et al. 1987, 1989) and was observed for this project with the ESO-VLT near-infrared integral field instrument SINFONI-SPIFFI. The subsequent data reduction was performed in parts with a self-written pipeline to obtain flux-calibrated spectra of all objects detected in the imaged field of view. First results of the observation were compiled and published in a spectral catalog of 160 flux-calibrated K-band spectra in the range of 1.95 to 2.45 micron, containing 85 early-type (OB) stars, 62 late-type (KM) stars, and 13 Wolf-Rayet stars. About 100 of these stars are cataloged for the first time.
The main part of the thesis project was concentrated on the analysis of the WR stars of the nitrogen sequence and one further identified emission line star (Of/WN) with tailored Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres (Hamann et al. 1995) that are applied to derive the stellar parameters of these stars. For this purpose, the atomic input data of the PoWR models had to be extended by further line transitions in the near-infrared spectral range to enable adaequate model spectra to be calculated. These models were then fitted to the observed spectra, revealing typical paramters for this class of stars.
A significant amount of hydrogen of up to XH ~ 0.2 by mass fraction is still present in their stellar atmospheres. The stars are also found to be very luminous (log(L/Lsun) > 6.0) and show mass-loss rates and wind characteristics typical for radiation-driven winds. By comparison with stellar evolutionary models (Meynet & Maeder 2003a; Langer et al. 1994), the initial masses were estimated and indicate that the Quintuplet WN stars are descendants from the most massive O stars with Minit > 60 Msun and their ages correspond to a cluster age of 3 - 5 million years.
The analysis of the individual WN stars revealed an average extinction of AK = 3.1 +- 0.5 mag (AV = 27 +- 4) towards the Quintuplet cluster. This extinction was applied to derive the stellar luminosities of the remaining early-type and late-type stars in the catalog and a Hertzsprung-Russell diagram could be compiled. Surprisingly, two stellar populations are found, a group of main sequence OB stars and a group of evolved late-type stars, i.e. red supergiants (RSG). The main sequence stars indicate a cluster age of 4 million years, which would be too young for red supergiants to be already present. A star formation event lasting for a few million years might possibly explain the Quintuplet's population and the cluster would still be considered coeval. However, the unexpected and simultaneous presence of red supergiants and Wolf-Rayet stars in the cluster points out that the details of star formation and cluster evolution are not yet well understood for the Quintuplet cluster.
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