The formation of the Galaxy: constraints from globular clusters

Project Information

GALFOR

Grant agreement ID: 716082

DOI

10.3030/716082

Project closed

EC signature date 1 December 2016

Start date 1 October 2017

End date 30 September 2022

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 717 246,00

EU contribution

€ 717 246,00

Coordinated by

UNIVERSITA DEGLI STUDI DI PADOVA
Italy

Periodic Reporting for period 4 - GALFOR (The formation of the Galaxy: constraints from globular clusters)

Reporting period: 2022-04-01 to 2022-09-30

Summary of the context and overall objectives of the project

For decades Globular Clusters (GCs), which are spheroidal systems composed of ancient stars, were considered prototypes of simple stellar populations. The unexpected discovery that their photometric diagrams are composed of distinct stellar sequences has recently shattered this traditional idea and demonstrated that GCs are complex stellar systems (Fig. 1, Milone et al. 2019). Each sequence corresponds to a distinct stellar population with different chemical content: a first population of stars (1P) with the same chemical composition as stars in the Galactic halo, and one or more second populations (2p) with a bizarre chemical composition.

The origin of multiple populations (MPs) is one of the main open issues of stellar astrophysics. Some scenarios suggest that MPs correspond to distinct stellar generations with different ages and that 2P stars formed from the ashes of more-massive 1P stars. GCs were much more massive at the formation and lost the majority of their 1P, thus providing a significant contribution to the formation of the Galactic halo and to the reionization of the Universe. As an alternative, all GC stars are coeval, and MPs are the products of exotic phenomena that occurred in the proto-GCs.
Understanding the origin of MPs is crucial to enhance knowledge of the formation and assembly of the Galaxy. The MPs would shed light on star formation and evolution in dense environments and possibly address some open long-standing problems of GC studies such as the second parameter phenomenon of the horizontal-branch (HB) morphology.

GALFOR is a project to analyze the largest sample of star clusters studied so far, by using innovative methods and unprecedented datasets. It studies the clusters that we observe today to shed light on the events that occurred at high redshift and led to the GC formation.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Based on data from the Hubble Space Telescope (HST) and from ground-based facilities, we derived high-precision multi-band photometry of ~200 Galactic and extragalactic star clusters that span a wide range of ages and metallicities.
Our analysis comprises all images of Magellanic Cloud clusters collected with the modern cameras of HST and all images of Galactic GCs with UV observations.
We corrected the photometry for differential reddening and calculated proper motions when multi-epoch images are available.

We inspected the catalogs to identify the MPs by using appropriate diagrams that maximize the separation among the distinct populations. These include the pseudo-two-color diagram dubbed 'chromosome map' (ChM), introduced by the PI
We detected MPs in more than 70 GCs older than 2 Gyr and extended main-sequence turn-off (eMSTOs) in all clusters younger than 2Gyr.

The investigation of GCs includes:
- determination of the relative numbers of 1P and 2P stars
- determination of the chemical composition of the stellar population. To do this, we followed complementary approaches i) we combined information from high-resolution spectroscopy and the ChM to derive detailed abundances in 30 GCs ii) we adopted a technique introduced by our team based on the synergy of multi-band photometry, synthetic spectra, and stellar models to infer the abundances of He, C, N, O, Mg of MPs of all GCs (Milone et al. 2018a, 2020a,b; Lagioia et al. 2019a,b).
- Results on chemical composition allowed us to infer the amount of mass loss and derive a new mass-loss law.
- we developed a new technique, based on observations from HST, ground-based facilities, and the GAIA satellite, to measure the MP kinematics and constrain their spatial distributions (Milone et al. 2018b, Cordoni et al. 2020a,b, Dondoglio et al. 2021).
- we developed new methods to infer the frequency of binaries among the distinct stellar populations.
- we investigated MPs along the main evolutionary stages, from the tip of the red-giant branch to the upper main sequence (MS). We detected MPs among very low-mass stars and inferred their chemical composition and mass functions.
- we investigate the relations between the properties of the MPs, the main cluster parameters, the host galaxy, and the environment.

Our project comprises the study of more than 100 young clusters. Results include:
- we identified MPs in the photometric diagrams. We discovered that split MSs are present in all clusters younger than 800 Myr, whereas eMSTOs are common features in clusters younger than 2 Gyr.
- We provide direct evidence, by means of both photometry and high-resolution spectroscopy, that the split MS corresponds to stellar populations with different rotation rates (Milone et al. 2018c, Cordoni et al. 2018, Marino, Milone, et al. 2018a,b).
- We investigate the properties of MPs in young clusters and explored the relations with the stellar mass and the properties of the host cluster.
Results on stellar populations in young clusters and GCs are compared with each other to understand whether or not the young clusters are the counterparts of GCs with MPs.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The outcomes of the project provide a major step towards the solution of open issues of stellar astrophysics:
Most results come from the first homogeneous surveys of MPs in a large sample of GCs, which we derived from this project and allowed us to characterize the main MP properties with unprecedented precision. We provide four major results below and refer to the review by Milone & Marino (2022) and references for further results from the GALFOR project.

i) we estimated for the first time the He content of MPs in ~70 GCs (thus increasing by ten times the number of GCs with He determinations) and
ii) we discovered that the He variation correlates with GC mass and HB extension.
These discoveries allowed us to break the degeneracy between He and mass-loss, and infer for the first time the RGB mass loss for 1P and 2P stars. We demonstrate that for a fixed metallicity, the HB is explained in terms of He and mass loss. Hence, we provide a solution to the long-held second parameter problem of the HB (Milone et al. 2018, 2022, Tailo, Milone et al. 2020).

We find that the fraction of 1P stars depends on GC mass, cluster orbit, and host galaxy (e.g. Milone et al. 2020).
We discovered that the 1P stars are not chemically homogeneous and defined a new method to infer the metallicity distributions of the original clouds where MP formed. The implications are:
i) 2P stars formed in a very-dense environment; ii) GCs preferentially lose 1P stars iii) 1P stars were 6-10 times more massive than today at formation. These conclusions, which are corroborated by results on the spatial distribution of MPs, and on binaries among MPs, are consistent with the predictions from the multiple-generation scenarios.

We identified and characterized MPs along stars with different masses and evolutionary phases. The discovery that the main MP properties (e.g. mass function and chemical composition) do not depend on stellar mass rules out the accretion scenarios.

The survey of young clusters allowed us to discover that multiple sequences and eMSTOs are typical features of their CMDs. We discovered that open clusters are not consistent with simple isochrones thus shuttering out the long-held notion that they are simple objects. At odds with GCs, MPs in young clusters depend on stellar mass. We provide direct evidence that stellar rotation is the main responsible for MPs in young clusters. Hence, different physical reasons are responsible for MPs in GCs and young clusters.

Correlation between the fraction of first-population stars and the mass of the host cluster.

The inner core of NGC6352 imaged by Hubble (left) and artist impression of mixed binaries (right)

Color-magnitude diagram of the globular cluster NGC6752 from deep Hubble Space Telescope photometry.