arXiv subjects

T. Çakır Alsaç

Publications and source records attributed to T. Çakır Alsaç.

3 recordsLinked to original sources

arXiv2026

New scaling relations from MESA models for mass and radius of subgiant stars: application to Kepler targets

The precise determination of the fundamental parameters of stars is crucial for understanding stellar structure and evolution. In this regard, the asteroseismic parameters of solar-like oscillating stars obtained from space telescopes are very useful for determining the mass ($M$) and radius ($R$) of single stars using classical scaling relations. These relations still need to be improved. In this study, we develop alternative scaling relations based on reference frequencies ($ν_{\rm min}$) due to helium ionization zone glitches, specifically for subgiant (SG) stars. We compile main-sequence and SG {\small{MESA}} models from the literature and derive new scaling relations for stellar $R$ and surface gravity ($g$) of evolved stars. The expressions for $R$ and $g$ are obtained simultaneously as functions of the large frequency separation, $ν_{\rm min}$, and metallicity. These new relations allow $M$ and $R$ to be reliably determined only from observational parameters, without the need for detailed stellar modelling. The resulting $ν_{\rm min}$-based relations are applied to a sample of 31 SG stars observed by the \emph{Kepler} mission, with masses in the range 0.85-1.74 M$_\odot$ and radii between 1.03 and 2.82 R$_\odot$. Using simultaneous solutions, we estimate new mass and radius ranges of 0.89-1.62 M$_\odot$ and 1.04-2.95 R$_\odot$, respectively. The results demonstrate significant improvements in the determination of $M$ and $R$ for evolved \emph{Kepler} target stars, highlighting the potential of the new scaling relations in asteroseismic analyses.

astro-ph.SR↗

arXiv2026

Asteroseismic analysis of red giants in eclipsing binaries using two methods: implications for scaling relations and chemical composition

The study of solar-like oscillating red giants in eclipsing binaries (EBs) provides a unique opportunity to advance stellar astrophysics by combining dynamical mass and radius measurements with asteroseismic constraints. EBs provide precise fundamental parameters (e.g. mass, radius, and luminosity) independent of distance, while solar-like oscillations probe stellar interiors and enable tests of asteroseismic scaling relations used to determine stellar masses and radii. {We apply two different methods to estimate the initial chemical composition of the systems. In Method I, the initial helium abundance ($Y_0$) is treated as the free parameter, whereas in Method II the free parameter is the initial metallicity ($Z_0$), assuming a relation between $Y_0$ and $Z_0$. We construct interior models individually for the components of 11 EBs and obtain coeval solutions for eight systems.} The ages and chemical compositions derived from the two methods are generally consistent with each other. Our results provide important clues about the chemical evolution of a part of the Galactic disk. Moreover, using the parameters obtained for two oscillating stars, Tek Ayak (KIC 8410637) and KIC 9970396, instead of solar reference values in the scaling relations yields masses and radii that are in much better agreement with the dynamical solutions without requiring additional corrections.

astro-ph.SR↗

arXiv2026

Planetary systems in the light of asteroseismology: metallicity threshold for the planetary systems and age-metallicity relation

We compiled data for 127 hosts (plus six candidates) and used them as constraints to construct interior models of the hosts using the {\small MESA} code. Two significant conclusions emerge from these models. First, except for a few stars, the hosts' metallicity ($Z_0$) is greater than 0.007. This suggests a possible suppression of the occurrence of planets below $Z_0\approx0.007$. Second, it concerns how chemical evolution unfolds in the galactic disc. For a given $Z_0$ value, considering the oldest stars, there is a linear relationship between $Z_0$ and age ($t_9$). This line is around $t_9=13.4$ Gyr at $Z_0=0$, a value consistent with the age of the Galaxy. The linear relationship continues until around $t_9=6$ Gyr, and the maximum value of $Z_0$ remains constant between $t_9=2-6$ Gyr. We further modelled 12 hosts classified as red clump (RC) stars in the literature, explicitly accounting for mass loss along the red giant branch. These models highlight the critical role of mass-loss assumptions in determining the initial masses and ages of RC hosts, and their implications for the survival and evolution of close-in planets. Another key outcome of this study is the discovery of the relationship between $Z_0$ and the observed metallicity ($Z_{\rm s}$) for the hosts. We obtain a useful expression for $Z_0$, the input parameter for the models, as a function of stellar mass, radius, and $Z_{\rm s}$. This expression can be used to estimate $Z_0$ based on the reduced surface metallicity due to microscopic diffusion. We also derive an expression for planetary mass relative to the orbital semimajor axis and host mass. This expression may indicate a mass distribution near the inner disc where these planets formed, except for hot-Jupiters. Planet radii appear to depend on the planet's mass and irradiation energy, as well as the orbital period.

astro-ph.SR↗