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Register a new userAluminium abundances in five discrete stellar populations of the globular cluster NGC 2808
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Authors
Eugenio Carretta
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We observed a sample of 90 red giant branch (RGB) stars in NGC 2808 using FLAMES/GIRAFFE and the high resolution grating with the set up HR21. These stars have previous accurate atmospheric parameters and abundances of light elements. We derived aluminium abundances for them from the strong doublet Al I 8772-8773 Angstrom as in previous works of our group. In addition, we were able to estimate the relative CN abundances for 89 of the stars from the strength of a large number of CN features. When adding self consistent abundances from previous UVES spectra analysed by our team, we gathered [Al/Fe] ratios for a total of 108 RGB stars in NGC 2808. The full dataset of proton-capture elements is used to explore in details the five spectroscopically detected discrete components in this globular cluster. We found that different classes of polluters are required to reproduce the (anti)-correlations among all proton-capture elements in the populations P2, I1, and I2 with intermediate composition. This is in agreement with the detection of lithium in lower RGB second generation stars, requiring at least two kind of polluters. To have chemically homogeneous populations the best subdivision of our sample is into six components, as derived from statistical cluster analysis. By comparing different diagrams [element/Fe] vs [element/Fe] we show for the first time that a simple dilution model is not able to reproduce all the sub-populations in this cluster. Polluters of different masses are required. NGC 2808 is confirmed to be a tough challenge to any scenario for globular cluster formation.
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NGC 4833 is a metal-poor Galactic globular cluster (GC) whose multiple stellar populations present an extreme chemical composition. The Na-O anti-correlation is quite extended, which is in agreement with the long tail on the blue horizontal branch, and the large star-to-star variations in the [Mg/Fe] ratio span more than 0.5 dex. Recently, significant excesses of Ca and Sc with respect to field stars of a similar metallicity were also found, signaling the production of species forged in H-burning at a very high temperature in the polluters of the first generation in this cluster. Since an enhancement of potassium is also expected under these conditions, we tested this scenario by analysing intermediate resolution spectra of 59 cluster stars including the K I resonance line at 7698.98 A. We found a wide spread of K abundances, anti-correlated to Mg and O abundances, as previously also observed in NGC 2808. The abundances of K are found to be correlated to those of Na, Ca, and Sc. Overall, this chemical pattern confirms that NGC 4833 is one of the relatively few GCs where the self-enrichment from first generation polluters occurred at such high temperatures that proton-capture reactions were able to proceed up to heavier species such as K and possibly Ca. The spread in K observed in GCs appears to be a function of a linear combination of cluster total luminosity and metallicity, as other chemical signatures of multiple stellar populations in GCs.
The presence of multiple populations in globular clusters has been well established thanks to high-resolution spectroscopy. It is widely accepted that distinct populations are a consequence of different stellar generations: intra-cluster pollution episodes are required to produce the peculiar chemistry observed in almost all clusters. Unfortunately, the progenitors responsible have left an ambiguous signature and their nature remains unresolved. To constrain the candidate polluters, we have measured lithium and aluminium abundances in more than 180 giants across three systems: NGC~1904, NGC~2808, and NGC~362. The present investigation along with our previous analysis of M12 and M5 affords us the largest database of simultaneous determinations of Li and Al abundances. Our results indicate that Li production has occurred in each of the three clusters. In NGC~362 we detected an M12-like behaviour, with first and second-generation stars sharing very similar Li abundances favouring a progenitor that is able to produce Li, such as AGB stars. Multiple progenitor types are possible in NGC~1904 and NGC~2808, as they possess both an intermediate population comparable in lithium to the first generation stars and also an extreme population, that is enriched in Al but depleted in Li. A simple dilution model fails in reproducing this complex pattern. Finally, the internal Li variation seems to suggest that the production efficiency of this element is a function of the clusters mass and metallicity - low-mass or relatively metal-rich clusters are more adept at producing Li.
We present the UV photometry of the globular cluster NGC 1261 using images acquired with the Ultraviolet Imaging Telescope (UVIT) on-board ASTROSAT. We performed PSF photometry on four near-UV (NUV) and two far-UV (FUV) images and constructed UV colour-magnitude diagrams (CMDs), in combination with HST, Gaia, and ground-based optical photometry for member stars. We detected the full horizontal branch (HB) in NUV, blue HB in the FUV and identified two extreme HB (EHB) stars. HB stars have a tight sequence in UV-optical CMDs well-fitted with isochrones generated (12.6 Gyr age, [Fe/H] = -1.27 metallicity) using updated BaSTI-IAC models. Effective temperatures (Teff), luminosities and radii of bright HB stars were estimated using spectral energy distribution. As we detect the complete sample of UV bright HB stars, the hot end of the HB distribution is found to terminate at the G-jump (Teff ~ 11500 K). The two EHB stars, fitted well with single spectra, have Teff= 31,000 K and a mass = 0.495Msun and follow the same Teff-Radius relation of the blue HB stars. We constrain the formation pathways of these EHB stars to extreme mass loss in the RGB phase (either due to rotation or enhanced Helium), OR early hot-flash scenario.
In this study, we identified and characterized the hot and luminous UV-bright stars in the globular cluster NGC 2808. We combined data from the Ultra Violet Imaging Telescope (UVIT) on-board the Indian space satellite, AstroSat, with the Hubble Space Telescope UV Globular Cluster Survey (HUGS) data for the central region (within $sim$ $ang[angle-symbol-over-decimal]{;2.7;} times ang[angle-symbol-over-decimal]{;2.7;}$) and Gaia and ground-based optical photometry for the outer parts of the cluster. We constructed the UV and UV-optical color-magnitude diagrams, compared the horizontal branch (HB) members with the theoretical zero-age HB and terminal-age HB models and identified 34 UV-bright stars. The spectral energy distributions of the UV-bright stars were fitted with theoretical models to estimate their effective temperatures (12500 K - 100,000 K), radii (0.13 to 2.2 $R_{odot}$), and luminosities ($sim 40$ to $3000$ $L_{odot}$) for the first time. These stars were then placed on the H-R diagram, along with theoretical post-HB (pHB) evolutionary tracks to assess their evolutionary status. The models suggest that most of these stars are in the AGB-manque phase and all, except three, have evolutionary masses $<$ 0.53 $M_{odot}$. We also calculated the theoretically expected number of hot post-(early)-AGB (p(e)AGB) stars in this cluster and found the range to match our observations. Seven UV-bright stars located in the outer region of the cluster, identified from the AstroSat/UVIT images, are ideal candidates for detailed follow-up spectroscopic studies.
NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters which share many properties, including the presence of an extended horizontal branch (HB), quite unexpected because of their high metal content. In this paper we use HSTs WFPC2, ACS, and WFC3 images and present a broad multicolor study of their stellar content, covering all main evolutionary branches. The color-magnitude diagrams (CMDs) give compelling evidence that both clusters host at least two stellar populations, which manifest themselves in different ways. NGC 6388 has a broadened main sequence (MS), a split sub-giant branch (SGB), and a split red giant branch (RGB) that becomes evident above the HB in our data set; its red HB is also split into two branches. NGC 6441 has a split MS, but only an indication of two SGB populations, while the RGB clearly splits in two from the SGB level upward, and no red HB structure. The multicolor analysis of the CMDs confirms that the He difference between the two main stellar populations in the two clusters must be similar. This is observationally supported by the HB morphology, but also confirmed by the color distribution of the stars in the MS optical band CMDs. However, a MS split becomes evident in NGC 6441 using UV colors, but not in NGC 6388, indicating that the chemical patterns of the different populations are different in the two clusters, with C, N, O abundance differences likely playing a major role. We also analyze the radial distribution of the two populations.
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