By Francesco Mauro, researcher of MAS and Universidad Católica del Norte´s Institute of Astronomy

Stellar clusters are groups of stars born approximately all-together from the same molecular cloud, enormous clouds of cool gas that live within galaxies. As they age, gravity keeps them bound forming a clear isolated system. There two main types: open and globular clusters. Globular cluster are typically older and more compact. Galactic globular clusters have approximately the same age of the Galaxy itself. For this reason they are used to study its formation and evolution.

Astronomers are able to measure the intrinsic brightness of stars at a given wavelength and call it “luminosity”. They call “color” the difference in luminosity between two different wavelength (the reason is simple: a blue star is more luminous in the blue part of the spectrum than a red one, and vice-versa). A plot of the stars’ luminosity against their color is a powerful analysis tool called Color-Luminosity Diagram (or Color-Magnitude diagram if using magnitudes instead of luminosities). For the stars members of a cluster, the graph results in a very distinct group of “lines”. The two brighter parts are the red giant branch (stars that have finished burning hidrogen in the core and are preparing to burn helium), and the horizontal branch (stars that are are already burning helium in their cores).

NGC6440 is an interesting globular cluster, located in the inner part of the Galactic bulge, that seems to have two horizontal branches, located a two different luminosities. This is an unusual feature, which is present only in two other Galactic globular clusters. A slightly not-uniform stellar chemical composition among the stars, probably due to a inhomogeneity in its stellar formation, could be an explanation.

To test this hypothesis, we selected seven stars of the red giant branch to determine their precise chemical composition through the analysis of their spectra, a piece of information that will clarify the history of the cluster environment and its formation.

The results show uniform values for the contents of iron group of chemical elements (made up of iron and other atoms with similar atomic number). In other words: no internal spread of iron group elements within the cluster. We did find a significant spread in the contents of sodium and aluminum, but no sign of the typical inverse proportionality between sodium and oxygen, or aluminum and manganese, which is generally present in the Galactic globular clusters. The abundance of chemical elements formed using helium nuclei as building blocks is similar to that of typical stars in the Milky Way bulge but higher than those of similar globular clusters.

The similarity of overall chemical composition of the cluster and Galactic bulge stars suggests a common origin. The contents of sodium, aluminum, manganese, and iron group elements is determined by different types of supernovae explosions within the parent cloud. The uncovered pattern of small differences and general similarities in chemical composition suggests that NGC6440 formed at the very beginning of the bulge history, in a region of a molecular cloud that had previously undergone a high supernova activity.

Color-magnitude diagram of NGC 6440 with data from the VVV survey. The stars selected for our spectroscopic study are marked by blue circles.