Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419

Recent work revealed that both the helium variation within globular clusters (GCs) and the relative numbers of first and second-generation stars (1G, 2G) depend on the mass of the host cluster. Precise determination of the internal helium variations and of the fraction of 1G stars are crucial constraints to the formation scenarios of multiple populations (MPs). We exploit multi-band Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one of the most-massive and distant GCs of the Galaxy, almost isolated from its tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the observed colors of these four populations with the colors derived from appropriate synthetic spectra to infer the relative helium abundances. We find that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY ~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium (Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most of the current scenarios for MPs. Furthermore, the relatively larger fraction of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting literature results, we find that the fractions of 1G stars of GCs with large perigalactic distance are typically higher than in the other GCs with similar masses. This suggests that NGC 2419, similarly to other distant GCs, lost a lower fraction of 1G stars.Comment: 10 pages, 8 figures, submitted to MNRAS January 22n

Extended main sequence turnoff as a common feature of Milky Way open clusters

We present photometric analysis of twelve Galactic open clusters and show that the same multiple-population phenomenon observed in Magellanic Clouds (MCs) is present in nearby open clusters. Nearly all the clusters younger than $\sim$2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and all the cluster younger than $\sim$700 Myr show broadened/split main sequences (MSs). High-resolution spectroscopy has revealed that these clusters host stars with a large spread in the observed projected rotations. In addition to rotation, internal age variation is indicated as a possible responsible for the eMSTOs, making these systems the possible young counterparts of globular clusters with multiple populations. Recent work has shown that the eMSTO+broadened MSs are not a peculiarity of MCs clusters. Similar photometric features have been discovered in a few Galactic open clusters, challenging the idea that the color-magnitude diagrams (CMDs) of these systems are similar to single isochrones and opening new windows to explore the eMSTO phenomenon. We exploit photometry+proper motions from Gaia DR2 to investigate the CMDs of open clusters younger than $\sim$1.5 Gyr. Our analysis suggests that: (i) twelve open clusters show eMSTOs and/or broadened MSs, that cannot be due neither to field contamination, nor binaries; (ii) split/broadened MSs are observed in clusters younger than $\sim$700 Myr, while older objects display only an eMSTO, similarly to MCs clusters; (iii) the eMSTO, if interpreted as a pure age spread, increases with age, following the relation observed in MCs clusters and demonstrating that rotation is the responsible for this phenomenon.Comment: 17 pages, 42 figures, 1 table, accepted for publication in ApJ (31/10/2018

Multiple stellar populations in Magellanic Cloud clusters. V. The split main sequence of the young cluster NGC1866

One of the most unexpected results in the field of stellar populations of the last few years, is the discovery that some Magellanic-Cloud globular clusters younger than ~400 Myr, exhibit bimodal main sequences (MSs) in their color-magnitude diagrams (CMDs). Moreover, these young clusters host an extended main sequence turn off (eMSTO) in close analogy with what is observed in most ~1-2 Gyr old clusters of both Magellanic Clouds. We use high-precision Hubble-Space-Telescope photometry to study the young star cluster NGC1866 in the Large Magellanic Cloud. We discover an eMSTO and a split MS. The analysis of the CMD reveals that (i) the blue MS is the less populous one, hosting about one-third of the total number of MS stars; (ii) red-MS stars are more centrally concentrated than blue-MS stars; (iii) the fraction of blue-MS stars with respect to the total number of MS stars drops by a factor of ~2 in the upper MS with F814W <~19.7. The comparison between the observed CMDs and stellar models reveals that the observations are consistent with ~200 Myr old highly-rotating stars on the red-MS, with rotation close to critical value, plus a non-rotating stellar population spanning an age interval between ~140 and 220 Myr, on the blue-MS. Noticeable, neither stellar populations with different ages only, nor coeval stellar models with different rotation rates, properly reproduce the observed split MS and eMSTO. We discuss these results in the context of the eMSTO and multiple MS phenomenon.Comment: 11 pages, 11 figures, accepted for publication in MNRA

The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XIX. A Chemical Tagging of the Multiple Stellar Populations Over the Chromosome Maps

The HST UV Survey of Globular Clusters (GCs) has investigated GCs and their stellar populations. In previous papers of this series we have introduced a pseudo two-color diagram, "chromosome map" (ChM), that maximises the separation between the multiple populations. We have identified two main classes of GCs: Type I (~83% of the objects) and Type II, both hosting two main groups of stars, referred to in this series as first (1G) and second generation (2G). Type II clusters exhibit two or more parallel sequences of 1G and 2G stars in their ChMs. We exploit elemental abundances from literature to assign the chemical composition to the distinct populations as identified on the ChMs of 29 GCs. We find that stars in different regions of the ChM have different composition: 1G stars share the same light-element content as field stars, while 2G stars are enhanced in N, Na and depleted in O. Stars enhanced in Al and depleted in Mg populate the extreme regions of the ChM. We investigate the color spread among 1G stars observed in many GCs, and find no evidence for variations in light elements, whereas either a 0.1 dex Fe spread or a variation in He remain to be verified. In the attempt of analysing the global properties of the multiple populations, we have constructed a universal ChM, which highlights that, though variegate, the phenomenon has some common pattern. The universal ChM reveals a tight connection with Na, for which we have provided an empirical relation. The additional ChM sequences typical of Type II GCs are enhanced in metallicity and, often, in s elements. Omega Cen can be classified as an extreme Type II GC, with a ChM displaying three main streams, each with its own variations in chemical abundances. One of the most noticeable differences is between the lower and upper streams, with the latter (associated with higher He) having higher Fe and lower Li. We publicly release ChMs.Comment: 35 pages, 28 figures, 3 tables. Submitted to MNRA

A new visual-near-infrared diagnostic to estimate the metallicity of cluster and field dwarf stars

We present a theoretical calibration of a new metallicity diagnostic based on the Strömgren index m1 and on visual-near-infrared (NIR) colors to estimate the global metal abundance of cluster and field dwarf stars. To perform the metallicity calibratio

The temperature distribution of horizontal branch stars: methods and first results

As part of a large project aimed at characterizing the ultraviolet (UV) properties of globular clusters, we present here a theoretical and observational analysis aimed at setting the framework for the determination of horizontal branch (HB) temperature distributions. Indeed this is a crucial information to understand the physical parameters shaping the HB morphology in globular clusters and to interpret the UV emission from unresolved stellar systems. We found that the use of zero age HB color-Teff relations is a robust way to derive effective temperatures of individual HB stars. We investigated the most suitable colors for temperature estimates, and the effect on the color-Teff relations of variations of the initial chemical composition, and of the evolution off the zero age horizontal branch. As a test case, we applied our color-Teff calibrations to the Galactic globular cluster M15. The photometry of M15 has been obtained with the Wide Field and Planetary Camera 2 on board the HST. The HB of M15 turned out to have a multimodal distribution, with a main component peaked at Teff~8,000 K and confined below Teff~10,000 K. The second component is peaked at Teff~14,000 K and extends up to Teff~20,000 K. The vast majority (~95%) of the HB stars in M15 is below 20,000 K, in agreement with the lack of a well populated extreme HB observed in other metal-poor globular clusters. We also verified that the temperatures derived with our analysis are consistent with spectroscopic estimates available in the literature.Comment: 13 pages, 13 figures. ApJ accepte

Mass loss of different stellar populations in Globular Clusters: the case of M4

In a Globular Cluster (GC), the mass loss during the red-giant branch (RGB) phase and the helium content are fundamental ingredients to constrain the horizontal branch (HB) morphology. While many papers have been dedicated to the helium abundance in the different stellar populations, small efforts have been done to disentangle the effects of mass loss and helium content. We exploit the nearby GC NGC6121 (M4), which hosts two well-studied main stellar populations, to infer both helium and RGB mass loss. We combine multi-band Hubble Space Telescope photometry of RGB and main sequence (MS) stars of M4 with synthetic spectra to constrain the relative helium content of its stellar populations. We find that the second generation stars in M4 is enhanced in helium mass fraction by $\rm \Delta Y = 0.013 \pm 0.002$ with respect to the remaining stars that have pristine helium content. We then infer the mass of the HB stars by searching for the best match between the observations and HB populations modelled assuming the helium abundance of each population estimated from the MS. By comparing the masses of stars along the HB, we constrain the mass loss of first- and second-generation stars in M4. We find that the mass lost by the helium enriched population is $\sim 13$% larger than the mass lost by the first generation stars ($\rm \Delta \mu = 0.027 \pm 0.006 \ M_\odot$). We discuss the possibility that this mass loss difference depends on helium abundance, the different formation environment of the two generations, or a combination of both.Comment: 10 pages, 4 figures, 1 table. Accepted for publication in Ap