Magnesium isotopes: a tool to understand self-enrichment in Globular Clusters

A critical issue in the asymptotic giant branch (AGB) self-enrichment scenario for the formation of multiple populations in Globular Clusters (GCs) is the inability to reproduce the magnesium isotopic ratios, despite the model in principle can account for the depletion of magnesium. In this work we analyze how the uncertainties on the various p-capture cross sections affect the results related to the magnesium content of the ejecta of AGB stars. The observed distribution of the magnesium isotopes and of the overall Mg-Al trend in M13 and NGC 6752 are successfully reproduced when the proton-capture rate by 25Mg at the temperatures 100 MK, in particular the 25Mg(p, gamma)26Alm channel, is enhanced by a factor 3 with respect to the most recent experimental determinations. This assumption also allows to reproduce the full extent of the Mg spread and the Mg-Si anticorrelation observed in NGC 2419. The uncertainties in the rate of the 25Mg(p,gamma)26Alm reaction at the temperatures of interest here leave space for our assumption and we suggest that new experimental measurements are needed to settle this problem. We also discuss the competitive model based on the super massive star nucleosynthesis.Comment: accepted for publication on MNRA

Stars caught in the braking stage in young Magellanic Clouds clusters

The color-magnitude diagrams of many Magellanic Cloud clusters (with ages up to 2 billion years) display extended turnoff regions where the stars leave the main sequence, suggesting the presence of multiple stellar populations with ages which may differ even by hundreds million years (Mackey et al. 2008, Milone et al. 2009, Girardi et al. 2011). A strongly debated question is whether such an extended turnoff is instead due to populations with different stellar rotations (Girardi et al. 2011, Goudfrooij et al. 2011, Rubele et al. 2013, Li et al. 2014). The recent discovery of a `split' main sequence in some younger clusters (about 80--400Myr) added another piece to this puzzle. The blue (red) side of the main sequence is consistent with slowly (rapidly) rotating stellar models (D'Antona et al. 2015, Milone et al. 2016, Correnti et al. 2017, Milone et al 2016), but a complete theoretical characterization of the observed color-magnitude diagram appeared to require also an age spread (Correnti et al. 2017). We show here that, in three clusters so far analyzed, if the blue main sequence stars are interpreted with models that have been always slowly rotating, they must be about 30% younger than the rest of the cluster. If they are instead interpreted as stars initially rapidly rotating, but that have later slowed down, the age difference disappears, and "braking" also helps to explain the apparent age differences of the extended turnoff. The age spreads in Magellanic Cloud clusters are a manifestation of rotational stellar evolution. Observational tests are suggested.Comment: Accepted for publication and in state of Advance Online Publication (from 24 July 2017) on Nature Astronom

Self-enrichment in Globular Clusters: the extreme He-rich population of NGC 2808

Almost several decades after the discovery of the first multiple populations in galactic globular clusters (GC) the debate on their formation is still extremely current and NGC2808 remains one of the best benchmark to test any scenario for their origin and the evolution. In this work we focus on the chemical composition of stars belonging to the extreme He-rich population populated by stars with the most extreme abundance of Mg, Al, Na, O and Si. We checked whether the most recent measures are consistent with the AGB yields of stars of $6.5-8~M_{\odot}$. These stars evolve on time scales of the order of 40-60 Myr and eject matter strongly enriched in helium, owing to a deep penetration of the surface convective zone down to regions touched by CNO nucleosynthesis occurring after the core He-burning phase. Since the big unknown of the AGB phase of massive stars is the mass loss, we propose a new approch that takes into account the effects of the radiation pressure on dust particles. We show that this more realistic description is able to reproduce the observed abundances of Mg, Al, Na and Si in these extreme stars. The large spread in the oxygen abundances is explained by invoking deep mixing during the RGB phase. It will be possible to check this work hypothesis as soon as the oxygen measurements of the main sequence stars of NGC2808 will be available.Comment: 12 pages, 5 figures, accepted 2018 June 29 by MNRA

A single model for the variety of multiple-population formation(s) in globular clusters: A temporal sequence

We explain the multiple populations recently found in the ‘prototype’ globular cluster (GC) NGC 2808 in the framework of the asymptotic giant branch (AGB) scenario. The chemistry of the five – or more – populations is approximately consistent with a sequence of star formation events, starting after the Type II supernova epoch, lasting approximately until the time when the third dredge-up affects the AGB evolution (age ∼90–120 Myr), and ending when the Type Ia supernovae begin exploding in the cluster, eventually clearing it from the gas. The formation of the different populations requires episodes of star formation in AGB gas diluted with different amounts of pristine gas. In the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV Legacy Survey of GCs, the nitrogen increase is due to the third dredge-up in the smallest mass AGB ejecta involved in the star formation of this population. The possibly iron-rich small population in NGC 2808 may be a result of contamination by a single Type Ia supernova. The NGC 2808 case is used to build a general framework to understand the variety of ‘second-generation’ stars observed in GCs. Cluster-to-cluster variations are ascribed to differences in the effects of the many processes and gas sources which may be involved in the formation of the second generation. We discuss an evolutionary scheme, based on pollution by delayed Type II supernovae, which accounts for the properties of s-Fe-anomalous clusters

Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen

Horizontal Branch stars belong to an advanced stage in the evolution of the oldest stellar galactic population, occurring either as field halo stars or grouped in globular clusters. The discovery of multiple populations in these clusters, that were previously believed to have single populations gave rise to the currently accepted theory that the hottest horizontal branch members (the blue hook stars, which had late helium-core flash ignition, followed by deep mixing) are the progeny of a helium-rich "second generation" of stars. It is not known why such a supposedly rare event (a late flash followed by mixing) is so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal branch stars 10 , or why the blue hook luminosity range in this massive cluster cannot be reproduced by models. Here we report that the presence of helium core masses up to \sim 0.04 solar masses larger than the core mass resulting from evolution is required to solve the luminosity range problem. We model this by taking into account the dispersion in rotation rates achieved by the progenitors, whose premain sequence accretion disc suffered an early disruption in the dense environment of the cluster's central regions where second-generation stars form. Rotation may also account for frequent late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201