arXiv subjects

Timur Şahin

Publications and source records attributed to Timur Şahin.

7 recordsLinked to original sources

arXiv2026

Unravelling Mass Transfer in Algols from Surface Abundances. I. Z Vulpeculae

The photospheric abundance pattern of the components is a sensitive probe of mass and angular momentum transfer in Algols. With the aim to trace the evolutionary history and disclosed efficiency of mass transfer in Algols we collected new high-resolution spectra for about a dozen Algols with the {\sc hermes} spectrograph at the Mercator Telescope on La Palma, Canary Islands. In the present paper, which is the first in the series, we present the results of a comprehensive analysis of Z Vul, a hot Algol-type system. We derived the absolute stellar quantities and elemental abundances for the components. The carbon-to-nitrogen (C/N) ratio is the most sensitive tracer of thermonuclear processing and mass transfer. It is found to be within expected value for gainer but donor lacks the severe C/N inversion traditionally expected for a deeply stripped star. An extensive grid of over three million evolutionary models was calculated with the Cambridge \texttt{STARS} code which allows determination of the best-fitting and all surviving models. Constraints from the He and CNO abundances indicate that Z Vul underwent an episode of nearly conservative mass transfer during the Main Sequence life-time of the originally more massive component (Case A). Our detailed chemical tagging confirms that the surface abundances of the mass-losing component represent the delicately stripped outer layers still not reaching the CNO-rich core. This underscores the need of coupling high-resolution spectroscopic abundances with interior stellar profiles to accurately reconstruct the evolutionary history of interacting binaries.

astro-ph.SR↗

arXiv2026

Spectral Disentangling Reveals Deep CNO-cycle Exposure in ET Cru

Binary stars undergoing mass transfer provide unique laboratories for testing stellar evolution. Here, we present a comprehensive photometric and spectroscopic analysis of the semi-detached system ET Cru. Using spectral disentangling, we independently determined the effective temperatures and chemical abundances of both components with high precision, including nine elements (eleven species). We find masses of $13.41\,M_\odot$ and $6.00\,M_\odot$ for the primary and secondary, respectively, with uncertainties of only $\sim$1.3%. The radii are $5.58\,R_\odot$ and $5.68\,R_\odot$, measured to within 0.4% and 0.5%. Surface gravities are constrained to better than 1%, while effective temperatures are determined to within 3-5%. The secondary exhibits extreme chemical anomalies, with severe carbon depletion and nitrogen enrichment far exceeding those reported in classical Algol systems. Multi-wavelength spectral energy distribution modelling yields a distance of $\sim$2.5 kpc, inconsistent with the $Gaia$ DR3 parallax, suggesting systematic astrometric uncertainties in the parallax distance. Together, these results establish ET Cru as a benchmark Algol-type binary, revealing direct spectroscopic evidence of deep CNO-cycle exposure in the donor and confirming the primary star as a rejuvenated gainer. ET Cru thus provides a chemically and dynamically illustrative case for understanding advanced binary interactions and the late evolutionary stages of massive-star evolution.

astro-ph.SR↗

arXiv2026

Chemical Taxonomy of $ω$~Centauri: Ten Populations Reveal a Multi-Phase Enrichment History

$ω$~Centauri, the most massive globular cluster in the Milky Way, exhibits a level of stellar population complexity that has long resisted a unified chemical characterisation. We exploit high-resolution near-infrared spectroscopy from the Milky Way Mapper survey (MWM DR19) to construct one of the largest homogeneously analysed samples of $ω$~Cen members to date. Applying Ward-linkage hierarchical clustering in a seven-dimensional chemical abundance space, without prior assumptions on population number or boundaries, we identify ten chemically distinct stellar populations. Their nucleosynthetic signatures trace four enrichment channels: iron-peak, $α$-element, CNO-cycle, and high-temperature proton-capture processes. The populations organise into two dominant groups separated by a large light-element spread at a modest iron baseline, consistent with AGB-driven self-enrichment. This dichotomy reflects distinct enrichment pathways: core-collapse supernovae establish the iron baseline, while AGB stars dominate light-element and $s$-process enrichment. A decoupled rise in $s$-process abundances relative to iron-peak elements, together with sub-dominant Type~Ia contributions across all metallicities, indicates evolution on timescales shorter than the characteristic Type~Ia delay time. One intermediate-metallicity population retains a primordial composition, providing evidence for spatially segregated enrichment within the progenitor. The most metal-rich component may trace star formation continuing after accretion into the Milky Way halo. All populations lie in the accreted regime of the $[\mathrm{Al/Fe}]$--$[\mathrm{Mg/Mn}]$ plane, supporting an ex-situ origin. These results reinforce the interpretation of $ω$~Cen as the remnant nucleus of an accreted dwarf galaxy and provide a framework for future chemo-dynamical studies.

astro-ph.GA↗

arXiv2026

The First Infrared Portrait of A Solar-Like Host Star with Debris Disk: Pioneering High-Resolution H- and K-Band Spectroscopy of HD115617 with Comparative Optical Spectrum Analysis

We present the first high-resolution near-infrared spectroscopic analysis of the solar analog HD115617 (61 Virginis), complemented by optical spectroscopy, asteroseismology, and spectral energy distribution modeling. Using ESPRESSO and IGRINS spectra with a newly calibrated NIR line list, we derived atmospheric parameters that revealed notable differences between spectral regions: the optical analysis yielded Teff = 5500 +- 140 K, log g = 4.40 +- 0.16, and solar metallicity, whereas the NIR yielded Teff = 5750 +- 140 K. We tested this 250 K discrepancy using the independent line depth ratio (LDR) method for both spectra. When applied to the optical lines, LDR confirmed the cooler scale (5553 +- 73 K); for the NIR lines, it provided an intermediate temperature (5636 +- 15 K). Asteroseismic scaling with TESS data yielded a radius of 0.98 +- 0.09 R_sun, consistent with SED fitting and confirming the star's main-sequence solar-like status. However, the age estimates diverged between methods, with optical and NIR analyses yielding ages of 10.97 and 8.04 Gyr, respectively. Critically, a condensation temperature analysis revealed no significant trend, confirming the star's bulk solar-like composition and showing no chemical signature of planetary formation processes. Kinematic diagnostics place HD115617 in the thin Galactic disk, with a birth radius of approximately 5.7-8.0 kpc. Although the spectral differences may be linked to the star's multi-planet system or debris disk, our analysis highlights the critical challenge of distinguishing such effects from methodological systematics in multi-wavelength studies. Consequently, we propose a systematic, homogeneous optical-NIR survey of solar-type stars to resolve this ambiguity, which could ultimately inform novel indirect methods for characterizing stellar environments.

astro-ph.SR↗

arXiv2025

Decoding the RVb Enigma of DF Cyg: Pulsations, Circumstellar Disks, and Post-RGB Evolution Revealed by Multi-Wavelength Observations

This investigation integrates five decades of ground-based photometric data from the AAVSO, AFOEV, ASAS, ASAS-SN, and SuperWASP surveys with recently acquired TESS observations, conducting a multi-wavelength and multi-phase photometric analysis to elucidate the complex nature of DF Cyg. For the first time, we analyze approximately four years of Kepler data alongside TESS observations. Furthermore, TESS observations obtained in 120-second cadence mode, constitute the highest-precision photometric dataset for DF Cyg to date. By isolating long-term trends in the TESS data, we quantified short-term variations in the fundamental pulsation frequency and its integer harmonics. The two alternating short-term cycles in the TESS light curve facilitated the unambiguous identification of seven previously undetected integer harmonics of the fundamental frequency in the power spectrum, providing critical new insights into the star's complex pulsation dynamics. A periodogram analysis of the combined ground- and space-based datasets revealed approximately 35 frequencies linked to both long- and short-term variability mechanisms. Stellar parameters derived from Gaia DR3 data specifically, luminosity and radius estimates based on Type II Cepheid PL and PR relations demonstrate consistency with values obtained through SED modeling. Complementary high-resolution spectroscopic analysis of DF Cyg yielded atmospheric parameters of Teff = 4781 K, logg = 1.74 dex, and [FeH] = -0.07 dex, further constraining physical characteristics of the star. This synthesis of multi-epoch, multi-instrument data advances the understanding of pulsational behavior and evolutionary status in RV Tauri systems. Our findings highlight DF Cyg as a critical benchmark for its class, as its characteristics bridge the gap between post-RGB systems in the Magellanic Clouds and higher-luminosity post-AGB stars.

astro-ph.SR↗

arXiv2025

Stellar Ancestry Unlocked: Chemical and Orbital Clues Link Metal Poor Stars to Globular Clusters and the Galaxy's Thick Disk

This study presents a detailed chemical, kinematic, and orbital dynamic analysis of five metal poor stars in the solar neighborhood: HD 2665, HD 5916, HD 122956, HD 189349, and HD 218857. Using high-resolution spectroscopic data from the ELODIE and ESPaDOnS instruments, we derived elemental abundances for 29 species (25 elements: C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Ba, Ce, Nd, and Sm) via LTE based analysis with ATLAS9 model atmospheres. Notably, we report first time detections of Ce and Nd in HD2665; Al, V, Sm, and Mn in HD5916; Al in HD 122956; and C, O, S, Sc, Mn, Co, Cu, Zn, Sr, Zr, Nd, and Sm in HD 189349. Dynamical and chemical diagnostics reveal distinct origins: HD 2665 shows strong orbital and chemical similarity to GC NGC 5139 ($ω$ Cen), while HD 218857 exhibits chemodynamic signatures consistent with NGC 5634. HD 122956 aligns with NGC 6864 (M75), though intriguingly shares age, metallicity ([Fe/H]), and [Mg/Fe] ratios with NGC 6517, a cluster whose reported abundances are derived solely from one star with APOGEE $H$-band spectroscopic measurements, as no optical spectroscopic data exist for its members. In contrast, HD 5916 and HD 189349 exhibit kinematic and chemical properties consistent with the field-star population and are classified as thick disk members.

astro-ph.GA↗

arXiv2025

Tracing the Galactic Origins of Selected Four G-type Stars in the Solar Neighbourhood

We present a multi-method investigation of four metal-poor G-type main-sequence stars to resolve their Galactic origins. By combining high-resolution spectroscopy from PolarBase, photometric/astrometric data from Gaia DR3, and spectral energy distribution (SED) modelling, we derive precise stellar parameters, chemical abundances, kinematics and Galactic orbital parameters. The stars HD 22879, HD 144579, HD 188510, and HD 201891 show effective temperatures of 5855 +/- 110, 5300 +/- 160, 5370 +/- 60, and 5880 +/- 90 K; surface gravities of 4.40 +/- 0.18, 4.52 +/- 0.37, 4.57 +/- 0.13, and 4.48 +/- 0.18 (in cgs units); and metallicities of -0.86 +/- 0.08, -0.55 +/- 0.12, -1.60 +/- 0.07, and -1.15 +/- 0.07 dex, respectively. Kinematic analysis suggests that HD 22879, HD 144579, and HD 201891 are potential bulge-origin escapees, possibly ejected by the Galactic bar or spiral arm perturbations. HD 188510, however, shows halo-like dynamics, including a retrograde orbit. Chemical abundance trends ([alpha/Fe] vs. [Fe/H]) reveal mixed origins, challenging kinematic classifications. This discrepancy highlights the importance of integrative methodologies in Galactic archaeology. We associate HD 22879, HD 144579, and HD 201891 with the bulge globular clusters NGC 6441, NGC 5927, and NGC 6544, respectively. HD 188510's retrograde motion and low metallicity align with ejection from the halo globular cluster NGC 5139 (omega Centauri). These results illustrate the complex interplay of dynamical processes -- including bar resonances, spiral arm perturbations, and tidal stripping -- in depositing metal-poor stars into the solar neighborhood.

astro-ph.SR↗