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Pfuhl, Oliver (2012): The GRAVITY interferometer and the Milky Way’s nuclear star cluster. Dissertation, LMU München: Faculty of Physics
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Abstract

This thesis is divided into two parts: an instrumentation part and an astrophysical part. The instrumentation part describes the development and implementation of the fiber coupler and guiding subsystems of the 2nd generation VLTI instrument GRAVITY. The astrophysical part describes the derivation of the star formation history of the Milky Way’s nuclear star cluster based on imaging and spectroscopic data obtained at the Very Large Telescope. The future VLTI instrument GRAVITY will deliver micro-arcsecond astrometry, using the interferometric combination of four telescopes. The instrument is a joint project of several European institutes lead by the Max Planck Institut f¨ur extraterrestrische Physik. The instrumental part of this thesis describes the fiber coupler unit and the guiding system. They serve for beam stabilization and light injection in GRAVITY. In order to deliver micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the VLTI optical train. We therefore developed a dedicated guiding system, correcting the longitudinal and lateral pupil wanderas well as the image jitter in VLTI tunnel. The actuators for the correction are provided by four fiber coupler units located in the GRAVITY cryostat. Each fiber coupler picks the light of one telescope and stabilizes the beam. Furthermore each unit provides field de-rotation, polarization adjustment as well as atmospheric piston correction. A novel roof-prism design offers the possibility of on-axis as well as off-axis fringe tracking. Finally the stabilized beam is injected with minimized losses into singlemode fibers via parabolic mirrors. We present lab results of the first guiding- as well as the first fiber coupler prototype, in particular the closed loop performance and the optical quality. Based on the lab results we derive the on-sky performance of the systems and the implications concerning the sensitivity of GRAVITY. The astrophysical part of this thesis presents imaging and integral field spectroscopy data for 450 cool giant stars within 1 pc from Sgr A*. We use the prominent CO bandheads to derive effective temperatures of individual giants. Additionally we present the deepest spectroscopic observation of the Galactic Center so far, probing the number of B9/A0 main sequence stars (2.2 − 2.8M) in two deep fields. From spectro-photometry we construct a Hertzsprung-Russell diagram of the red giant population and fit the observed diagram with model populations to derive the star formation history of the nuclear cluster. We find that (1) the average nuclear star-formation rate dropped from an initial maximum 10Gyrs ago to a deep minimum 1-2Gyrs ago and increased again during the last few hundred Myrs, and (2) that roughly 80% of the stellar mass formed more than 5Gyrs ago; (3) mass estimates within R 1 pc from Sgr A* favor a dominant star formation mode with a normal Chabrier/Kroupa initial mass function for the majority of the past star formation in the Galactic Center. The bulk stellar mass seems to have formed under conditions significantly different from the observed young stellar disks, perhaps because at the time of the formation of the nuclear cluster the massive black hole and its sphere of influence was much smaller than today.