arXiv subjects

Furkan Akbaba

Publications and source records attributed to Furkan Akbaba.

6 recordsLinked to original sources

arXiv2026

Kings of the Milky Way: A Homogeneous Gaia DR3 Analysis of King Open Clusters and the Galactic Disc Metallicity Gradient

This study aims to establish an internally homogeneous set of structural, astrophysical, and kinematical parameters for King open clusters and to assess their role as tracers of the Galactic thin disc. We present a homogeneous structural, astrophysical, and kinematical analysis of 27 King open clusters using high-precision Gaia DR3 astrometry, photometry, and spectroscopy. Cluster reddenings, distances, metallicities, and ages are determined via a homogeneous Markov Chain Monte Carlo isochrone-fitting approach applied to Gaia colour-magnitude diagrams. The derived parameters span E(G_BP-G_RP) = 0.113-1.933 mag, metallicities of [Fe/H] = -0.40 to +0.28 dex, ages from 17 to 6166 Myr, and heliocentric distances between 739 and 6272 pc. A comparison between astrometric and isochrone-based distances yields a median parallax difference of $-13\mu$as, indicating a good level of internal consistency between the photometric and astrometric distance scales within the adopted homogeneous framework. The radial metallicity behaviour of the King clusters is also examined using three complementary Galactic distance definitions and compared with the metallicity-radius relations obtained independently from 164 spectroscopically analysed open clusters from the literature. The King-cluster gradients are close to $-0.060$ dex kpc$^{-1}$ and are broadly consistent with those of the external reference sample, indicating that the homogeneously analysed King clusters follow the negative metallicity-radius trend traced by the broader thin-disc open-cluster population. Flatter gradients are found for young clusters, suggesting more homogeneous chemical enrichment at recent epochs. When dynamical constraints are applied to reduce the effects of radial migration and orbital blurring, the inferred gradient remains close to $-0.059$ dex kpc$^{-1}$.

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arXiv2026

Unsupervised Chemo-Dynamical Dissection of the Inner Galactic Halo: Discovery of Five Accreted Substructures with SDSS-V and Gaia

The inner Galactic halo is a complex graveyard of the Milky Way's earliest accretion events, where severe orbital phase-mixing challenges traditional dynamical stream-finding techniques. We present a purely data-driven, 12-dimensional chemo-dynamical analysis of the inner halo using \textsl{SDSS-V Milky Way Mapper} (DR19) and \textsl{Gaia} DR3. Utilizing an unsupervised machine learning framework based on UMAP and HDBSCAN, we perform a blind search for clustered populations within a chemically selected \textit{ex-situ} sample of 2,185 stars without kinematic pre-selection. Our pipeline recovers nine kinematic groupings corresponding to seven known substructures (including \textsl{Gaia}-Enceladus/Sausage, the Helmi Streams, and Sequoia), validating the robustness of the high-dimensional feature space. We also report five new tightly bound candidate substructures, designated FO1--FO5 ($E_{\rm tot} \leq -1.8 \times 10^5~\mathrm{km^2~s^{-2}}$). Four candidates (FO1, FO3, FO4, FO5) are confirmed as robust chemo-dynamical overdensities, while FO2 exhibits a striking nitrogen enhancement ($[\mathrm{N/Fe}] = +0.83 \pm 0.16$) suggestive of tidal debris from a disrupted massive globular cluster. Finally, we demonstrate that high-dimensional chemical information is critical for resolving dynamical degeneracies in the crowded inner halo, differentiating structures sharing similar orbits but distinct chemistry (e.g., FO5 and Shiva), and the reverse (e.g., FO3 and the Helmi Streams). These findings confirm that the deepest regions of the Galactic potential preserve a rich record of the Galaxy's assembly history.

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arXiv2026

Astrophysical Parameters of 5056 Open Star Clusters from Bayesian Nested Sampling with PARSEC Isochrones

We present a homogeneous catalogue of fundamental astrophysical parameters -- age, metallicity ([Fe/H]), heliocentric distance, and colour excess $E(G_{\mathrm{BP}}-G_{\mathrm{RP}})$ -- for 5,056 open star clusters drawn from the Unified Cluster Catalogue (UCC). All parameters are derived uniformly from Gaia Data Release 3 (DR3) colour-magnitude diagrams via Bayesian Nested Sampling with PARSEC stellar isochrones, with no manual intervention on individual clusters. Initial metallicity $Z_{\mathrm{ini}}$ is treated as a free parameter throughout, yielding a photometric [Fe/H] estimate for every cluster. Physically motivated priors -- parallax-based distances from Gaia DR3 astrometry, spectrophotometric metallicity constraints from Gaia XP spectra where available, and interstellar reddening from the Schlegel-Finkbeiner-Davis dust map -- reduce CMD degeneracies without anchoring the fit to any external parameter catalogue. Of the 5,056 clusters, 3,766 (74.5\%) satisfy the fit-quality criterion $\eta_{\mathrm{fit}} \ge 0.80$. This high-quality subset spans ages 0.003-5.5~Gyr ($\log(\mathrm{Age/yr})$ median $8.33 \pm 0.34$~dex), heliocentric distances 88-19,011 pc (median 2,150~pc), metallicities $-1.17 \le \mathrm{[Fe/H]} \le +0.42$~dex (median $+0.002$ dex), and extinctions up to $A_G = 7.37$~mag (median 1.07~mag). The catalogue is made publicly available via CDS/VizieR; the complete nested-sampling posterior chains are archived on Zenodo.

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arXiv2026

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

$\omega$~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 $\omega$~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, $\alpha$-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 $\omega$~Cen as the remnant nucleus of an accreted dwarf galaxy and provide a framework for future chemo-dynamical studies.

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arXiv2026

The structure and evolution of the Galactic high-$\alpha$ disc I. Chemical and age orbital cartography

We present a comprehensive chemical and age orbital cartography of the Galactic high-$\alpha$ disc using subgiant stars with precise ages, element abundances, and full phase-space information from the \textsl{LAMOST--Gaia} data set. Specifically, we map how average [Fe/H], [$\alpha$/Fe], and age vary across present-day kinematic and orbital coordinates. We analyse the data in full and across mono-abundance populations to measure element abundance-orbital and age-orbital gradients across orbital actions and angular-momenta. Our results show that the high-$\alpha$ disc exhibits clear and coherent gradients in [Fe/H], [$\alpha$/Fe], and age with orbits; these gradients are much stronger and sharper in orbital space than in present-day kinematics, showing that orbital diagnostics recover the intrinsic disc structure of old disc populations more effectively than instantaneous kinematic coordinates. We find that older high-$\alpha$ populations display qualitatively similar element abundance--orbital and age--orbital trends to stars in the low-$\alpha$ disc, although the high-$\alpha$ gradients are generally shallower. The presence of these ordered correlations indicates that the old high-$\alpha$ disc is structured, and preserved a strong fossil record of its early assembly despite the Milky Way's subsequent accretion history. This result implies that later mergers did not fully erase the chemical-orbital and age-orbital structure imprinted during the high-$\alpha$ disc's earliest formation epoch. All together, these findings indicate that the Galactic high-$\alpha$ disc formed mainly through inside-out and upside-down growth.

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arXiv2025

Estimating Initial Mass of Gaia-Enceladus Dwarf Galaxy with Chemical Evolution Model

This work investigates the initial mass and chemical evolution history of the Gaia-Enceladus dwarf galaxy. We combine spectroscopic data from APOGEE with astrometric data from Gaia DR3 to identify Gaia-Enceladus candidate stars via a machine-learning pipeline using t-SNE and HDBSCAN. By focusing on kinematic and chemical parameters, especially $\mathrm{[Fe/H]}$, $\mathrm{[Mg/Fe]}$, $\mathrm{[Al/Fe]}$, and $\mathrm{[Mn/Fe]}$, we uncover a population of metal-poor, high-eccentricity stars that align with literature criteria for Gaia-Enceladus debris. We then apply the \textit{OMEGA+} chemical evolution model, incorporating MCMC fitting of the observed abundance trends in the $\mathrm{[Mg/Fe]\times[Fe/H]}$ plane. Our best-fitting model indicates a gas mass of $4.93_{-0.72}^{+0.32}\times10^9\,{M_{\odot}}$ for Gaia-Enceladus, placing it at the higher end of previously suggested mass ranges. The model scenario suggests a short star formation timescale, substantial outflows, and a rapid build-up of metals mainly driven by core-collapse supernovae, with a lesser contribution from Type~Ia supernovae. Comparison with observational data in other chemical planes (e.g., $\mathrm{[Mg/Mn]\times[Al/Fe]}$) supports this scenario, emphasizing a distinct evolution path relative to the Milky Way. Additionally, our results provide indirect evidence that star formation in Gaia-Enceladus likely ceased within the first 4 Gyr, consistent with earlier inferences of an early merger event. These findings highlight the power of chemical evolution modeling in reconstructing the origin and mass of ancient accreted systems. Overall, we show that Gaia-Enceladus, through a rapid star formation and strong outflows, contributed a significant fraction of the metal-poor stellar halo of the Milky Way.

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