An observational study of G type stars in the solar neighborhood: Investigation of the existence of the magnetic field and its effect on the stellar atmosphere

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 Mediterranean University HRSS High-Resolution Stellar Spectroscopy Basic Science Research Laboratory. The same spectral analysis methods were also applied to the project's 90 metal-poor (−2.5 < [Fe/H] < −0.5) G-spectral type stars. Consequently, the results of this thesis constitute a preliminary study for the planned magnetic field-based analyses of these stars. To investigate the effect of the magnetic field on atmospheric parameters, neutral iron (FE I) transitions used in excitation and ionization equilibrium calculations were identified as the focal point. FE I transitions with high magnetic sensitivity (Lande g > 1.9) located within the 5000–6000 Å range were tested on the solar spectrum. It was found that lines at, for example, 5225.53, 5247.06, 5250.22 and 5506.79 Å wavelengths lead to abundance increases on the order of ∼0.1 dex. For the target stars, the calculated abundance values exhibiting similar observed differences could be compensated for by decreases in the calculated effective temperature values assuming no magnetic field effect: This decrease was calculated as 100 K for HD 189349, 30 K for HD 186306, and 120 K for BD+443197. At this stage, it is essential that non-LTE effects on the calculated elemental abundances are meticulously corrected for on a line-by-line basis. While the magnetic field effect, manifested as a temperature decrease for neutral Fe lines, was represented by a change in the surface gravity log g value for ionized Fe transitions. Another significant effect, equally important as non-LTE effects during the investigation of the magnetic field impact and requiring examination of its influence on calculated abundances, is the convection effect. For the stars analyzed within the thesis, both prescriptions available in the literature (Magic2015 and Ludwig1999) were used to determine the effect of convection on elemental abundances. Additionally, a new methodology was developed within the thesis to detect the magnetic field strength of stars lacking spectropolarimetric observational data. Comparing synthetic spectropolarimetric line profiles calculated for the stars with the derivatives of the intensity spectra of the studied stellar spectra enables a numerical estimation of the possible effective magnetic field strength for the studied stellar atmosphere. The method was found to be successfully applicable for the thesis stars, HD 189349 and BD+443197, giant and subgiant stars. In this context, the magnetic field strength value detected for HD 189349 was determined to be 500 G, and for the BD+443197 subgiant star, it was 600 G. These reported numerical values for the magnetic field represent surface magnetic field strength values. These results will only be verifiable through high-precision spectropolarimetric observations of the stars.