oMEGACat. IX. Chemical Tagging of Omega Centauri Populations with Machine-Learning-Inferred Abundances from the MUSE Spectrograph

Abstract

We present chemical abundance measurements for 7,302 red giant branch stars within the half-light radius (~5') of $ω$ Centauri ($ω$ Cen), derived from MUSE spectra using the neural network model DD-Payne. DD-Payne effectively identifies spectral features of C, N, and O for [Fe/H]>-1.0 dex; Mg for [Fe/H]>-1.5 dex; and Na, Ca, and Ba for all metallicities. By combining these measurements with previous high-resolution studies, we create the most comprehensive picture of $ω$ Cen's rich chemical evolutionary history. For the first time, we map elemental variations across the entire chromosome diagram, which is widely used to identify multiple populations. We analyze the median chemical abundance trends as functions of age and metallicity for different subpopulations. The DD-Payne measurements of [C/Fe], [N/Fe], and [O/Fe] extend literature trends to higher metallicities and show continuous abundance-metallicity relations, with [(C+N+O)/Fe] increasing steadily with [Fe/H]. [Ca/Fe] and the s-process element [Ba/Fe] also increase with metallicity across all populations. For [Ba/Fe], the chemically enhanced (P2) populations are more enriched than primordial (P1) and the intermediate (Im) populations. Furthermore, [N/Fe] correlates strongly with stellar age while [Ca/Fe] and [Ba/Fe] exhibits a weaker age dependence. Using these abundance-metallicity-age relations, we evaluate different formation scenarios of $ω$ Cen proposed in the literature. Our study demonstrates that combining MUSE with machine learning enables large-sample stellar abundance measurements in crowded cluster cores, overcoming the limitations of fiber-fed spectroscopy for studying multiple stellar populations and their evolutionary histories.

Figures (13)

• Figure 1: Left: Colour-magnitude diagram (CMD) of the oMEGACat DD-Payne sample using photometry from Anderson2010ApJ, with stars color-coded by the MUSE spectral signal-to-noise ratio (S/N). The two stars shown in the right-hand panels are highlighted as filled star markers. Right: Example DD-Payne spectral fits for two stars. We show a primordial (P1) star and a chemically enhanced (P2) star (as defined in Section \ref{['subsec:discuss-stream']}), fitted with the DD-Payne-A and DD-Payne-G models in AO and non-AO observations, respectively. Note that all stars, in both observing modes, are fitted with all DD-Payne models; the cases shown here are representative examples of the fitting performance.
• Figure 2: Direct comparison of DD-Payne abundance measurements for oMEGACat DD-Payne stars with literature values for the cross-matched samples. Common stars between oMEGACat and each literature catalog are shown in different colors, with point size indicating $\rm{S/N}_{\rm{MUSE}}$. The number of common stars is provided in the legend for each literature work, and the black dashed lines are the one-to-one relation for visual reference.
• Figure 3: Median (top panel) and standard deviation (bottom panel) of the differences between oMEGACat DD-Payne abundances and literature studies for the cross-matched stars. The color scheme for each literature is the same as in Fig. \ref{['fig:abundances_1_1_literature']}. Gray markers with different shapes indicate comparisons of cross-matched stars between different literature, with the number of stars shown in parentheses. The open circle marker labeled "Selected" indicates the reference literature used for the abundance calibration of each element. As discussed in detail in Section \ref{['subsec:validation-calibration']}, we adopt $[\mathrm{Fe/H}]$, and [Na/Fe] from Johnson2010ApJ; [C/Fe], [O/Fe], [N/Fe], and [Mg/Fe] from Schiavon2024MNRAS; and [Ba/Fe] from Marino2011ApJ as references to calibrate DD-Payne abundances.
• Figure 4: Difference between oMEGACat DD-Payne abundances and literature measurements ($\Delta$[X/Fe]) as a function of $[\mathrm{Fe/H}]$ (original DD-Payne measurements without calibration). For each element, we use the standard reference literature adopted in Section \ref{['subsec:validation-calibration']}. The stars are divided into nine $[\mathrm{Fe/H}]$ bins, and the median $\Delta$[X/Fe] values are plotted as black points with error bars represented by the $16^{th}$ and $84^{th}$ percentiles. The gray dashed lines represent the fitted second-degree polynomial relations used to calibrate the DD-Payne abundances to the literature scale, as described in Section \ref{['subsec:validation-calibration']}. In each panel, the annotation in the lower-left corner reports the overall median $\Delta$[X/Fe] and $\sigma\equiv(p_{84}-p_{16})/2$ computed from all stars shown in that panel; these values are also listed in Table \ref{['tab:literature_counts']}.
• Figure 5: Chemical abundances of $\omega$ Cen stars as a function of $[\mathrm{Fe/H}]$. The DD-Payne measurements are shown as black density contours (50%, 80%, and 95% enclosed probability), while only the lowest-density of stars are over-plotted in gray to show the outer envelope. Colored points show literature measurements used as calibration references in Section \ref{['subsec:validation-calibration']}, using the same color scheme as Fig. \ref{['fig:abundances_1_1_literature']}. For the [Mg/Fe]-[Fe/H] relation, only stars with DD-Payne$[\mathrm{Fe/H}]{}_{\rm uncal} > -1.5$ dex are shown.