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NLTE analysis of the Gaia-ESO spectroscopic survey.

Kovalev, Mikhail

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Abstract

In this work, we develop the spectral analysis pipeline that combines the NLTE synthetic spectra withthe Payne spectral model (Ting et al.2018). We compute two spectral grids, one grid with all elements treated in LTE and a second grid with iron, magnesium, titanium and manganese modelled in NLTE, to study the NLTE effects on the determination of the stellar parameters and chemical abundances. This pipeline is applied to spectra from the third public data release of the Gaia-ESO (Gilmore et al. 2012). We validate the method on the subsample of standard stars and cluster’s members and find significant differences between NLTE and LTE in the metal-poor regime. All clusters are homogeneous in Fe and Ti, but several globular clusters showed significant dispersion in [Mg/Fe]. The depletion of the mean [Mg/Fe] in several globular clusters, compared to field stars of the same metallicity, may indicate their ex-situ formation history. We also combine our NLTE results with Gaia DR2 (Gaia Collaboration et al.2018) astrometric data to study Galactic chemo-dynamic evolution. We apply chemical separation to select high-[α/Fe] and low-[α/Fe] disk populations, with halo stars selected using kinematics. We confirm previous results like super-solar Mg abundance in the metal-poor regime and decrease of [Mg/Fe] to the solar abundances in the metal-rich regime. We find a constant NLTE [Mg/Fe]∼0.3 dex ratio for both metal-poor disk and halo with relatively small star-to-star scatter. The halo stars with solar-like [Mg/Fe] are probably accreted from the other galaxies. The observed eccentricity distribution for high-[α/Fe] disk population rules out a violent thick disk formation mechanisms like direct accretion and dynamic heating. The measured chemical and kinematic gradients and velocity dispersions of the high-[α/Fe] population can be explained by the gas-rich merger scenario with thenon-negligible contribution from the radial migration.

Item Type: Dissertation
Supervisor: Bergemann, Dr. Maria
Place of Publication: Heidelberg
Date of thesis defense: 11 November 2019
Date Deposited: 25 Nov 2019 10:51
Date: 2019
Faculties / Institutes: The Faculty of Physics and Astronomy > Dekanat der Fakultät für Physik und Astronomie
Subjects: 520 Astronomy and allied sciences
Controlled Keywords: NLTE spectroscopy, globular clusters
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