Clusters twins, with different DNA
Gaia -ESO, the public spectroscopic survey carried out in one of the four telescopes at ESO VLT, has successfully completed its first two years of observations, for a total of 135 nights on the 300 planned. More than 5,000 high-resolution stellar spectra have been made available since last November to the international scientific community.
Gaia -ESO, the public spectroscopic survey carried out in one of the four telescopes at ESO VLT, led by Sofia Randich INAF – Arcetri and Gerry Gilmore (IoA, Cambridge), and has successfully completed its first two years of observations, for a total of 135 nights, on the 300 planned. Many achievements in this initial period, among which the first delivery of particular relevance to ESO “products” of the Survey, more than 5,000 high-resolution stellar spectra have been made available since last November to the international scientific community. The INAF contribution to the achievement of these objectives is very significant because researchers in Arcetri, Bologna, Catania and Palermo coordinate critical activities within the project and, more generally, about 40 people in 7 institutions (Capodimonte, Padua and Turin, in addition to those already mentioned) are actively involved in the survey.
In parallel with the release of the spectra at ESO, has begun the phase of scientific exploitation inside the consortium, which has led to the achievement of the first scientific results, many of which are headed by an Italian. In particular, the first two articles of the consortium to be accepted by the journal Astronomy & Astrophysics were coordinated by Paolo Donati (INAF, Bologna Observatory) and Laura Magrini (INAF, Arcetri Observatory).
In the first case, the Gaia -ESO data have allowed us to determine very accurately and safely parameters (age, distance, extinction) of Trumpler 20, one of the first open clusters observed by the survey; another, has reconstructed the distribution of abundances of chemical elements of three open clusters, including the Trumpler 20, located in the inner part of our Galaxy.
Open clusters are groups of stars held together by mutual gravitational attraction, born from the same cloud, and then having the same age and chemical composition. Open clusters reside in the thin disk of the Milky Way and are very important tracers of the spatial distribution of chemical elements in the disk and of their evolution over time. Each open cluster has, in fact, a pattern of abundances (a bit like every cell has a DNA sequence), which is the imprint “fossil” of the composition of the interstellar medium from which the cluster was formed.
Open clusters are among the main targets of the Gaia -ESO Survey that will keep more than 50. During the first six months were obtained spectra of three clusters (NGC 6705, NGC 4815 and Trumpler 20), very important because they are relatively close to the Galactic Center, i.e., in a hostile environment, where it is unlikely that a cluster survives for a long time, due to of the numerous collisions with molecular clouds. These three items are therefore probably part of what remains of a population of massive clusters, survived thanks to their mass and an orbit that allowed them to avoid collisions.
The three open clusters have very similar properties, in particular, are located approximately the same distance from the Galactic Center and they are all much younger than the Sun. The predictions of the models commonly used to study the formation and chemical evolution of the Milky Way, probably too simplified indicate that they should also share the same distribution of chemical elements, the same “DNA”.
The team coordinated by Magrini has rather surprisingly revealed diversity in the “DNA” of the three clusters. In other words, the quality of the spectra obtained from the Gaia -ESO Survey has allowed us to reveal differences in the distribution of chemical elements, which is quite distinct for each of the three objects. This result on the one hand indicates that the clusters were formed in different parts of the Milky Way and have subsequently moved from the place of birth to their current position on the other, suggests that the process of evolution of the thin disk is much more complex than expected.