Chemical Abundances in the Metal-Poor Globular Cluster ESO 280-SC06: A Formerly Massive, Tidally Disrupted Globular Cluster

TL;DR

The paper investigates chemical abundances in ESO 280-SC06, a very metal-poor and low-mass Milky Way globular cluster, using high-resolution MIKE spectra for 10 RGB stars to map 21 elements and assess multiple populations. It finds a mean metallicity of $[Fe/H] \approx -2.54$ with minimal dispersion and identifies eight 2P stars ($f_{\rm enrich} \approx 0.80$), alongside a nitrogen-enhanced, s-process–enriched NEMP star resulting from binary mass transfer. A lack of detectable neutron-capture dispersion (excluding the NEMP star) challenges some expectations from similar-metallicity clusters. Kinematic modeling indicates the cluster originated with $M_i \sim 10^{5.4-5.7}$ M$_\odot$ and has lost $\sim$95–98% of its mass, implying extreme tidal disruption and possible preferential loss of 1P stars; overall the work emphasizes incorporating mass loss into interpretations of GC chemical evolution and enrichment patterns.

Abstract

We present the first high-resolution abundance study of ESO 280-SC06, one of the least luminous and most metal-poor gravitationally bound Milky Way globular clusters. Using Magellan/MIKE spectroscopy for ten stars, we confirm the cluster's low metallicity as [Fe/H] = $-2.54 \pm 0.06$ and the presence of a nitrogen-enhanced star enriched by binary mass transfer. We determine abundances or abundance upper limits for 21 additional elements from the light, alpha, odd-Z, iron peak, and neutron-capture groups for all ten stars. We find no spread in neutron-capture elements, unlike previous trends identified in some metal-poor globular clusters such as M15 and M92. Eight of the ten stars have light-element abundance patterns consistent with second-population globular cluster stars, which is a significantly larger second-population fraction than would be expected from the low present-day mass of $10^{4.1}$ Msun. We estimate the initial mass of the cluster as $10^{5.4 - 5.7}$ Msun based on its orbit in the Milky Way. A preferential loss of first-population stars could explain the high fraction of second-population stars at the present time. Our results emphasize the importance of considering mass loss when studying globular clusters and their enrichment patterns.

Figures (6)

• Figure 1: The normalized spectra of the ESO 280-SC06 stars around the measured sodium doublet at 5890 and 5896 Å (top) and the measured aluminum line at 3962 Å (bottom). The thin, dotted vertical lines indicate the precise locations of the sodium lines and aluminum line in their respective panels. Stars 001, 005 and 025 are represented by a gold solid line, a blue dashed line, and a dotted red line, respectively. All stars have a temperature around 5180 K. Despite the similar temperatures, stars 005 and 025 clearly display much weaker sodium and aluminum absorption lines. This demonstrates that both stars are 1P stars, while star 001 is a 2P star. The other stars' spectra are shown as thin gray lines in the background. In the bottom panel, ISM Na indicates an absorption feature in the spectrum due to sodium in the interstellar medium.
• Figure 2: The color-magnitude diagram for ESO 280-SC06 stars. The dotted line represents a MIST isochrone with $\log$ age = 10.15 and metallicity $\mathrm{\,[Fe/H]}$ = $-2.5$Dotter2016Choi2016. We shift the isochrone to the red side by 0.08 (as was done in previous analyses such as Simpson2018). The first-population / 1P stars, AGB star, NEMP star and remaining second-population / 2P stars are denoted to blue hollow circles, a red hollow star, a green hollow square and a yellow hollow plus symbols.
• Figure 3: Key abundances for identifying chemical patterns found in multiple populations. 2P stars are consistently enriched in sodium, aluminum, and nitrogen, and depleted in magnesium and oxygen. The top left plot compares the LTE sodium and aluminum abundances. Directly below this, the bottom left plot adjusts these abundances to incorporate nLTE corrections. The top center, top right and bottom center plots compare the carbon abundances (measured using the molecular C-H band), nitrogen abundances (measured using the molecular C-N band) and magnesium abundances, to the nLTE aluminum abundances. Lastly, the bottom right plot compares the nLTE sodium abundances to magnesium abundances. We identify eight stars with chemical enrichment patterns indicative of second-population/2P stars. Stars 005 and and 025 (represented by hollow blue circles) are the only stars that have low sodium and aluminum abundances, and are therefore classified as first-population/1P stars. Star 184, which has been identified as an AGB, is represented by a red hollow star. The NEMP star (identified in Simpson2019) is represented by a green hollow square. The remaining second-population stars are represented by dark yellow plus symbols. For comparison, we also show abundances from M15 Sobeck2011, represented by solid green triangles, and M92 Kirby2023, represented by solid blue squares with error bars. For nLTE-corrected sodium abundances and magnesium abundances, we overlay a blue region and a light red hatched region to represent the diagnostic thresholds used in Kirby2023 to classify 1P stars: [Na/Fe] $<$ 0.1 and [Mg/Fe] $>$ 0.45 indicate a star is 1P. The authors suggest the sodium threshold is more reliable than the magnesium threshold. By these categorizations, 005 is the only star that is sodium-poor enough to definitively be considered 1P. Star 025, however, has very similar sodium and aluminum abundances, and is likely also a 1P star, despite being slightly over the threshold. Star 184 is magnesium-rich enough to be classified as a 1P star, however, its high aluminum abundance is incongruent with being a 1P star; we therefore consider it a 2P star.
• Figure 4: Neutron-capture abundances measured in the ESO 280-SC06 stars. The first-populations/1P stars, the AGB star, the NEMP star, and the remaining second-population/2P stars are represented by blue hollow circles, a red hollow star, a green hollow square and yellow hollow plus symbols, respectively. For comparison, we also compare to abundances from M15 Sobeck2011, represented by green triangles, and M92 Kirby2023, represented by blue squares with error bars. The NEMP star, 026, has significantly higher s-process abundances (strontium, barium, and yttrium) than the other stars. This is consistent with s-process enhancement predicted from accretion from an asymptotic giant branch binary companion. Among the other stars, we find no significant spread in neutron-capture abundances, contrary to patterns previously identified in other systems such as M92 Kirby2023.
• Figure 5: Three views of the Galactic orbit of ESO 280-SC06. The present position of the cluster is marked with a red pentagon, and its orbit over the past 1 Gyr is shown as a blue line. The high eccentricity and inclination of ESO 280-SC06's orbit mean that it has significant disk crossings at each pericenter passage, roughly every 150 Myr. The position of the Sun is shown with a yellow circle. The Milky Way is represented by the gray circle in the XY plane, and the thick gray line for side-on views in XZ and YZ planes. This orbit was integrated using gala with the default MilkyWayPotential.