We present yields from stars of mass in the range M ≤ M ≤ 8 M of metallicities Z = 3 × 10−4 and 8 × 10−3, thus encompassing the chemistry of low- and high-Z globular clusters. The yields are based on ...full evolutionary computations, following the evolution of the stars from the pre-main sequence through the asymptotic giant branch phase, until the external envelope is lost.
Independent of metallicity, stars with M < 3 M are dominated by third dredge-up, thus ejecting into their surroundings gas enriched in carbon and nitrogen. Conversely, hot bottom burning is mainly responsible for the modification of the surface chemistry of more massive stars, whose mass exceeds 3 M: their gas shows traces of proton-capture nucleosynthesis.
The extent of hot bottom burning turns out to be strongly dependent on metallicity. Models with Z = 8 × 10−3 achieve a modest depletion of oxygen, barely reaching −0.3 dex, and do not activate the Mg-Al chain. Low-Z models with Z = 3 × 10−4 achieve a strong nucleosynthesis at the bottom of the envelope, with a strong destruction of the surface oxygen and magnesium; the most extreme chemistry is reached for models of mass ∼6 M, where δO/Fe ∼ −1.2 and δMg/Fe ∼ −0.6. Sodium is found to be produced in modest quantities at these low Zs, because the initial increase due to the combined effect of the second dredge-up and of 22Ne burning is compensated by the later destruction via proton capture. A great increase by a factor of ∼10 in the aluminium content of the envelope is also expected. These results can be used to understand the role played by intermediate-mass stars in the self-enrichment scenario of globular clusters: the results from spectroscopic investigations of stars belonging to the second generation of clusters with different metallicity will be used as an indirect test of the reliability of the present yields.
The treatment of mass loss and convection is confirmed as the main uncertainties affecting the results obtained in the context of the modelling of the thermal pulses phase. An indirect proof of this comes from the comparison with other investigations in the literature, based on a different prescription for the efficiency of convection in transporting energy and using a different recipe to determine the mass-loss rate.
We study the interaction of the early spherical GC wind powered by Type II supernovae (SNe II) with the surrounding ambient medium consisting of the gaseous disc of a star-forming galaxy at redshift ...z ... 2. The bubble formed by the wind eventually breaks out of the disc, and most of the wind moves directly out of the galaxy and is definitively lost. The fraction of the wind moving nearly parallel to the galactic plane carves a hole in the disc which will contract after the end of the SN activity. During the interval of time between the end of the SN explosions and the 'closure' of the hole, very O-poor stars (the Extreme population) can form out of the super-AGB (asymptotic giant branch) ejecta collected in the GC centre. Once the hole contracts, the AGB ejecta mix with the pristine gas, allowing the formation of stars with an oxygen abundance intermediate between that of the very O-poor stars and that of the pristine gas. We show that this mechanism may explain why Extreme populations are present only in massive clusters, and can also produce a correlation between the spread in helium and the cluster mass. Finally, we also explore the possibility that our proposed mechanism can be extended to the case of multiple populations showing bimodality in the iron content, with the presence of two populations characterized by a small difference in Fe/H. Such a result can be obtained taking into account the contribution of delayed SN II. (ProQuest: ... denotes formulae/symbols omitted.)
We investigate the physical and chemical evolution of Population II stars with initial masses in the range 6.5-8 M⊙, which undergo an off-centre carbon ignition under partially degenerate conditions, ...followed by a series of thermal pulses, and supported energetically by a CNO burning shell, above an O-Ne degenerate core. In agreement with the results by other research groups, we find that the O-Ne core is formed via the formation of a convective flame that proceeds to the centre of the star. The evolution, which follows, is strongly determined by the description of the mass-loss mechanism. Use of the traditional formalism with the superwind phase favours a long evolution with many thermal pulses, and the achievement of an advanced nucleosynthesis, due to the large temperatures reached by the bottom of the external mantle. Use of a mass-loss recipe with a strong dependence on the luminosity favours an early consumption of the stellar envelope, so that the extent of the nucleosynthesis, and thus the chemical composition of the ejecta, is less extreme. The implications for the multiple populations in globular clusters are discussed. If the 'extreme' populations present in the most massive clusters are a result of direct formation from the super asymptotic giant branch (SAGB) ejecta, their abundances may constitute a powerful way of calibrating the mass-loss rate of this phase. This calibration will also provide information on the fraction of SAGBs exploding as single e-capture supernovae, leaving a neutron star remnant in the cluster.
We use high-precision photometry of red-giant-branch (RGB) stars in 57 Galactic globular clusters (GCs), mostly from the 'Hubble Space Telescope (HST) UV Legacy Survey of Galactic GCs', to identify ...and characterize their multiple stellar populations. For each cluster the pseudo-two-colour diagram (or 'chromosome map') is presented, built with a suitable combination of stellar magnitudes in the F275W, F336W, F438W, and F814W filters that maximizes the separation between multiple populations. In the chromosome map of most GCs (type-I clusters), stars separate in two distinct groups that we identify with the first (1G) and the second generation (2G). This identification is further supported by noticing that 1G stars have primordial (oxygen-rich, sodium-poor) chemical composition, whereas 2G stars are enhanced in sodium and depleted in oxygen. This 1G-2G separation is not possible for a few GCs where the two sequences have apparently merged into an extended, continuous sequence. In some GCs (type-II clusters) the 1G and/or the 2G sequences appear to be split, hence displaying more complex chromosome maps. These clusters exhibit multiple subgiant branches (SGBs) also in purely optical colour-magnitude diagrams, with the fainter SGB joining into a red RGB which is populated by stars with enhanced heavy-element abundance. We measure the RGB width by using appropriate colours and pseudo-colours. When the metallicity dependence is removed, the RGB width correlates with the cluster mass. The fraction of 1G stars ranges from ~8 per cent to ~67 per cent and anticorrelates with the cluster mass, indicating that incidence and complexity of the multiple population phenomenon both increase with cluster mass.
The majority of globular clusters show chemical inhomogeneities in the composition of their stars, apparently due to a second stellar generation in which the forming gas is enriched by hot-CNO cycled ...material processed in stars belonging to a first stellar generation. Clearly this evidence prompts questions on the modalities of formation of globular clusters. An important preliminary input to any model for the formation of multiple generations is to determine which is today the relative number fraction of ‘normal’ and anomalous stars in each cluster. As it is very difficult to gather very large spectroscopic samples of globular cluster stars to achieve this result with good statistical significance, we propose to use the horizontal branch (HB). We assume that, whichever the progenitors of the second generation, the anomalies also include enhanced helium abundance. In fact, helium variations have been recently recognized to be able to explain several puzzling peculiarities (gaps, RR Lyr periods and period distribution, ratio of blue to red stars, blue tails) in HBs. We summarize previous results and extend the analysis in order to infer the percentage in number of the first and second generation in as many clusters as possible. We show that, with few exceptions, approximately 50 per cent or more of the stars belong to the second generation. In other cases, in which at first sight one would think of a simple stellar population, we give arguments and suggest that the stars might all belong to the second generation. We provide in the appendix a detailed discussion and new fits of the optical and ultraviolet data of NGC 2808, the classic example of a multiple helium populations cluster, consistently including a reproduction of the main-sequence splittings and an examination of the problem of ‘blue hook’ stars. We also show a detailed fit of the totally blue HB of M13, one among the clusters that are possibly fully made up by second generation stars. We conclude that the formation of the second generation is a crucial event in the life of globular clusters. The problem of the initial mass function required to achieve the observed high fraction of second generation stars can be solved only if the initial cluster was much more massive than the present one and most of the first generation low-mass stars have been preferentially lost. As shown by D'Ercole et al., by modelling the formation and dynamical evolution of the second generation, the mass loss due to the explosions of the Type II supernovae of the first generation may be the process responsible for triggering the expansion of the cluster, the stripping of its outer layers and the loss of most of the first generation low-mass stars.
We present high-precision multiband photometry for the globular cluster (GC) M2. We combine the analysis of the photometric data obtained from the Hubble Space Telescope UV Legacy Survey of Galactic ...GCs GO-13297, with chemical abundances by Yong et al., and compare the photometry with models in order to analyse the multiple stellar sequences we identified in the colour–magnitude diagram. We find three main stellar components, composed of metal-poor, metal-intermediate, and metal-rich stars (hereafter referred to as population A, B, and C, respectively). The components A and B include stars with different s-process element abundances. They host six sub-populations with different light-element abundances, and exhibit an internal variation in helium up to ΔY ∼ 0.07 dex. In contrast with M22, another cluster characterized by the presence of populations with different metallicities, M2 contains a third stellar component, C, which shows neither evidence for sub-populations nor an internal spread in light-elements. Population C does not exhibit the typical photometric signatures that are associated with abundance variations of light elements produced by hydrogen burning at hot temperatures. We compare M2 with other GCs with intrinsic heavy-element variations and conclude that M2 resembles M22, but it includes an additional stellar component that makes it more similar to the central region of the Sagittarius galaxy, which hosts a GC (M54) and the nucleus of the Sagittarius galaxy itself.
Abstract
We present a spectroscopic analysis of main sequence (MS) stars in the young globular cluster NGC 1818 (age ∼40 Myr) in the Large Magellanic Cloud. Our photometric survey of Magellanic ...Clouds clusters has revealed that NGC 1818, similar to other young objects with ages ≲600 Myr, displays not only an extended MS turnoff (eMSTO), as observed in intermediate-age clusters (age ∼1–2 Gyr), but also a split MS. The most straightforward interpretation of the double MS is the presence of two stellar populations: a sequence of slowly rotating stars lying on the blue-MS (bMS) and a sequence of fast rotators, with rotation close to the breaking speed, defining a red-MS (rMS). We report the first direct spectroscopic measurements of projected rotational velocities
for the double MS, eMSTO, and Be stars of a young cluster. The analysis of line profiles includes non-local thermodynamic equilibrium effects, required for correctly deriving
values. Our results suggest that: (i) the mean rotation for bMS and rMS stars is
=71 ± 10 km s
−1
(
σ
= 37 km s
−1
) and
= 202 ± 23 km s
−1
(
σ
= 91 km s
−1
), respectively; (ii) eMSTO stars have different
, which are generally lower than those inferred for rMS stars, and (iii) as expected, Be stars display the highest
values. This analysis supports the idea that distinct rotational velocities play an important role in the appearance of multiple stellar populations in the color–magnitude diagrams of young clusters, and poses new constraints on the current scenarios.
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.
We present photometric analysis of 12 Galactic open clusters and show that the same multiple-population phenomenon observed in Magellanic Clouds (MCs) is present in nearby open clusters. Nearly all ...the clusters younger than ∼2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and all the cluster younger than ∼700 Myr show broadened/split main sequences (MSs). High-resolution spectroscopy has revealed that these clusters host stars with a large spread in the observed projected rotations. In addition to rotation, internal age variation is indicated as possibly responsible for the eMSTOs, making these systems the possible young counterparts of globular clusters with multiple populations. Recent work has shown that the eMSTO+broadened MSs are not a peculiarity of MCs clusters. Similar photometric features have been discovered in a few Galactic open clusters, challenging the idea that the color-magnitude diagrams (CMDs) of these systems are similar to single isochrones and opening new windows to explore the eMSTO phenomenon. We exploit photometry+proper motions from Gaia DR2 to investigate the CMDs of open clusters younger than ∼1.5 Gyr. Our analysis suggests that: (i) 12 open clusters show eMSTOs and/or broadened MSs, which cannot be due to either field contamination or binaries; (ii) split/broadened MSs are observed in clusters younger than ∼700 Myr, while older objects display only an eMSTO, similarly to MCs clusters; (iii) the eMSTO, if interpreted as a pure age spread, increases with age, following the relation observed in MCs clusters and demonstrating that rotation is responsible for this phenomenon.
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