oMEGACat. VIII. A Subpopulation Census of ω Centauri

TL;DR

The paper tackles the complex assembly history of Omega Centauri by constructing a high-precision, multi-probe census of subpopulations from the upper RGB to below the MSTO using the oMEGACat dataset. It introduces a four-phase subpopulation parsing pipeline that fuses photometric chromosome diagrams with spectroscopic metallicities in a 3D space ($[\mathrm{Fe/H}]$, $\Delta_{F336W,F814W}$, $\Delta_{C_{F275W,F336W,F435W}}$) to identify 14 discrete subpopulations and connect them to the age-metallicity relation via SGB ages. The study finds that chemically enhanced (P2) populations are ~1 Gyr younger than primordial (P1) populations, with larger intrinsic age spreads, while intermediate (Im) populations lie in between; it also links the chromosome diagram to the two-stream AMR, suggesting complex, possibly multi-environment formation channels. Overall, the results constrain formation scenarios for Omega Centauri, revealing that no single model explains all observed features and highlighting the need for high-precision abundances and updated isochrones to further resolve the cluster’s assembly history.

Abstract

An understanding of the assembly history of the complex star cluster Omega Centauri has long been sought after, with many studies separating the stars on the color-magnitude diagram into multiple groupings across small magnitude ranges. Utilizing the oMEGACat combined astro-photometric and spectroscopic dataset we parse 14 subpopulations from the upper red-giant branch to below the main-sequence turnoff. We combine our results with previous works to estimate the age and age spread of each population. We find that the chemically enhanced (P2) populations are all ~1 Gyr younger (~11.6 Gyr old) and have significantly higher intrinsic age spreads (0.6 Gyr) than the primordial (P1) populations (~12.6 Gyr old, 0.3 Gyr spread), with the intermediate (Im) populations falling in between the two. Additionally, we connect for the first time the Chromosome Diagram to the two-stream age-metallicity relation, allowing us to link the P1 and P2 stars to the distinct star formation tracks, proposed to be in-situ and ex-situ contributions to the cluster's assembly. Our results are consistent with some suggested formation models and rule out others but no current model can explain all observed features of the subpopulations.

Figures (8)

• Figure 1: Color-Magnitude Diagrams:(both panels) The black dotted line traces the overdense region of this plot when considering a subset of stars centered around the median metallicity ([Fe/H] $\sim$-1.7). (left panel) The full subset of high-quality member stars is plotted with light grey markers. All SGB stars with age determinations are overplotted in black. The upper black box delineates the region where the RGB ChD is generated while the lower black box delineates where the age-metallicity relation is calculated. (right panel) The full subset of high-quality member stars is plotted with individual markers colored by their [Fe/H] value, following the colorbar at the top of this panel. The black box delineates where the initial RGB clustering is performed. The upper grey shaded region shows the extent of the first propagation step centroiding sample while the lower grey shaded region shows the extent of the first propagation step assignment data. The scaling data is constituted by the combination of the centroiding and assignment data.
• Figure 2: Subpopulation Parsing Algorithm Flowchart: The four phases of our subpopulation parsing algorithm are outlined vertically, top to bottom, with each phase being delineated by a horizontal colored band. Within each white box a bulleted list of grouped steps in the procedure is given. Black solid line arrows connect steps performed in order, and black dash-dot line arrows show steps performed iteratively. The grey lined boxes and arrows indicate steps which are only considered in our second-pass version of the subpopulation parsing algorithm.
• Figure 3: Subpopulation Parsing:(all panels) Each small marker point is a single star, colored by its subpopulation label. The large colored circle marks the associated cluster and within each is the annotation of the cluster number, in order of metallicity, as well as the ChD stream label, as a subscript. The 1- and 2-$\sigma$ density contours are shown in thick and thin black lines, respectively. (upper left) The RGB chromosome map is constructed from the two delta-colors. The centroid of each subpopulation is given by the large colored markers. (upper right) The pseudo-color vs. [Fe/H] space shows the distinction of the subpopulations in this space. (lower panels) Same as upper panels except now for the SGB. Here, the large colored markers are offset to aid visibility and are connected to the centroid of each cluster via a solid line with the same color.
• Figure 4: Chromosome Diagram Grouped CMDs: The F814W versus F606W - F814W CMD is shown for each set of populations grouped on the ChD (see Fig.\ref{['fig:rgb_clustering']}). The grouping (P1, Im, or P2) is annotation in the bottom right of each panel. Each data point is colored by its subpopulation assignment and indicated in the relevant legends. A fiducial line is given by a black dashed line, which traces the median metallicity population. Through comparison with the fiducial line we can confirm continuous sequences with CMD morphologies reflective of the expected variations in helium and alpha element abundances.
• Figure 5: [Na/Fe] Verification of Subpopulation Consistency: (both panels) Shown here is the median DD-Payne derived sodium abundance for the each subpopulations versus the respective median iron abundance. The left panel contains values for stars on the RGB (12 $<$ F814W $<$ 16 mag) and the right panel contains those for SGB stars (16 $<$ F814W $<$ 17 mag). We see consistency in the separation of P1, Im, and P2 across the panels with the overall range of sodium values decreasing on the SGB due to increased uncertainties.