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Register a new userHomogeneous Analysis of Globular Clusters from the APOGEE Survey with the BACCHUS Code. II. The Southern Clusters and Overview
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We investigate the Fe, C, N, O, Mg, Al, Si, K, Ca, Ce and Nd abundances of 2283 red giant stars in 31 globular clusters from high-resolution spectra observed in both the northern and southern hemisphere by the SDSS-IV APOGEE-2 survey. This unprecedented homogeneous dataset, largest to date, allows us to discuss the intrinsic Fe spread, the shape and statistics of Al-Mg and N-C anticorrelations as a function of cluster mass, luminosity, age and metallicity for all 31 clusters. We find that the Fe spread does not depend on these parameters within our uncertainties including cluster metallicity, contradicting earlier observations. We do not confirm the metallicity variations previously observed in M22 and NGC 1851. Some clusters show a bimodal Al distribution, while others exhibit a continuous distribution as has been previously reported in the literature. We confirm more than 2 populations in $omega$ Cen and NGC 6752, and find new ones in M79. We discuss the scatter of Al by implementing a correction to the standard chemical evolution of Al in the Milky Way. After correction, its dependence on cluster mass is increased suggesting that the extent of Al enrichment as a function of mass was suppressed before the correction. We observe a turnover in the Mg-Al anticorrelation at very low Mg in $omega$ Cen, similar to the pattern previously reported in M15 and M92. $omega$ Cen may also have a weak K-Mg anticorrelation, and if confirmed, it would be only the third cluster known to show such a pattern.
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We analyze a large sample of 885 GCs giants from the APOGEE survey. We used the Cannon results to separate the red giant branch and the asymptotic giant branch stars, not only allowing for a refinement of surface gravity from isochrones, but also providing an independent H-band spectroscopic method to distinguish stellar evolutionary status in clusters. We then use the BACCHUS code to derive metallicity, microturbulence, acroturbulence and many light-element abundances as well as the neutron-capture elements Nd and Ce for the first time from the APOGEE GCs data. Our independent analysis helped us to diagnose issues regarding the standard analysis of the APOGEE DR14 for low-metallicity GC stars. Furthermore, while we confirm most of the known correlations and anti-correlation trends (Na-O, Mg-Al,C-N), we discover that some stars within our most metal-poor clusters show an extreme Mg depletion and some Si enhancement but at the same time show some relative Al depletion, displaying a turnover in the Mg-Al diagram. These stars suggest that Al has been partially depleted in their progenitors by very hot proton-capture nucleosynthetic processes. Furthermore, we attempted to quantitatively correlate the spread of Al abundances with the global properties of GCs. We find an anti-correlation of the Al spread against clusters metallicity and luminosity, but the data do not allow to find clear evidence of a dependence of N against metallicity in the more metal-poor clusters. Large and homogeneously analyzed samples from on-going spectroscopic surveys unveil unseen chemical details for many clusters, including a turnover in the Mg-Al anti-correlation, thus yielding new constrains for GCs formation evolution models.
We study the multiple populations of $omega$ Cen by using the abundances of Fe, C, N, O, Mg, Al, Si, K, Ca, and Ce from the high-resolution, high signal-to-noise (S/N$>$70) spectra of 982 red giant stars observed by the SDSS-IV/APOGEE-2 survey. We find that the shape of the Al-Mg and N-C anticorrelations changes as a function of metallicity, continuous for the metal-poor groups, but bimodal (or unimodal) at high metallicities. There are four Fe populations, similar to what has been found in previously published investigations, but we find seven populations based on Fe, Al, and Mg abundances. The evolution of Al in $omega$ Cen is compared to its evolution in the Milky Way and in five representative globular clusters. We find that the distribution of Al in metal-rich stars of $omega$ Cen closely follows what is observed in the Galaxy. Other $alpha-$elements and C, N, O, and Ce are also compared to the Milky Way, and significantly elevated abundances are observed over what is found in the thick disk for almost all elements. However, we also find some stars with high metallicity and low [Al/Fe], suggesting that $omega$ Cen could be the remnant core of a dwarf galaxy, but the existence of these peculiar stars needs an independent confirmation. We also confirm the increase in the sum of CNO as a function of metallicity previously reported in the literature and find that the [C/N] ratio appears to show opposite correlations between Al-poor and Al-rich stars as a function of metallicity.
In this paper we describe the photometric and spectroscopic properties of multiple populations in seven northern globular clusters. In this study we employ precise ground based photometry from the private collection of Stetson, space photometry from the Hubble Space Telescope, literature abundances of Na and O, and APOGEE survey abundances for Mg, Al, C, and N. Multiple populations are identified by their position in the CU,B,I -V pseudo-CMD and confirmed with their chemical composition determined using abundances. We confirm the expectation from previous studies that the RGB in all seven clusters are split and the different branches have different chemical compositions. The Mg-Al anti-correlations were well explored by the APOGEE and Gaia-ESO surveys for most globular clusters, some clusters showing bimodal distributions, while others continuous distributions. Even though the structure (i.e., bimodal vs. continuous) of Mg-Al can greatly vary, the Al-rich and Al-poor populations do not seem to have very different photometric properties, agreeing with theoretical calculations. There is no one-to-one correspondence between the Mg-Al anticorrelation shape (bimodal vs. continuous) and the structure of the RGB seen in the HST pseudo-CMDs, with the HST photometric information usually implying more complex formation/evolution histories than the spectroscopic ones. We report on finding two second generation HB stars in M5, and five second generation AGB stars in M92, which is the most metal-poor cluster to date in which second generation AGB stars have been observed.
We use Hubble Space Telescope (HST) imaging from the ACS Treasury Survey to determine fits for single population isochrones of 69 Galactic globular clusters. Using robust Bayesian analysis techniques, we simultaneously determine ages, distances, absorptions, and helium values for each cluster under the scenario of a single stellar population on model grids with solar ratio heavy element abundances. The set of cluster parameters is determined in a consistent and reproducible manner for all clusters using the Bayesian analysis suite BASE-9. Our results are used to re-visit the age-metallicity relation. We find correlations with helium and several other parameters such as metallicity, binary fraction, and proxies for cluster mass. The helium abundances of the clusters are also considered in the context of CNO abundances and the multiple population scenario.
We study the formation of multiple populations in globular clusters (GC), under the hypothesis that stars in the second generation formed from the winds of intermediate-mass stars, ejected during the asymptotic giant branch (AGB) phase, possibly diluted with pristine gas, sharing the same chemical composition of first-generation stars. To this aim, we use the recent APOGEE data, which provide the surface chemistry of a large sample of giant stars, belonging to clusters that span a wide metallicity range. The APOGEE data set is particularly suitable to discriminate among the various pollution scenarios proposed so far, as it provides the surface abundances of Mg and Al, the two elements involved in a nuclear channel extremely sensitive to the temperature, hence to the metallicity of the polluters. The present analysis shows a remarkable agreement between the observations and the theoretical yields from massive AGB stars. In particular, the observed extension of the depletion of Mg and O and the increase in Al is well reproduced by the models and the trend with the metallicity is also fully accounted for. This study further supports the idea that AGB stars were the key players in the pollution of the intra-cluster medium, from which additional generations of stars formed in GC.
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