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ESO 280-SC06 is a tidally disrupted globular cluster that has lost almost all its mass, observations reveal

Using the Magellan Telescopes at Las Campanas Observatory in Chile, astronomers have performed a chemical study of a globular cluster known as ESO 280-SC06. As a result, they found that ESO 280-SC06 is a tidally disrupted cluster that was once massive but has lost at least 95% of its initial mass. The new findings were June 18 on the arXiv pre-print server.

Globular clusters (GCs) are spherical collections of tightly gravitationally bound stars, highly concentrated at the center and spread over a volume ranging from a few dozen up to more than 300 light years in diameter. Studies of GCs, which contain hundreds of thousands, and sometimes millions of stars, could help us better understand the formation, history and evolution of early-type galaxies.

In general, GCs harbor stars with light element abundances similar to solar abundances (the first population of stars, or 1P) and stars with correlated enhancements in nitrogen, sodium and aluminum, and depletions in carbon, oxygen and magnesium (the second population, or 2P).

Located some 67,000 light years away, ESO 280-SC06 is one of the faintest and most metal-poor globular clusters in the Milky Way. The cluster's mass is estimated to be about 12,500 solar masses.

A team of astronomers led by Sam A. Usman of the University of Chicago, Illinois, decided to take a closer look at the chemical composition of ESO 280-SC06. They employed the Magellan Inamori Kyocera Echelle (MIKE) spectrograph to conduct a high-resolution abundance study of ten red giant branch (RGB) stars in this cluster.

"Here we present the chemical abundances of these 10 stars with high-resolution spectroscopy using the Magellan Telescopes," the researchers wrote in the paper.

Using MIKE, Usman's team found that ESO 280-SC06 has a metallicity of approximately -2.54 dex and that it has a surprisingly high fraction of 2P stars—about 80%. The astronomers note that clusters with similar mass usually have a fraction of 2P stars at a level of 40%.

Furthermore, the observations found that the initial mass of ESO 280-SC06 was about 250,000–500,000 solar masses. This means that the cluster has lost some 95–98% of its mass, which is significantly higher than mass loss observed in similar GCs.

The study also detected the presence of a nitrogen-enhanced star enriched by binary mass transfer in ESO 280-SC06. Moreover, the researchers determined abundances or abundance upper limits for 21 additional elements from the light, alpha, odd-Z, iron peak, and neutron-capture groups for all the 10 investigated RGB stars.

Trying to explain the unusual properties of ESO 280-SC06, the authors conclude that the 1P stars may have been stripped first from the cluster, leaving a higher fraction of 2P stars relative to the known clusters that were fully mixed prior to experiencing significant tidal disruption.