Intermediate-mass Asymptotic Giant Branch Stars Research Articles

Asteroseismology Sheds Light on the Origin of Carbon-deficient Red Giants: Likely Merger Products and Linked to the Li-rich Giants

Carbon-deficient red giants (CDGs) are a peculiar class of stars that have eluded explanation for decades. We aim to better characterize CDGs by using asteroseismology (Kepler, TESS) combined with spectroscopy (APOGEE, LAMOST), and astrometry (Gaia). We discovered 15 new CDGs in the Kepler field, and confirm that CDGs are rare, as they are only 0.15% of our background sample. Remarkably, we find that our CDGs are almost exclusively in the red clump (RC) phase. Asteroseismic masses reveal that our CDGs are primarily low-mass stars (M ≲ 2 M ⊙), in contrast to previous studies, which suggested they are intermediate mass (M = 2.5–5.0 M ⊙) based on HR diagrams. A very high fraction of our CDGs (50%) are also Li-rich giants. We observe a bimodal distribution of luminosity in our CDGs, with one group having normal RC luminosity and the other being a factor of 2 more luminous than expected for their masses. We find demarcations in chemical patterns and luminosities, which lead us to split them into three groups: (i) normal-luminosity CDGs, (ii) overluminous CDGs, and (iii) overluminous highly polluted CDGs. We conclude that a merger of a helium white dwarf with a red giant branch star is the most likely scenario for the two groups of overluminous stars. Binary mass-transfer from intermediate-mass asymptotic giant branch stars is a possibility for the highly polluted overluminous group. For the normal-luminosity CDGs, we cannot distinguish between core He-flash pollution or lower-mass merger scenarios. Due to the overlap with the CDGs, Li-rich giants may have similar formation channels.

A comparative study of three modes of s-process nucleosynthesis in extremely metal-poor AGB stars

Abstract Carbon-enhanced metal-poor (CEMP) stars in the Galactic halo have a wide range of neutron-capture element abundance patterns. To identify their origin, we investigate three modes of s-process nucleosynthesis that have been proposed to operate in extremely metal-poor asymptotic giant branch (AGB) stars: convective 13C burning, which occurs when hydrogen is engulfed by helium-flash convection in low-mass AGB stars; convective 22Ne burning, which occurs in helium-flash convection of intermediate-mass AGB stars; and radiative 13C burning, which occurs in the 13C pocket that is formed during inter-pulse periods. We show that the production of s-process elements per iron seed (s-process efficiency) does not depend on metallicity below [Fe$/$H] = −2, because 16O in the helium zone dominates the neutron poison. The convective 13C mode can produce a variety of s-process efficiencies for Sr, Ba, and Pb, including the maxima observed among CEMP stars. The 22Ne mode only produces the lowest end of s-process efficiencies among CEMP models. We show that the combination of these two modes can explain the full range of observed enrichment of s-process elements in CEMP stars. In contrast, the 13C pocket mode can hardly explain the high level of enrichment observed in some CEMP stars, even if considering star-to-star variations of the mass of the 13C pocket. These results provide a basis for discussing the binary mass transfer origin of CEMP stars and their subgroups.

Modelling of the post-asymptotic giant branch phase as a tool to understand asymptotic giant branch evolution and nucleosynthesis

ABSTRACT We study a sample of single Galactic post-asymptotic giant branch (post-AGB) stars with known surface chemical composition. Gaia EDR3 and Gaia DR3 parallaxes have enabled accurate determinations of the luminosities of these objects, thus making it possible to characterize them in terms of their initial masses, chemical compositions, and progenitor ages. We used extant evolutionary sequences of asymptotic giant branch (AGB) stars produced using the ATON stellar evolutionary code extended to the post-AGB phase, specifically for this study. The luminosity and surface carbon abundance together prove to be the most valuable indicator of the previous evolution and nucleosynthetic history of the star, particularly regarding the efficiency of mixing mechanisms and the growth of the core. This analysis allows us to place the post-AGB sample into classes based on their evolutionary history on the AGB. This includes low-mass objects that evolved as M-type stars without third dredge-up, carbon stars, and intermediate-mass AGB stars with hot bottom burning (HBB). Additionally, our analysis reveals that AGB nucleosynthesis is not homogeneous, even for stars with similar initial masses and metallicities. The approach of using observations of post-AGB stars to constrain AGB and post-AGB models shed new light on still debated issues related to the AGB evolution, such as the threshold mass required to become a C-star, the minimum mass for HBB and the amount of carbon that can be accumulated in the surface regions of the star during the AGB lifetime, as well as deep mixing experienced during the red giant branch phase.

Lithium abundances as a tracer of asymptotic giant branch star pollution in the globular cluster NGC 6752

ABSTRACT This paper presents a chemical abundance analysis of 217 stars in the metal-poor globular cluster NGC 6752, distributed from the turn-off to the lower red giant branch. Al and Li abundances were derived through spectral synthesis applied to spectra collected with FLAMES, in both GIRAFFE and UVES modes. The work aims to gain insight into the nature of the polluter(s) responsible for the abundance variations and the C-N, Na-O, Al-Mg anticorrelations associated with the multiple-population phenomenon. We found a plateau at A(Li) = 2.33 ± 0.06 dex in unevolved stars, with the average Li content decreasing continuously down to ∼1.25 dex at the bottom of the red giant branch. As expected in the classic anticorrelation scenario, we found stars low in Al and high in Li abundance, and stars high in Al and low in Li. However, in addition, we also found evidence of Al-rich, second-generation stars with high Li content. This finding suggests the need for Li production, known to happen in intermediate-mass (∼4–8 M⊙) asymptotic giant branch stars through the Cameron–Fowler mechanism. It is worth noting that the Li abundance observed in Al-rich stars never exceeds that in Al-poor stars.

Microstructural and Chemical Investigations of Presolar Silicates from Diverse Stellar Environments

Abstract We report the structural and chemical investigation of nine presolar silicate grains from the CH3/CBb3 chondrite Isheyevo and CR2 chondrite Northwest Africa (NWA) 801. Five of these grains belong to group 1, likely condensed in low- to intermediate-mass asymptotic giant branch (AGB) stars, super-AGB stars, or core-collapse supernovae, while the remaining four grains belong to group 4 and have a supernova origin. The advanced transmission electron microscopy and associated electron spectroscopy analyses show a diverse range of chemical and structural compositions for presolar silicates. Two GEMS (glass with embedded metal and sulfide)-like silicates, each from different groups, condensed under nonequilibrium conditions in stellar outflows. Two nonstoichiometric silicates from group 1 have dissimilar formation and alteration histories. An amorphous silicate from group 1 with olivine-like [(Mg,Fe)2SiO4] composition likely formed as a crystalline olivine that subsequently amorphized in the interstellar medium. An oldhamite (CaS) grain within a stoichiometric enstatite (MgSiO3) from group 1 probably formed by heterogeneous condensation in circumstellar outflows. Of the two crystalline grains from group 4, one is an antigorite [(Mg,Fe)3Si2O5(OH)4], while the other is a nontronite [Na,Fe2(Si,Al)4O10(OH)2.nH2O], both formed as a crystalline forsterite and later altered to have hydrated silicate composition. A group-4 silicate has a chemical composition similar to a low Ca-pyroxene [(Ca,Mg)(Si,Al)2O6]. Our data imply that presolar grains from different groups can have a similar range of grain-formation conditions.

The Abundance of S-Process Elements: Temporal and Spatial Trends from Open Cluster Observations

Spectroscopic observations of stars belonging to open clusters, with well-determined ages and distances, are a unique tool for constraining stellar evolution, nucleosynthesis, mixing processes, and, ultimately, Galactic chemical evolution. Abundances of slow (s) process neutron capture elements in stars that retain their initial surface composition open a window into the processes that generated them. In particular, they give us information on their main site of production, i.e., the low- and intermediate-mass Asymptotic Giant Branch (AGB) stars. In the present work, we review some observational results obtained during the last decade that contributed to a better understanding of the AGB phase: the growth of s-process abundances at recent epochs, i.e., in the youngest stellar populations; the different relations between age and [s/Fe] in distinct regions of the disc; and finally the use of s-process abundances combined with those of α elements, [s/α], to estimate stellar ages. We revise some implications that these observations had both on stellar and Galactic evolution, and on our ability to infer stellar ages.

The most metal-rich stars in the universe: chemical contributions of low- and intermediate-mass asymptotic giant branch stars with metallicities within 0.04 ≤ Z ≤ 0.10

ABSTRACT Low- and intermediate-mass stars with supersolar metallicities comprise a known portion of the universe. Yet yields for asymptotic giant branch (AGB) stars with metallicities greater than Z = 0.04 do not exist in the literature. This contributes a significant uncertainty to galactic chemical evolution simulations. We present stellar yields of AGB stars for $M=1\!-\!8\, {\rm M}_\odot$ and Z = 0.04–0.10. We also weight these yields to represent the chemical contribution of a metal-rich stellar population. We find that as metallicity increases, the efficiency of the mixing episodes (known as the third dredge-up) on the thermally pulsing AGB (TP-AGB) decrease significantly. Consequently, much of the nucleosynthesis that occurs on the TP-AGB is not represented on the surface of very metal-rich stars. It instead remains locked inside the white dwarf remnant. The temperatures at the base of the convective envelope also decrease with increasing metallicity. For the intermediate-mass models, this results in the occurrence of only partial hydrogen burning at this location, if any burning at all. We also investigate heavy element production via the slow neutron capture process (s-process) for three 6-$\, {\rm M}_\odot$ models: Z = 0.04, 0.05, and 0.06. There is minor production at the first s-process peak at strontium, which decreases sharply with increasing metallicity. We find the chemical contributions of our models are dominated by proton capture nucleosynthesis, mixed to the surface during the first and second dredge-up events. This conclusion is mirrored in our stellar population yields, weighted towards the lower mass regime to reflect the mass distribution within a respective galaxy.

Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances*

Abstract We combine asteroseismology, optical high-resolution spectroscopy, and kinematic analysis for 26 halo red giant branch stars in the Kepler field in the range of −2.5 < [Fe/H] < −0.6. After applying theoretically motivated corrections to the seismic scaling relations, we obtain an average mass of 0.97 ± 0.03 M ⊙ for our sample of halo stars. Although this maps into an age of ∼7 Gyr, significantly younger than independent age estimates of the Milky Way stellar halo, we considered this apparently young age to be due to the overestimation of stellar mass in the scaling relations. There is no significant mass dispersion among lower red giant branch stars (log g > 2), which constrains the relative age dispersion to <18%, corresponding to <2 Gyr. The precise chemical abundances allow us to separate the stars with [Fe/H] > −1.7 into two [Mg/Fe] groups. While the [α/Fe] and [Eu/Mg] ratios are different between the two subsamples, [s/Eu], where s stands for Ba, La, Ce, and Nd, does not show a significant difference. These abundance ratios suggest that the chemical evolution of the low-Mg population is contributed by Type Ia supernovae, but not by low- to intermediate-mass asymptotic giant branch stars, providing a constraint on its star formation timescale as 100 Myr < τ < 300 Myr. We also do not detect any significant mass difference between the two [Mg/Fe] groups, thus suggesting that their formation epochs are not separated by more than 1.5 Gyr.

Isotope Systematics of Presolar Silicate Grains: New Insights from Magnesium and Silicon

Abstract We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) “normal,” (ii) 25Mg-rich, (iii) 26Mg-rich, and (iv) 25Mg-poor. Normal Group 1 grains (∼60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ∼0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The 25Mg-rich grains (∼25%) show enrichments in 25Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The 26Mg-rich and 25Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that >30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought.

Period-luminosity diagram of long period variables in the Magellanic Clouds

Context. The period-luminosity diagram (PLD) has proven to be a powerful tool for studying populations of pulsating red giants. Gaia Data Release 2 (DR2) provides a large data set including many long-period variables (LPVs) on which this tool can be applied. Aims. We investigate the location of LPVs from the Large and Small Magellanic Clouds in the PLD using various optical and infrared luminosity indicators from Gaia and 2MASS, respectively. We thereby distinguish between stars of different masses and surface chemistry. Methods. The data set taken from the Gaia DR2 catalogue of LPVs allows for a homogeneous study from low- to high-mass LPVs. These sources are divided into sub-populations of asymptotic giant branch (AGB) stars according to their mass and their O- or C-rich nature using the Gaia-2MASS diagram developed by our group. This diagram uses a Wesenheit index WBP, RP based on Wesenheit functions in the Gaia and 2MASS photometric bands. Four different luminosity indicators are used to study the period-luminosity (P–L) relations. Results. We provide the first observational evidence of a P–L relation offset for both fundamental and 1O pulsators between low- and intermediate-mass O-rich stars, in agreement with published pulsation predictions. Among the luminosity indicators explored, sequence C′ is the narrowest in the P–WBP, RP diagram, and is thus to be preferred over the other PLDs for the determination of distances using LPVs. The majority of massive AGB stars and red supergiants form a smooth extension of sequence C of low- and intermediate-mass AGB stars in the P–WBP, RP diagram, suggesting that they pulsate in the fundamental mode. All results are similar in the two Magellanic Clouds.

Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins

Abstract We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (T MAX) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (∼1.5–3 M ⊙) rather than intermediate-mass (>3–∼9 M ⊙) AGB stars. Because of the low efficiency of 22Ne(α, n)25Mg at the low T MAX values attained in low-mass AGB stars, model calculations cannot explain the large 30Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.

Asymmetric Drift in the Andromeda Galaxy (M31) as a Function of Stellar Age

Abstract We analyze the kinematics of Andromeda’s disk as a function of stellar age by using photometry from the Panchromatic Hubble Andromeda Treasury survey and spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. We use H i 21 cm and CO ( ) data to examine the difference between the deprojected rotation velocity of the gas and that of the stars. We divide the stars into four stellar age bins, from shortest lived to longest lived: massive main-sequence stars (0.03 Gyr), more luminous intermediate-mass asymptotic giant branch (AGB) stars (0.4 Gyr), less luminous intermediate-mass AGB stars (2 Gyr), and low-mass red giant branch stars (4 Gyr). There is a clear correlation between the offset of the stellar and the gas rotation velocity, or the asymmetric drift: the longer-lived populations lag farther behind the gas than short-lived populations. We also examine possible causes of the substructure in the rotation curves (RCs) and find that the most significant cause of scatter in the RCs comes from the tilted ring model being an imperfect way to account for the multiple warps in Andromeda’s disk.

When binaries keep track of recent nucleosynthesis

Context. Barium stars are s-process enriched giants. They owe their chemical peculiarities to a past mass transfer phase. During this phase they were polluted by their binary companion, which at the time was an asymptotic giant branch (AGB) star, but is now an extinct white dwarf. Barium stars are thus ideal targets for understanding and constraining the s-process in low- and intermediate-mass AGB stars. Aims. We derive the abundances of a large number of heavy elements in order to shed light on the conditions of operation of the neutron source responsible for the production of s-elements in the former companions of the barium stars. Methods. Adopting a recently used methodology, we analyse a sample of eighteen highly enriched barium stars observed with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma). We determine the stellar parameters and abundances using MARCS model atmospheres. In particular, we derive the Nb–Zr ratio which was previously shown to be a sensitive thermometer for the s-process nucleosynthesis. Indeed, in barium stars, 93Zr has fully decayed into mono-isotopic 93Nb, so Nb/Zr is a measure of the temperature-sensitive 93Zr/Zr isotopic ratio. Results. HD 28159, previously classified as K5III and initially selected to serve as a reference cool K star for our abundance analysis, turns out to be enriched in s-process elements, and as such is a new barium star. Four stars are characterised by high nitrogen abundances, and among those three have high [Nb/Zr] and [hs/ls] ratios. The derived Zr and Nb abundances provide more accurate constraints on the s-process neutron source, identified to be 13C(α, n)16O for barium stars. The comparison with stellar evolution and nucleosynthesis models shows that the investigated barium stars were polluted by a low-mass (M ∼ 2 − 3 M⊙) AGB star. HD 100503 is potentially identified as a high metallicity analogue of carbon-enhanced metal-poor star enriched in both r- and s-process elements (CEMP-rs).

High-temperature Dust Condensation around an AGB Star: Evidence from a Highly Pristine Presolar Corundum

Abstract Corundum (α-Al2O3) and amorphous or metastable Al2O3 are common components of circumstellar dust observed around O-rich asymptotic giant branch (AGB) stars and found in primitive meteorites. We report a detailed isotopic and microstructural investigation of a unique presolar corundum grain, QUE060, identified in an acid residue of the Queen Alexandra Range 97008 (LL3.05) meteorite. Based on its O and Mg isotopic compositions, this 1.4 μm diameter grain formed in a low- or intermediate-mass AGB star. It has four developed rhombohedral {011} faces of corundum and a rough, rounded face with cavities. High Mg contents (Mg/Al > 0.004) are due to the decay of radioactive 26Al. No spinel (MgAl2O4) inclusions that might have exsolved from the corundum are observed, but there are several high-Mg domains with modulated structures. The subhedral shape of grain QUE060 is the first clear evidence that corundum condenses and grows to micrometer sizes in the extended atmospheres around AGB stars. The flat faces indicate that grain QUE060 experienced little modification by gas–grain and grain–grain collisions in the interstellar medium (ISM) and solar nebula. The Mg distribution in its structure indicates that grain QUE060 has not experienced any severe heating events since the exhaustion of 26Al. However, it underwent at least one very transient heating event to form the high-Mg domains. A possible mechanism for producing this transient event, as well as the one rough surface and cavity, is a single grain–grain collision in the ISM. These results indicate that grain QUE060 is the most pristine circumstellar corundum studied to date.

The Universality of the Rapid Neutron-capture Process Revealed by a Possible Disrupted Dwarf Galaxy Star*

Abstract The rapid neutron-capture or r-process is thought to produce the majority of the heavy elements ( ) in extremely metal-poor stars. The same process is also responsible for a significant fraction of the heavy elements in the Sun. This universality of the r-process is one of its characteristic features, as well as one of the most important clues to its astrophysical origin. We report the discovery of an extremely metal-poor field giant with and , the lowest abundances of strontium and barium relative to iron ever observed. Despite its low abundances, the star 2MASS J151113.24–213003.0 has , therefore its neutron-capture abundances are consistent with the main solar r-process pattern that has . It has been suggested that extremely low neutron-capture abundances are a characteristic of dwarf galaxies, and we find that this star is on a highly eccentric orbit with an apocenter ≳100 kpc that lies in the disk of satellites in the halo of the Milky Way. We show that other extremely metal-poor stars with low [Sr, Ba/H] and [Sr, Ba/Fe] plus solar [Sr/Ba] tend to have orbits with large apocenters, consistent with a dwarf galaxy origin for this class of object. The nucleosynthesis event that produced the neutron-capture elements in 2MASS J151113.24–213003.0 must produce both strontium and barium together in the solar ratio. We exclude contributions from the s-process in intermediate-mass asymptotic giant branch or fast-rotating massive metal-poor stars, pair-instability supernovae, the weak r-process, and neutron-star mergers. We argue that the event was a Pop III or extreme Pop II core-collapse supernova explosion.

LITHIUM-RICH GIANTS IN GLOBULAR CLUSTERS*

ABSTRACT Although red giants deplete lithium on their surfaces, some giants are Li-rich. Intermediate-mass asymptotic giant branch (AGB) stars can generate Li through the Cameron–Fowler conveyor, but the existence of Li-rich, low-mass red giant branch (RGB) stars is puzzling. Globular clusters are the best sites to examine this phenomenon because it is straightforward to determine membership in the cluster and to identify the evolutionary state of each star. In 72 hours of Keck/DEIMOS exposures in 25 clusters, we found four Li-rich RGB and two Li-rich AGB stars. There were 1696 RGB and 125 AGB stars with measurements or upper limits consistent with normal abundances of Li. Hence, the frequency of Li-richness in globular clusters is (0.2 ± 0.1)% for the RGB, (1.6 ± 1.1)% for the AGB, and (0.3 ± 0.1)% for all giants. Because the Li-rich RGB stars are on the lower RGB, Li self-generation mechanisms proposed to occur at the luminosity function bump or He core flash cannot explain these four lower RGB stars. We propose the following origin for Li enrichment: (1) All luminous giants experience a brief phase of Li enrichment at the He core flash. (2) All post-RGB stars with binary companions on the lower RGB will engage in mass transfer. This scenario predicts that 0.1% of lower RGB stars will appear Li-rich due to mass transfer from a recently Li-enhanced companion. This frequency is at the lower end of our confidence interval.

LOW-RESOLUTION SPECTROSCOPIC STUDIES OF GLOBULAR CLUSTERS WITH MULTIPLE POPULATIONS

Recent narrow-band Ca photometry discovered two distinct red giant branch (RGB) populations in some massive globular clusters (GCs) including M22, NGC 1851, and NGC 288. In order to investigate the differences in light/heavy elements abundances between the two subpopulations, we have performed low-resolution spectroscopy for stars on the two RGBs in these GCs. We find a significant difference (more than <TEX>$4{\sigma}$</TEX>) in calcium abundance from the spectroscopic HK' index for both M22 and NGC 1851. We also find a more than <TEX>$8{\sigma}$</TEX> difference in CN band strength between the Ca-strong and Ca-weak subpopulations. For NGC 288, however, we detect the presence of a large difference only in the CN strength. The calcium abundances of the two subpopulations in this GC are identical within errors. We also find interesting differences in CN-CH relations among these GCs. While CN and CH indices are correlated in M22, they show an anti-correlation in NGC 288. However, NGC 1851 shows no difference in CH between two groups of stars having different CN strengths. The CN bimodality in these GCs could be explained by pollution from intermediate-mass asymptotic giant branch stars and/or fast-rotating massive stars. For the presence or absence of calcium bimodality and the differences in CN-CH relations, we suggest these would be best explained by how strongly type II supernovae enrichment has contributed to the chemical evolutions of these GCs.

EVOLUTION AND NUCLEOSYNTHESIS OF ASYMPTOTIC GIANT BRANCH STELLAR MODELS OF LOW METALLICITY

We present stellar evolutionary tracks and nucleosynthetic predictions for a grid of stellar models of low- and intermediate-mass asymptotic giant branch (AGB) stars at $Z=0.001$ ([Fe/H]$=-1.2$). The models cover an initial mass range from 1 M$_{\odot}$ to 7 M$_{\odot}$. Final surface abundances and stellar yields are calculated for all elements from hydrogen to bismuth as well as isotopes up to the iron group. We present the first study of neutron-capture nucleosynthesis in intermediate-mass AGB models, including a super-AGB model, of [Fe/H] = $-1.2$. We examine in detail a low-mass AGB model of 2 M$_{\odot}$ where the $^{13}$C($\alpha$,$n$)$^{16}$O reaction is the main source of neutrons. We also examine an intermediate-mass AGB model of 5 M$_{\odot}$ where intershell temperatures are high enough to activate the $^{22}$Ne neutron source, which produces high neutron densities up to $\sim 10^{14}$ n cm$^{-3}$. Hot bottom burning is activated in models with $M \geq 3$ M$_{\odot}$. With the 3 M$_{\odot}$ model we investigate the effect of varying the extent in mass of the region where protons are mixed from the envelope into the intershell at the deepest extent of each third dredge-up. We compare the results of the low-mass models to three post-AGB stars with a metallicity of [Fe/H] $\simeq -1.2$. The composition is a good match to the predicted neutron-capture abundances except for Pb and we confirm that the observed Pb abundances are lower than what is calculated by AGB models.

An HST/WFC3 view of stellar populations on the horizontal branch of NGC 2419

We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the Horizontal Branch (HB) population of the We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the horizontal branch (HB) population of the Galactic globular cluster (GC) NGC 2419. A detailed analysis of the composite HB highlights three populations:(1) the blue luminous HB, hosting standard helium stars (Y=0.25) with a very small spread of mass, (2) a small population of stars with intermediate helium content (0.26<Y<=0.29), and (3) the well-populated extreme HB. We can fit the last group with models having high helium abundance (Y \sim 0.36), half of which (the hottest part, 'blue hook' stars) are identified as possible 'late flash mixed stars'. The initial helium abundance of this extreme population is in nice agreement with the predicted helium abundance in the ejecta of massive asymptotic giant branch (AGB) stars of the same metallicity as NGC 2419. This result further supports the hypothesis that second-generation stars in GCs formed from the ashes of intermediate-mass AGB stars. We find that the distribution in magnitude of the blue hook stars is larger than that predicted by theoretical models. We discuss the possible uncertainties in the magnitude scales and different attempts to model this group of stars. Finally, we suggest that consistency can be better achieved if we assume core masses larger than predicted by our models. This may be possible if the progenitors were fast rotators on the main sequence. If further study confirms this interpretation, a fast initial rotation would be a strong signature of the peculiarity of extreme second-generation stars in GCs.

A Study of the λ10830 He I Line Among Red Giants in Messier 13 1 1The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial

Two properties of Messier 13 are pertinent to the study of mass loss among metal-poor stars and the chemical evolution of globular clusters: (1) an extended blue horizontal branch, which seems to demand mass loss from red giant progenitor stars and possibly an enhanced helium abundance, and (2) the presence of internal abundance inhomogeneities of elements in the mass range from C to Al. A popular explanation for this second phenomenon is that M13 was self-enriched by intermediate-mass asymptotic giant branch (IM-AGB) stars of a type that may also have been able to instigate helium enrichment. Spectra of the λ10830 absorption feature produced by He I have been obtained by using the NIRSPEC spectrometer on the Keck 2 telescope for seven red giants in M13 chosen to have a range in λ3883 CN band strengths, oxygen, and sodium abundances. Whereas these spectra do reveal the presence of fast winds among some M13 red giants, they provide little support for helium abundance differences of the type that might have been generated by a burst of IM-AGB star activity within the M13 protocluster.