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Register a new userExcess of Ca (and Sc) produced in globular cluster multiple populations: a first census in 77 Galactic globular clusters
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Eugenio Carretta
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Multiple stellar populations in globular clusters (GCs) are distinct by their different abundances of light elements. The abundance anti-correlations point towards a nucleosynthesis origin due to high-temperature H burning, but it remains to be assessed which type of stars altered primordial abundances in GCs. In particular, the regime at very high temperature that shapes the variations in potassium as well as calcium and scandium, which has been detected in a few notable cases such as NGC 2419 and NGC 2808, is still poorly explored. We started a systematic search for excess of Ca (and Sc) in GC stars with respect to the level of unmodified field stars. Statistically robust evidence of such excess was found in a small number of GCs (NGC 4833, NGC 6715, NGC 6402, NGC 5296, NGC 5824, and NGC 5139/omega Centauri) that join the previously known two clusters. For the first time we show that NGC 4833 is likely to host anti-correlated K and Mg abundances. All these GCs are among the most massive ones in the Galaxy. We found that the fraction of stars with Ca enhancement at 3sigma above the field star distribution is a multivariate function of the GC mass and metallicity, as in other manifestations of the multiple population phenomenon in GCs. We argue that these alterations in only a few GCs can be reproduced by two different channels: either a class of ordinary stars, that is common to all GCs, acts only in particular environments, or an on-off mechanism is generated by the occurrence of a peculiar type of stars (or lack of such stars). Hot bottom burning in asymptotic giant branch stars in the low-metallicity regime is a good candidate for the first class. Alternatively, a metallicity dependence is also expected for supermassive stars, which are predicted to preferentially form in massive GCs. (abridged).
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We present a new photometric catalogue of the rich globular cluster (GC) system around M87, the brightest cluster galaxy in Virgo. Using archival Next Generation Virgo cluster Survey (NGVS) images in the ugriz bands, observed with CFHT/MegaPrime, we perform a careful subtraction of the galaxys halo light in order to detect objects at small galactocentric radii as well as in the wider field, and find 17620 GC candidates over a radius range from 1.3 kpc to 445 kpc with g < 24 magnitudes. By inferring their colour, radial and magnitude distributions in a Bayesian way, we find that they are well described as a mixture of two GC populations and two distinct contaminant populations, but confirm earlier findings of radius-dependent colour gradients in both GC populations. This is consistent with a picture in which the more enriched GCs reside deeper in the galaxys potential well, indicating a role for dissipative collapse in the formation of both the red and the blue GCs.
We have calculated synthetic spectra for typical chemical element mixtures (i.e., a standard alpha-enhanced distribution, and distributions displaying CN and ONa anticorrelations) found in the various subpopulations harboured by Galactic globular clusters. From the spectra we have determined bolometric corrections to the standard Johnson-Cousins and Stroemgren filters, and finally predicted colours. These bolometric corrections and colour-transformations, coupled to our theoretical isochrones with the appropriate chemical composition, provide a complete and self-consistent set of theoretical predictions for the effect of abundance variations on the observed cluster CMD. CNO abundance variations affect mainly wavelengths shorter than 400 nm, due to the arise of molecular absorption bands in cooler atmospheres. As a consequence, colour and magnitude changes are largest in the blue filters, independently of using broad or intermediate bandpasses. Colour-magnitude diagrams involving uvy and UB filters (and their various possible colour combinations) are thus the ones best suited to infer photometrically the presence of multiple stellar generations in individual clusters. They are particularly sensitive to variations in the N abundance, with the largest variations affecting the Red Giant Branch (RGB) and lower Main Sequence (MS). BVI diagrams are expected to display multiple sequences only if the different populations are characterized by variations of the C+N+O sum and helium abundance, that lead to changes in luminosity and effective temperature, but leave the flux distribution above 400 nm practically unaffected. A variation of just the helium abundance, up to the level we investigate here, affects exclusively the interior structure of stars, and is largely irrelevant for the atmospheric structure and the resulting flux distribution in the whole wavelength range spanned by our analysis.
Evidence that the multiple populations (MPs) are common properties of globular clusters (GCs) is accumulated over the past decades from clusters in the Milky Way and in its satellites. This finding has revived GC research, and suggested that their formation at high redshift must have been a much-more complex phenomenon than imagined before. However, most information on MPs is limited to nearby GCs. The main limitation is that most studies on MPs rely on resolved stars, facing a major challenge to investigate the MP phenomenon in distant galaxies. Here we search for integrated colors of old GCs that are sensitive to the multiple-population phenomenon. To do this, we exploit integrated magnitudes of simulated GCs with MPs, and multi-band Hubble Space Telescope photometry of 56 Galactic GCs, where MPs are widely studied, and characterized as part of the UV Legacy Survey of Galactic GCs. We find that both integrated $C_{rm F275W,F336W,F438W}$ and $m_{rm F275W}-m_{rm F814W}$ colors strongly correlate with the iron abundance of the host GC. In second order, the pseudo two-color diagram built with these integrated colors is sensitive to the MP phenomenon. In particular, once removed the dependence from cluster metallicity, the color residuals depend on the maximum internal helium variation within GCs and on the fraction of second-generation stars. This diagram, which we define here for Galactic GCs, has the potential of detecting and characterizing MPs from integrated photometry of old GCs, thus providing the possibility to extend their investigation outside the Local Group.
Since the discovery of chemically peculiar stars in globular clusters in the last century, the study of multiple populations has become increasingly important, given that chemical inhomogeneity is found in almost all globular clusters. Despite various proposed theories attempting to explain this phenomenon, fitting all the observational evidence in globular clusters with one single theory remains notoriously difficult and currently unsuccessful. In order to improve existing models and motivate new ones, we are observing globular clusters at critical conditions, e.g., metal-rich end, metal-poor end, and low mass end. In this paper, we present our first attempt to investigate multiple populations in low mass globular clusters. We obtained low-resolution spectra around 4000 A of 30 members of the globular cluster Palomar 13 using OSIRIS/Multi-object spectrograph mounted at the Gran Telescopio Canarias. The membership of red giant branch stars is confirmed by the latest proper motions from Gaia DR2 and literature velocities. After comparing the measured CN and CH spectral indices with those of the stellar models, we found a clear sign of nitrogen variation among the red giant branch stars. Palomar 13 may be the lowest mass globular cluster showing multiple populations.
Multiple stellar populations (MPs) are a distinct characteristic of Globular Clusters (GCs). Their general properties have been widely studied among main sequence, red giant branch (RGB) and horizontal branch (HB) stars, but a common framework is still missing at later evolutionary stages. We studied the MP phenomenon along the AGB sequences in 58 GCs, observed with the Hubble Space Telescope in ultraviolet (UV) and optical filters. By using UV-optical color-magnitude diagrams, we selected the AGB members of each cluster and identified the AGB candidates of the metal-enhanced population in type II GCs. We studied the photometric properties of AGB stars and compared them to theoretical models derived from synthetic spectra analysis. We observe the following features: i) the spread of AGB stars in photometric indices sensitive to variations of light-elements and helium is typically larger than that expected from photometric errors; ii) the fraction of metal-enhanced stars in the AGB is lower than in the RGB in most of the type II GCs; iii) the fraction of 1G stars derived from the chromosome map of AGB stars in 15 GCs is larger than that of RGB stars; v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent a clear evidence of the presence of MPs along the AGB of Galactic GCs and indicate that a significant fraction of helium-enriched stars, which have lower mass in the HB, does not evolve to the AGB phase, leaving the HB sequence towards higher effective temperatures, as predicted by the AGB-manque scenario.
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