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Register a new userModels for the lithium abundances of multiple populations in globular clusters and the possible role of the Big Bang lithium
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Globular cluster stars show chemical abundance patterns typical of hot-CNO processing. Lithium is easily destroyed by proton capture in stellar environments, so its abundance may be crucial to discriminate among different models proposed to account for multiple populations. In order to reproduce the observed O-Na anticorrelation and other patterns typical of multiple populations, the formation of second generation stars must occur from the nuclearly processed stellar ejecta, responsible of the chemical anomalies, diluted with pristine gas having the composition of first generation stars. The lithium abundance in the unprocessed gas -which is very likely to be equal to the lithium abundance emerging from the Big Bang- affects the lithium chemical patterns among the cluster stars. This paper focuses on a scenario in which processed gas is provided by asymptotic giant branch (AGB) stars. We examine the predictions of this scenario for the lithium abundances of multiple populations. We study the role of the non-negligible lithium abundance in the ejecta of massive AGB (A(Li)~2), and, at the same time, we explore how our models can constrain the extremely large ---and very model dependent--- lithium yields predicted by recent super--AGB models. We show that the super--AGB yields may be tested by examining the lithium abundances in a large set of blue main sequence stars in wCen and/or NGC2808. In addition, we examine the different model results obtained by assuming for the pristine gas either the Big Bang abundance predicted by the standard models (A(Li)=2.6-2.7), or the abundance detected at the surface of population II stars (A(Li)=2.2-2.3). Once a chemical model is well constrained, the O--Li distribution could perhaps be used to shed light on the primordial lithium abundance.
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Lithium is created during the Big Bang nucleosynthesis and it is destroyed in stellar interiors at relatively low temperatures. However, it should be preserved in the stellar envelopes of unevolved stars and progressively diluted during mixing processes. In particular, after the first dredge-up along the RGB, lithium should be completely destroyed, but this is not what we observe today in globular clusters. This element allows to test stellar evolutionary models, as well as different types of polluters for second population stars in the multiple population scenarios. Due to the difficulty in the measurement of the small available lithium line, few GCs have been studied in details so far. Literature results are not homogeneous for what concerns type of stars, sample sizes, and chemical analysis methods. The Gaia-ESO survey allows us to study the largest sample of GCs stars (about 2000, both dwarfs and giants) for which the lithium has been analysed homogeneously.
We present the third paper about our ongoing HST survey for the search for multiple stellar populations (MPs) within Magellanic Cloud clusters. We report here the analysis of NGC 419, a $sim 1.5$ Gyr old, massive ($gtrsim 2 times 10^5 , {rm M_{odot}}$) star cluster in the Small Magellanic Cloud (SMC). By comparing our photometric data with stellar isochrones, we set a limit on [N/Fe] enhancement of $lesssim$+0.5 dex and hence we find that no MPs are detected in this cluster. This is surprising because, in the first two papers of this series, we found evidence for MPs in 4 other SMC clusters (NGC 121; Lindsay 1, NGC 339, NGC 416), aged from 6 Gyr up to $sim 10-11$ Gyr. This finding raises the question whether age could play a major role in the MPs phenomenon. Additionally, our results appear to exclude mass or environment as the only key factors regulating the existence of a chemical enrichment, since all clusters studied so far in this survey are equally massive ($sim 1-2 times 10^5 , {rm M_{odot}}$) and no particular patterns are found when looking at their spatial distribution in the SMC.
We use measurements of nitrogen abundances in red giants to search for multiple stellar populations in the four most metal-poor globular clusters (GCs) in the Fornax dwarf spheroidal galaxy (Fornax 1, 2, 3, and 5). New imaging in the F343N filter, obtained with the Wide Field Camera 3 on the Hubble Space Telescope, is combined with archival F555W and F814W observations to determine the strength of the NH band near 3370 AA. After accounting for observational errors, the spread in the F343N-F555W colors of red giants in the Fornax GCs is similar to that in M15 and corresponds to an abundance range of Delta([N/Fe])=2 dex, as observed also in several Galactic GCs. The spread in F555W-F814W is, instead, fully accounted for by observational errors. The stars with the reddest F343N-F555W colors (indicative of N-enhanced composition) have more centrally concentrated radial distributions in all four clusters, although the difference is not highly statistically significant within any individual cluster. From double-Gaussian fits to the color distributions we find roughly equal numbers of N-normal and N-enhanced stars (formally about 40% N-normal stars in Fornax 1, 3, and 5 and 60% in Fornax 2). We conclude that GC formation, in particular regarding the processes responsible for the origin of multiple stellar populations, appears to have operated similarly in the Milky Way and in the Fornax dSph. Combined with the high ratio of metal-poor GCs to field stars in the Fornax dSph, this places an important constraint on scenarios for the origin of multiple stellar populations in GCs.
A number of scenarios have been put forward to explain the origin of the chemical anomalies (and resulting complex colour-magnitude diagrams) observed in globular clusters (GCs), namely the AGB, Fast Rotating Massive Star, Very Massive Star, and Early Disc Accretion scenarios. We compare the predictions of these scenarios with a range of observations (including young massive clusters (YMCs), chemical patterns, and GC population properties) and find that all models are inconsistent with observations. In particular, YMCs do not show evidence for multiple epochs of star-formation and appear to be gas free by an age of ~3 Myr. Also, the chemical patterns displayed in GCs vary from one to the next in such a way that cannot be reproduced by standard nucleosynthetic yields. Finally, we show that the mass budget problem for the scenarios cannot be solved by invoking heavy cluster mass loss (i.e. that clusters were 10-100 times more massive at birth) as this solution makes basic predictions about the GC population that are inconsistent with observations. We conclude that none of the proposed scenarios can explain the multiple population phenomenon, hence alternative theories are needed.
In the multiple population framework, a number of studies have been accomplished in order to explore the behaviour of lithium with proton-capture element abundances (e.g., Na, O, Al) in globular cluster stars. Lithium offers perhaps one of the most severe constraints on the stellar source of internal pollution in these complex systems. Given its vulnerability, we expect that material processed via the hot H-burning, re-cycled in the formation of the subsequent generation(s), is free of Li. However, Nature breaks our expectations. In this contribution, we will review the current status of this field, by examining the controversial, surprising results and implications.
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