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Building up the Milky Way Halo in the era of multiple stellar populations

Periodic Reporting for period 1 - Global-assembly (Building up the Milky Way Halo in the era of multiple stellar populations)

Reporting period: 2018-10-01 to 2020-09-30

"This project aims to advance our knowledge of the most ancient stellar systems in the Milky Way, the globular clusters (GCs). The study of the newly discovered generations of stars in GCs is an active branch of stellar and Galactic astrophysics. Global-assembly investigates this phenomenon, to address some of the fundamental issues: What is the nature of the first stars? How do galaxies evolve across cosmic time? How are elements produced by stars, recycled through galaxies, eventually producing the elements for life itself?

The outcome of this research will help us to advance in a vast range of fields, from cosmology to stellar physics, i.e.the contribution of GCs to the Galactic Halo, star formation in dense ancient stellar systems, the role of GCs to the cosmic reionization. Understanding the origin of stellar populations in GCs is a necessary step to constrain stellar evolution and nucleosynthesis, i.e. the production of the chemical elements in stars, then recycled into galaxies, and eventually producing the matter for life itself.

The objective is a detailed analysis of the multiple populations in star clusters to shed light on their origin. This analysis includes: 1) The chemical tagging of all the sub-populations observed through the “chromosome maps”, a photometric diagram which is a powerful population diagnostic unique to each GC; 2) The investigation of possible chemical variations in first population stars, crucial to constrain the GCs’ mass-loss into the field; 3) A survey of the anomalous GCs, i.e. GCs with a complex star-formation, which might be associated with former dwarf galaxies; 4) The construction of grids of synthetic spectra with the chemical composition typical of the different populations, useful for various purposes, like the determination of He variations in a given GC, and for the construction of more realistic isochrone databases; 5) Multiple populations in young clusters in the Magellanic Clouds to understand if they are the counterpart of old Milky Way GCs; 6) Kinematics of different populations combining radial velocities from spectroscopy and proper motions from Gaia.

Global-assembly has exploited chromosome maps to study the elemental abundances characterizing each stellar population, providing the first and most complete “chemical key"" to read these maps. It has been shown that the GCs with specific morphological features on the map, have an “anomalous” chemical composition with internal variations in iron. The project has provided evidence for chemical variations in the overall metallicity among stars associated with the main first populations. Small internal variations in helium have been found in younger clusters, with age of ~6-11 Gyr. The analysis of kinematic properties has shown for the first time a distinct overall rotation of different stellar populations in a GC."
"Global-assembly has pursued an analysis of chemical abundances in the GC systems, as well as in the metal-poor field stars observed in the Galactic Halo and in the Bulge. This project has resulted in 27 peer-reviewed papers, a successful proposal, with me as Principal Investigator, and several other proposals where I am co-Investigator.

As a major result, I have provided a first chemical atlas of the stellar populations observed along the GC chromosome maps. Despite the high level of heterogeneity, my work has introduced a ""Universal chromosome map"", represented in Figure 1, which highlights the common properties in the multiple population phenomenon in different GCs. Specifically: 1) two main peaks in the star density can be identified at the same location on the Universal map, one corresponding to the first population and the other to the second population; 2) the separation among these main populations occurs at the same level of the map in all GCs; 3) the first population in typically elongated suggesting a chemical inhomogeneity.

I have investigated two of the most intriguing and critical aspects revealed by the maps, namely the presence of two main morphologies of ""chromosome maps"", Type-I (the most common in the Milky Way) and Type-II maps, and the apparent lack of chemical homogeneity among stars that were commonly associated with a single stellar population, the first population stars. My work has demonstrated that all the Type-II GCs have internal variations in the heavy elements including iron, which is unusual for relatively-low mass stellar systems like GCs. Surprisingly, I have demonstrated that the elongation on the map observed in the first population is associated with internal variations in the overall metallicity. The challenge is now to understand how these variations originated in the first hundreds million years of the history of our Universe. To understand this issue I have been awarded of telescope time at the Very Large Telescope of the European Southern Observatory.

Going to the external galaxies, the analysis of GCs younger than typical Milky Way GCs has revealed that these objects, similarly to old GCs, display internal variations in He, although the level of enhancement is small. This result suggests that the typical chemical variations associated with the classical multiple populations phenomenon are not confined to a formation in the high redshift Universe.
A new line of research of this project has been the investigation of the kinematics of individual populations in GCs. By exploiting data from the Gaia satellite different populations have resulted to display various degrees of kinematic anysotropy, and one GC has been found with hints of different rotation in its two stellar populations.

I have co-supervised the work of one bachelor and one master thesis student (E. Ventura, E. Dondoglio), and I am currently supervising two PhD students (G. Cordoni, E. Dondoglio).
For the dissemination activities I have presented my work at the MODEST conference organized by the International Astronomical Union (May, 2019, Bologna - Italy), where I have given an invited talk; I have organized a workshop at the European Week of Astronomy (https://eas.unige.ch/EWASS2019/session.jsp?id=SS23) and I have been invited to give a seminar at the University of Sharjah (UAE).
As part of the outreach and public engagement activities I have participated the European Researchers’ Night, and I have given lectures of basic astronomy in primary schools for science week."
Global-assembly has established that GCs have a complex stellar population pattern. Beyond the Type II GCs, many other GCs, once considered mono-metallic, show small variations in metallicity. This discovery is a big step forward with respect to the state of the art in the field and has an impact not only in the GC research, but also in other fields, such as the star formation in the early Universe. Either the molecular cloud out of which GC stars formed was not chemically homogeneous, i.e. was not fully mixed, or that part of the stars formed while the first supernovae had already started to enrich the inter-stellar medium of the nascent GC. These options can provide us with new hints to understand the process of GC formation, its timescale, and help identifying the class of stars responsible for the further enrichment of the GC stars.

Overall, the use of advanced facilities for this project, such as the 8-10m telescopes, has guaranteed parallel technological development for top level research and the return of investment made to that development.
Fig.1: Two versions of the Universal chromosome map of Milky Way GCs.