Akademik Veri Yönetim Sistemi
Tezin Türü: Doktora
Tezin Yürütüldüğü Kurum: Akdeniz Üniversitesi, Fen Bilimleri Enstitüsü, Fen Bilimleri Enstitüsü, Türkiye
Tezin Onay Tarihi: 2025
Tezin Dili: Türkçe
Öğrenci: Gizay YOLALAN
Danışman: Timur Şahin
Özet:
Studies on the origin of stars in the solar neighborhood contribute to the development
of galactic evolution models. Therefore, the precise determination of stellar atmospheric
parameters and/or the reporting of metallicity ([Fe/H]) and α-element abundances are of
critical importance. Stellar atmosphere models constructed under the local thermodynamic
equilibrium (LTE) approximation with plane-parallel geometry do not account for polarized
light effects or atmospheric magnetic field gradients. In these models, the magnetic field
is assumed to be constant throughout the atmosphere. While simulations of magnetic
field-sensitive transitions show variations in individual elemental abundances in the
presence of a magnetic field, the effect of these variations on atmospheric parameters
has not been sufficiently investigated in the literature.
Within the scope of this study, as part of TÜBİTAK ARDEB 1001 project (121F265),
three stars (HD 189349, HD 186306, BD+443197) with both TESS and Kepler data
were selected from the 90 G-spectral type stars investigated for Galactic origins,
and high-resolution (R ∼ 80, 000) spectroscopic analyses were performed. Analyses
conducted using ESPaDOnS archive spectra included the following steps: determination of
atmospheric parameters under 1D-LTE conditions, determination of elemental abundances,
and modeling of the magnetic field effect on selected spectral lines.
One of the primary objectives of this study is to break the existing parametric
degeneracy between Teff and log g during the spectroscopic analysis phase, using scaling
relations in conjunction with Kepler and TESS photometric data. Asteroseismic analyses
require Teff as an input parameter for scaling relations, and these values are typically derived from photometric color-temperature calibrations. Within this thesis, it was aimed to
obtain more accurate surface gravity (log g) values by using precise Teff values determined
spectroscopically as input to the scaling relations. However, the inconsistent reporting of
the Sun’s large (∆ν ⊙ ) and small (δν ⊙ ) frequency separations – two fundamental constants
essential for the log g solution – by different groups in the literature introduces uncertainties
in the iterative parameter determination process. This situation carries the risk of producing
forced Teff and log g parameter sets constrained by these constants, rather than being
independently determined by the physical nature of the stars. The findings obtained have
provided valuable insight into breaking this methodological circularity.
In quantifying the magnetic field effect, solar spectropolarimetric data were used
as a reference. The effect of the magnetic field on abundance calculations for selected
atomic spectral lines was investigated using the high-resolution solar spectrum. The solar
spectrum serves as a standard laboratory reference in the literature for atomic data quality
validation, and this approach has been successfully applied in the work of members of the
Akdeniz University HRSS High-Resolution Stellar Spectroscopy Basic Science Research
Laboratory.