Chemical history of a star cluster in the inner regions of the Milky Way galaxy revealed

An international group of researchers, including astronomers from the Universidad Católica del Norte have conducted the first investigation of the chemical properties of the ancient globular cluster Tonantzintla 1, also known as NGC 6380. The researchers worked with data obtained with the Irénée du Pont telescope at Las Campanas Observatory.

Globular clusters are symmetric, compact collections of thousands or even millions of ancient stars, which have existed since the earliest epochs of galaxy formation, and have been considered important systems that provide important clues about their parent galaxies. In our Milky Way galaxy, approximately 170 of these systems have been cataloged, many of which remain poorly explored, including Tonantzintla 1. 

For approximately 195 years, the properties of Tonantzintla 1 have remained virtually hidden in the inner regions of our Galaxy, due to the large amount of gas and dust in the intergalactic medium that exists in the inner regions of the Milky Way, which blocks much of the visible light escaping from the stars of Tonantzintla 1. 

However, new near-infrared observations were able to penetrate these dense columns of gas and dust to capture great detail of the atmospheres of the Tonantzintla 1 stars. Thus, the researchers analyzed data from the advanced spectrograph of the Apache Point Observatory Galactic Evolution Experiment (APOGEE), which was installed on the Irénée du Pont telescope, which has a 2.5-meter main optics and operates from the Las Campanas Observatory in the third region of Chile. 

The stellar spectra obtained in the infrared band of the electromagnetic spectrum provided for the first time individual physical characteristics of the stars of Tonantzintla 1, such as their surface temperatures and gravities, but also their velocity in radial direction (i.e., we can know whether they are moving away from or towards us and how fast they are moving), and also their chemical composition. All these parameters provide the best estimates to date, revealing for the first time the chemical composition of Tonantzintla 1.

The new near-infrared spectra made it possible to investigate the abundances of a variety of chemical species, including elements lighter and heavier than iron. For example, among the light elements that were detected in the atmospheres of Tonantzintla 1 stars are Oxygen, Carbon, Nitrogen, Magnesium, and Aluminum, while Cerium is among the elements heavier than iron that were identified in the atmospheres of Tonantzintla 1 stars.  

The research has revealed that the iron content in the stars of Tonantzintla 1 is about six times lower than the iron content of the Sun, but is similar to the iron content identified in other ancient globular clusters in the inner regions of our Galaxy. 

But beyond iron, astronomers noticed that other chemical elements in Tonantzintla 1 show an atypical trend that distinguishes it from other globular clusters. The heavy element cerium was found to be correlated with the light elements nitrogen and aluminum, indicating a complex chemical evolutionary history in Tonantzintla 1. The discovery of these trends between light and heavy elements suggests that different episodes of star formation or rare astrophysical processes of stellar nucleosynthesis, such as massive stars with high rotation, evolved stars, among others, could have controlled the complex history of chemical evolution in ancient star clusters such as Tonantzintla 1. 

The team was led by José G. Fernández-Trincado, an assistant professor at the Universidad Católica del Norte, and included collaborators in several countries, including the U.S., Chile, Mexico, and Brazil. The results of the research have been published in the prestigious U.S. journal The Astrophysical Journal Letters:

This project is part of the Sloan Digital Sky Survey (SDSS for short), a large-scale project, of which the Universidad Católica del Norte is a partner along with about forty institutions around the world, and whose goal is to study the history of stars and galaxies in a considerable fraction of the sky. 

Panoramic image of the Milky Way showing the location of the globular cluster Tonantzintla 1 towards the inner region of our Galaxy. Image Credits: NASA/ESA/Hubble/E.Noyola (Tonantzintla 1/NGC 6380) and ESA/Gaia/DPAC, CC BY-SA 3.0 IGO (Milky Way panoramic image).

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