Chilean researchers study chemical composition of globular cluster NGC 6553

Using the Very Large Telescope (VLT), astronomers in Chile performed a chemical analysis of a sample of stars in the globular cluster NGC 6553. Results of this study, available in an article published on March 12 at the arXiv pre-print server, sheds more light on the chemical makeup of this cluster.

Globular clusters (GC) are collections of closely related stars orbiting galaxies. Astronomers see them as natural laboratories for studies of the evolution of stars and galaxies. In particular, globular clusters could help researchers better understand the history of the formation and evolution of early-type galaxies, as the origin of GC appears to be closely related to periods of intense star formation.

Located some 19,500 light years away, NGC 6553 is a metal-rich GC in the bulge of the Milky Way galaxy. The cluster is around 13 billion years old and has a mass of around 235,000 solar masses. Although several studies on NGC 6553 have been carried out, little is known about its chemical composition.

A team of astronomers, led by Carolina Montecinos from the University of Concepción, Chile, have now performed chemical analysis of NGC 6553 on a sample of over 100 giant stars, examining data primarily from the FLAMES / GIRAFFE spectrograph on VLT. The study was completed by the results of the VISTA variables from the Vía Láctea (VVV) Data Release 2 (DR2) survey of the Gaia satellite of ESA.

“In this work, we performed a chemical analysis of the convex globular cluster NGC 6553 on a sample of 116 giant stars both in the horizontal branch and in the giant branch, using the FLAMES / GIRAFE spectrograph. (…) Next, we performed a comparison between NGC 6553 and other globular bulge clusters nearby, as well as with other globular clusters of galaxies and field stars from the bulge, disk and halo of the Milky Way, ”the researchers wrote in the article.

The study found that the stars studied in NGC 6553 exhibit an overabundance of five alpha elements – oxygen, magnesium, silicon, calcium and titanium. This is in agreement with all GCs in the Milky Way bulge, as they also have a similar overabundance of alpha elements.

In addition, vertical sodium-oxygen anticolerration has been identified because there is sodium dispersion, but oxygen variation is minimal. Such anticolerration occurs in other clusters of the galactic bulge.

Additionally, research has not found any intrinsic variation in alpha and peak iron content. Astronomers have noted that this is consistent with the trend for galactic bulging stars, suggesting a similar origin and evolution.

The average iron content of the star sample in NGC 6553 was found to be at a level of -0.1. However, no significant spread of iron content was detected by the study.

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