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The formation of the Galaxy: constraints from globular clusters

Periodic Reporting for period 2 - GALFOR (The formation of the Galaxy: constraints from globular clusters)

Reporting period: 2019-04-01 to 2020-09-30

For decades Globular Clusters (GCs), which are spheroidal systems composed of ancient stars, were considered prototypes of simple populations of stars with the same metallicity and the same helium content.
The unexpected discovery that their photometric diagrams are composed of distinct sequences of stars has recently shattered this traditional idea and demonstrated that GCs are complex stellar systems. As an example, the diagram of Figure 1, which is obtained from extremely-deep observations collected with the Hubble Space Telescope (adapted from Milone et al. 2019, MNRAS, 484, 4046), shows that the GC NGC6752 exhibits three stellar sequences in the color-luminosity plane.
Each sequence corresponds to a distinct stellar population with different chemical content. Specifically, all GCs host a first population of stars with the same chemical composition as stars in the halo of the Galaxy, and second stellar populations with extreme chemical compositions that are not observed in any other place of the Universe.

The origin of multiple stellar populations, occurred at high redshift, is one of the main open issue of modern stellar astrophysics.
Some authors suggested that first and second-population stars correspond to distinct stellar generations with different ages and that second population stars formed from the ashes of more-massive first population stars. The nature of the polluted is still debated and asymptotic giant branch  (AGB) stars, fast-rotating massive stars (FRMSs) and supermassive stars are promising candidates.
These scenarios imply that globular clusters were much more massive at formation by a factor of ten or even more. These giant proto clusters may have lost the majority of their first population stars, thus providing a significant contribution to the formation of the Galactic halo. Clearly, such massive stellar systems that formed a few hundreds mega years after the Bing Bang may have provided a significant contribution to the reionization of the Universe.
As an alternative, all stars in the GCs are coeval, and multiple populations are the product of exotic phenomena that occurred in the unique environment of the proto GCs, possibly associated to mass transfer in binary systems or to the evolution of stars that are more than 1000 times more massive than the Sun.

Moreover, the discovery of a new class of globular clusters with internal variations of heavy elements has provided new and unexpected insight to the solution of the missing satellite problem, that is the lack of low-mass Milky-Way satellites compared with the predictions of the main cosmological models. Finally, the discovery that multiple populations have different helium contents may provide the opportunity to solve one of the main problems of stellar evolution that is the second parameter of the horizontal branch morphology.

Understanding the origin of multiple is crucial to enhance knowledge on the formation and the assembly of the Galaxy and on stellar evolution.
How did the globular clusters form in the early Universe?
What is their role in the assembly of the Galactic Halo?
Which is their contribution to the re-ionization of the Universe?

GALFOR is an ambitious project to address intriguing open questions.    To do this, we are using innovative methods of investigations and we are exploiting the unique dataset that the Hubble Space Telescope and the major ground-based facilities have collected for us to study the largest sample of Galactic and extra-Galactic star clusters never studied before.
In summary, we investigate the star clusters that we observe today to shed light on the series of events that occurred at high redshift and led to the formation of the globular clusters and of their multiple populations.
We have derived high precision multi-band photometry of 123 Galactic and extragalactic star clusters and corrected the stellar magnitudes for the effects of differential reddening to  provide a comprehensive characterization of the multiple stellar population phenomenon.  
Thanks to the Chromosome Map of Globular Clusters, which is the new photometric diagram introduced by the PI of the Galfor project, we have identified for the first time first and second population stars in a large sample of more than seventy GCs, thus providing the first atlas of multiple populations in Globular Clusters.
Thus, we started exploiting these catalogues, together with spectroscopic data and data from other facilities, to characterize stellar populations in terms of relative numbers of first-and second-population stars, chemical composition, spatial distribution, internal kinematics, binary fractions and mass functions. Such information is crucial to constraint the formation scenarios of multiple stellar populations and their contribution to the assembly of the Milky Way and the reionization of the Universe.

The chemical composition of the distinct stellar populations has been inferred by means of high-resolution spectroscopy of stars along the chromosome map (e.g. Marino et al. 2019), which provided detailed elemental abundances of bright stars in about thirty GCs. Moreover, we adopted a new technique based on the synergy of multi-band high-precision photometry, synthetic spectra and stellar models to infer the chemical compositions of thousands stars in all studied GCs. In particular, we have estimated the relative helium abundances (together with the abundances of C, N, O, Mg, Al) of the distinct stellar populations in more than seventy Galactic and extragalactic GCs. We find that the maximum helium abundance of second population stars ranges from almost the primordial value (Y=0.26) to more than 0.45 in helium mass fraction. These results  provide major challenges for the scenarios on the formation of multiple populations where either AGB stars or FRMSs are responsible for the chemical composition of the second populations.
The internal variation in helium and nitrogen correlate with the mass of the host cluster, with massive clusters hosting more-complex multiple-population patterns and extreme chemical compositions, when compared with low mass GCs. We have also detected a threshold in the initial mass that separates multiple population clusters from simple population ones. There is no evidence for correlation between the properties of multiple populations and cluster age. These facts provide crucial constraints on the formation scenarios.

The accurate estimates that we inferred for the helium content of a large sample of GCs have provided a major step towards the solution of the long held second parameter problem of the horizontal branch morphology. Indeed, the strong correlation between our helium determinations and the color extension of the horizontal branch demonstrates that helium is the main second parameter governing the HB shape. However, based on the comparison between the observations and simulated HBs that account for the helium abundances that we inferred from the main sequence and the red giant branch, helium alone does not entirely account for the color extension of the HB.
Driven by this result, we have developed a new method to disentangle between the effect of helium and mass loss on the HB. We have determined for the time accurate measurements of the mass loss of the distinct stellar populations of GCs and discovered that second-population stars are affected by larger amounts of mass loss, when compared with the first populations (Tailo, Milone et al. 2019). Results on a large sample of GCs indicate that mass loss together with helium variations entirely account for the HB morphology of GCs, thus providing the solution for the long held second parameter problem (Tailo, Milone et al. submitted to MNRAS).

The chromosome maps have allowed us to derive for the first time, the relative numbers of stars in the distinct stellar populations in a large sample of Galactic and extragalactic GCs. We find that the fraction of first population stars changes from one cluster to another and ranges from less than 10%, in Omega Centauri, which is the most massive GC of the Galaxy, to more than 60% in small-mass GCs. The fraction of first-population stars correlates with the mass of the host GC, thus challenging predictions from scenarios based on cooling flow. Intriguingly, we find that star clusters of the Large and the Small Magellanic Clouds, which have ages between 2 and 10 Gyr host larger fractions of first population stars than 12-13 Gyr old Galactic GCs (Milone et al. 2020). This finding is consistent with predictions from those scenarios where GCs have lost the majority of their first generations. We thus provide the first empirical evidence that first-population stars of GCs provided a significant contribution to the assembly of the Galactic halo.

One of the main challenges in understanding the origin of multiple populations, arises from the fact that most of the studies are based on Galactic and extragalactic GCs that formed at high redshift. Hence, the investigation of young stellar systems could provide an alternative approach to study multiple populations a few hundreds million years after their formation. In this context, the recent discovery of double main sequences and extended main-sequence turn off in Magellanic Cloud clusters provides a unique opportunity to investigate the multiple population phenomenon from a different perspective.
We used the Hubble Space Telescope to collect images of various young and intermediate age clusters in the Large and the Small Magellanic to investigate their stellar populations by mean of high precision photometry, and we collected high resolution spectra of one of them with the Very Large Telescope. Our survey with HST allowed us to fully characterize multiple sequences in the photometric diagrams of these young clusters and compare them with multiple populations in old globular clusters.
We find that multiple sequences in young clusters correspond to stellar populations with different rotation rates and confirmed this results by means of direct spectroscopic measurements of stare in the different main sequences and along the extended main-sequence turnoff of NGC1818. Moreover, we find that multiple sequences of young and old clusters have different properties in terms of radial distribution, mass fraction and population ratio (Milone et al. 2018).   We conclude that multiple populations in young Magellanic Cloud clusters and in old Galactic and extragalactic GCs are associated with different phenomena.
The findings from our ongoing research activity already include ground-breaking results in the context of stellar populations.

Our investigation on the helium content of the distinct stellar populations of GCs already provided a major advance in our understanding of multiple stellar populations and of stellar evolution.
Indeed, although helium is the second most-abundant element in the Universe and in stars, spectroscopic determination of helium abundances are feasible for few evolved HB stars of old stellar populations alone within a narrow range of effective temperature and gravity. For this reason, since a few years ago estimates of helium abundance of GCs were available for a few stars of five GCs alone.
The innovative approach adopted in the GALFOR project has allowed us to infer the helium content of stellar populations in all GCs with available chromosome maps or appropriate multi-band photometry.
The obtained results are based on RGB stars for the majority of the studied 70 clusters. By the end of the project we will also infer helium abundances from main sequence stars. We expect that the helium abundances obtained by combining results from the red-giant branch with the new findings will be unparalleled for decades.
Similarly, we plan to adapt to MS, red HB and AGB stars, the method to identify distinct stellar populations that we introduced for the red giant branch. Combined results will provide accurate determinations of the population ratio, thus enhancing our understanding on the role of mass, age, environment and host galaxy on the multiple population phenomenon. The comparison between the population ratios and the chemical compositions of stellar populations in Galactic and extragalactic clusters has provided direct evidence that Globular Clusters preferentially lose their mass (see Figure 2, adapted from Milone et al. 2020). Final results from the Galfor project will quantify this phenomenon and provide an estimate of the contribution from GC first population stars to the mass of the halo.

As discussed in the previous section, our homogeneous determination of helium abundances in a large sample of clusters already provided a major step towards the understanding of the multiple population phenomenon and seriously challenged some of the main formation scenarios. Moreover, we have provided firm empirical evidence that helium is a second parameter of the HB morphology.
Our helium determinations are now used as input parameters to derive the mass loss of the distinct stellar populations of a large sample of GCs, as we did in the pioneering paper on the GC M4 by Tailo, Milone et al. (2019). Results on all monometallic GCs with multiple populations, which are presented in a paper submitted to MNRAS, clearly show that mass loss togher with helium determine the HB shape, providing the solution for the long-held second parameter problem. We are confident that results will be published by the end of the project.

Our ongoing survey of Magellanic Cloud clusters allowed us to understand the physical mechanism that is responsible for the occurrence of multiple populations in young and intermediate clusters and provided direct evidence that the multiple populations in young and old clusters are different phenomena. These findings represent a major step towards the understanding of multiple populations and stellar evolution. Moreover, we discovered that multiple sequences and extended turn off are not peculiarity of massive Magellanic Cloud clusters but are common features of Galactic open clusters (Cordoni et al. 2019, Marino et al. 2019). This unexpected result, which is in contrast with the traditional picture that the color-magnitude diagrams of open clusters are consistent with simple isochrones, have provided a new picture of these objects. Results on Magellanic Cloud Clusters are currently based on about 20 objects. We are currently investigating a sample of 120 clusters of both Magellanic Clouds to fully characterize the phenomenon of multiple populations in clusters in a wide range of ages, masses, and environments. The resulting catalogs, together with those of Galactic clusters, will be publicly released thus providing high-precision photometry and reddening maps of a large sample of more than 250 Galactic and extragalactic clusters. We expect that these catalogs will be instrumental for studies on stellar populations of the next decades.
Moreover, we have been awarded observational time with the Hubble Space Telescope to get deep images of the Magellanic Cloud cluster NGC1818 (PI Giacomo Cordoni). This project is based on a feature of the cluster, the Turn On, which has never been used  in the context of multiple populations and that will allow us to understand whether multiple stellar populations with different rotation rates are coeval or exhibit different ages as suggested for multiple populations in old Globular Clusters.

The evidence that second-population stars formed in a very-dense environment is another intriguing early finding from this project. This conclusion, which provides crucial constraints for the formation scenarios, comes from the distribution of multiple population stars among binaries (see Figure 3) and from the discovery that second-population stars lose more-mass than the first population along the RGB phase.
We pushed the investigation of multiple populations in the regime of very low mass stars. First results are based on very deep near-infrared observations of the GC NGC6752, where we discovered three main populations among faint M-dwarfs (Milone et al. 2019). We measured for the first time the relative mass functions of the three stellar populations over a wide range of stellar masses. Moreover, by extending our method to the low mass regime, we inferred the chemical composition of the distinct stellar populations at the bottom of the MS. The discovery that the properties of the three stellar populations do not depend on stellar mass roles out the scenarios where second population stars formed from accretion of polluted material, unless the amount of accreted material is proportional to stellar mass. We are currently investigating deep archive images of additional clusters and we have been awarded HST observational time to get deep images of some Galactic GCs. The resulting data set will allow us to investigate the low stellar-mass regime in clusters with different properties and allow us to understand whether the conclusion that we obtained in the pioneering work on NGC6752 can be extended to other classes of globular 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.