Massive stars in the Galactic Center Quintuplet cluster
Adriane Liermann
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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