Aluminum‐, Calcium‐ and Titanium‐rich Oxide Stardust in Ordinary Chondrite Meteorites

Among presolar grains, oxide ones are made of oxygen, aluminum, and a small fraction of magnesium, produced by the 26Al decay. The largest part of presolar oxide grains belong to the so-called group 1 and 2, which have been suggested to form in Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, respectively. However, standard stellar nucleosynthesis models cannot account for the 17O/16O, 18O/16O, and 26Al/27Al values recorded in those grains. Hence, for more than 20 years, the occurrence of mixing phenomena coupled with stellar nucleosynthesis have been suggested to account for this peculiar isotopic mix. Nowadays, models of massive AGB stars experiencing Hot Bottom Burning or low mass AGB stars where Cool Bottom Process, or another kind of extra-mixing, is at play, nicely fit the oxygen isotopic mix of group 2 oxide grains. The largest values of the 26Al/27Al ratio seem somewhat more difficult to account for.

Ninety-two refractory oxide grains (primarily Al2O3) with highly unusual O-isotopic ratios have been found in acid-resistant residues of five primitive meteorites. Thirty-five of these also have large excesses of 26Mg, attributable to the in situ decay of radioactive 26Al. The extreme ranges of isotopic compositions of the grains indicate that they are unprocessed stellar condensates. The grains have been divided into four groups. Group 1 grains have 17O excesses and moderate 18O depletions, relative to solar, and most likely formed around red giants and asymptotic giant branch (AGB) stars. However, many individual stars with different masses and initial compositions are required to explain the range of O-isotopic ratios and inferred 26Al/27Al ratios observed in the grains. Group 3 grains, which have 17O and 18O depletions, probably originated in O-rich red giants of very low mass (M≲1.4M⊙) and low metallicity. The Group 3 grains’ compositions are probably strongly influenced by the chemical evolution of the Galaxy; they also provide a new method of determining the age of our Galaxy. Group 2 grains have large 18O depletions, 17O enrichments and high inferred 26Al/27Al ratios; they probably formed in low-mass AGB stars in which extra mixing (“cool bottom processing”) occurred. The four Group 4 grains have 18O enrichments. Possible explanations for these excesses include dredge-up of this isotope in early thermal pulses in AGB stars or an origin in low-mass red giants of unusually high metallicity. One grain, T54, is extremely enriched in 17O and depleted in 18O, and may have formed in an AGB star undergoing hot-bottom-burning. Presolar oxides are underabundant in meteorites, relative to presolar SiC, perhaps because Al condenses more readily into silicates than into refractory oxides or because presolar Al2O3 has a finer grain size distribution. No presolar oxide grains from supernovae have been identified, despite expectations that they should be present.

Context. Leo A is a gas-rich dwarf irregular galaxy of low stellar mass located in the outskirts of the Local Group. It has an extended star formation history with stellar populations spanning a wide age range (∼0.01−10 Gyr). As Leo A is a well-isolated dwarf galaxy, it is a perfect target to study a galactic structure formed entirely by processes of self-induced star formation. Aims. Our aim is to study populations of the brightest asymptotic giant branch (AGB) stars and red giant branch (RGB) stars over the entire extent of the Leo A galaxy. Methods. We analysed populations of AGB and RGB stars in the Leo A galaxy using multicolour photometry data obtained with the Subaru Suprime-Cam (B, V, R, I, Hα) and HST ACS (F475W, F814W) cameras. In order to separate the Milky Way and Leo A populations of red stars, we developed a photometric method that enabled us to study the spatial distribution of AGB and RGB stars within the Leo A galaxy. Results. We found a previously unknown sequence of 26 peculiar RGB stars which probably have a strong CN band in their spectra (∼380−390 nm). This conclusion is supported by the infrared CN spectral features observed in four of these stars with available spectra from the literature. Additionally, we present a catalogue of 32 luminous AGB stars and 3 candidate AGB stars. Twelve AGB stars (three of them might have dusty envelopes) from this sample are newly identified; the remaining 20 AGB stars were already presented in the literature based on near-infrared observations. By splitting the RGB sequence into blue and red parts, we revealed different spatial distributions of the two subsets, with the former being more centrally concentrated than the latter. Cross-identification with spectroscopic data available in the literature suggests that the bulk of blue and red RGB stars are, on average, similar in metallicity; however, the red RGB stars might have an excess of metal-deficient stars of [Fe/H] < −1.8. We also found that the distributions of luminous AGB and blue RGB stars have nearly equal scale lengths (0.′87 ± 0.′06 and 0.′89 ± 0.′09, respectively), indicating that they could belong to the same generation. This conclusion is strengthened by the similarities of the cumulative distributions of AGB and blue RGB stars, both showing more centrally concentrated populations compared to red RGB stars. There is also a prominent decline in the ratio of AGB to RGB stars with an increasing radius. These results suggest that the star-forming disk of Leo A is shrinking, which is in agreement with the outside-in star formation scenario of dwarf galaxy evolution.

Low-mass asymptotic giant branch (AGB) stars are the producers of the main component of cosmic s-process elements. This scenario has been independently confirmed by three different fields: the computation of theoretical AGB stellar models, measurements of the chemical composition of AGB stars, and the analysis of the isotopic composition of meteoritic SiC grains. We present the nucleosynthesis and evolution of lowmass AGB stars by computing three different models with the same initial mass ( ) and different metallicities M p 2 M, ( [equivalent to the initial metal content of the 2 Z p 1.5 # 10 Sun], , and ). In the first part, we 3 4 Z p 1 # 10 Z p 1 # 10 describe the main features of the s-process, illustrating the motivations at the base of this work and the main characteristics of the FRANEC stellar evolutionary code, stressing the treatment of convection and the adopted input physics. We then review extant theoretical investigations of the nucleosynthesis and evolution of low-mass AGB stars. The s-process is believed to occur in low-mass AGB stars after the activation of the reaction. However, to date, no one has been able 13 16 C(a, n) O to explain the formation of a consistent pocket, as required C by observations. In the FRANEC code, the introduction of an exponentially decaying velocity profile below the convective envelope during third–dredge-up episodes allows a small amount of protons to diffuse in the C-rich He intershell, leading to the formation of a C-rich layer. The resulting C pocket partially overlaps with a more external N pocket, followed by a further minor Na pocket; the mass extension of the C pocket decreases along the AGB phase (the first one extending for ) simultaneously with the shrinking of 3 DM ∼ 1 # 10 M, the He intershell region. The exponential decay of the velocity profile depends on a free parameter b, which has been calibrated by maximizing the amount of C inside the pocket. Once the neutron source is obtained, the next step is the construction of a full nuclear network, starting from hydrogen and extending up to the Pb-Bi s-process ending point. The procedure that is followed in preparing this is described in detail, with particular emphasis on the choice of the experimental and theoretical reaction rates for both strong and weak interactions. The evolution and nucleosynthesis of the three computed models are then presented; the final elemental distributions are representative of those expected for the intrinsic carbon stars observed in the disk and in the halo of the Milky Way. A comparison with available spectroscopic analyses of s-process–enriched C stars shows reasonable agreement at solar metallicity while pointing out some problems in the modeling of AGB stars at low metallicities. For the case of solar metallicity, we formulate a new hypothesis on the origin of short-lived radioactive isotopes at the epoch of early solar system formation. For the first time in the literature, we furnish a uniform set of yields, at different metallicities, containing all the chemical species. Moreover, it is demonstrated that a different treatment of the opacity coefficients in the cool envelopes of low-mass AGB stars at low metallicities has dramatic effects on their massloss rate, therefore implying large changes in their final surface overabundances. This problem proves to be unsolvable, because of the lack of opacity tables calculated with different C/O ratios. The present work demonstrates that present-day computational power allows the coupling of a stellar evolutionary code and a full nuclear network. The introduction of an exponentially decaying profile of the velocities at the inner border of the convective envelope allows the formation of the C pocket. We propose our mechanism as a valid tool for the creation of a self-consistent s-process model, although other physical mechanisms have to be taken into account (e.g., rotation and magnetic fields). The inclusion of such processes in our code could have important effects on the mixing, with interesting consequences on the formation and survival of the C pocket: we intend to pursue the evaluation of their global effect in future work. Finally, the importance of the adopted mass-loss rate and the molecular contribution to opacity is pointed out, stressing the need for opacity tables with enhanced carbon and nitrogen abundances.

Based on the s-process nucleosynthesis model with the 13C(α,n)16O reaction occurring under radiative conditions in the interpulse phases, we investigate the characteristics of the distribution of neutron exposure in low-mass Asymptotic Giant Branch (AGB) stars. We introduce a new concept, the distribution of neutron exposures of the Galaxy (NEG), to study the chemical evolution characteristics of the Galaxy for s-process elements. Using a chemical evolution model of the Galaxy, we develop a model for the NEG and obtain the evolution results of the NEG in different epochs. The present results appear to reasonably reproduce the distribution of neutron exposures of the solar system (hereafter NES). The main component and the strong component in the NES are built up in different epochs. The strong component of the s-process is mainly synthesised in the low-mass and metal-poor AGB stars, and the main component is produced by the s-process in the low-mass AGB stars with higher metallicities.

Ba isotopic compositions in stardust SiC grains from the Murchison meteorite: Insights into the stellar origins of large SiC grains

Presolar Al 2O 3 grains as probes of stellar nucleosynthesis and galactic chemical evolution

Aims.We investigate the Na abundance distribution of asymptotic giant branch (AGB) stars in Galactic globular clusters (GCs) and its possible dependence on GC global properties, especially age and metallicity.Methods.We analyze high-resolution spectra of a large sample of AGB and red giant branch (RGB) stars in the Galactic GCs NGC 104, NGC 6121, and NGC 6809 obtained with FLAMES/GIRAFFE at ESO/VLT, and determine their Na abundances. This is the first time that the AGB stars in NGC 6809 are targeted. Moreover, to investigate the dependence of AGB Na abundance dispersion on GC parameters, we compare the AGB [Na/H] distributions of a total of nine GCs, with five determined by ourselves with homogeneous method and four from literature, covering a wide range of GC parameters.Results.NGC 104 and NGC 6809 have comparable AGB and RGB Na abundance distributions revealed by the K−S test, while NGC 6121 shows a lack of very Na-rich AGB stars. By analyzing all nine GCs, we find that the Na abundances and multiple populations of AGB stars form complex picture. In some GCs, AGB stars have similar Na abundances and/or second-population fractions as their RGB counterparts, while some GCs do not have Na-rich second-population AGB stars, and various cases exist between the two extremes. In addition, the fitted relations between fractions of the AGB second population and GC global parameters show that the AGB second-population fraction slightly anticorrelates with GC central concentration, while no robust dependency can be confirmed with other GC parameters.Conclusions.Current data roughly support the prediction of the fast-rotating massive star (FRMS) scenario. However, considering the weak observational and theoretical trends where scatter and exceptions exist, the fraction of second-population AGB stars can be affected by more than one or two factors, and may even be a result of stochasticity.

Almost 30 years ago, stardust grains were identified in meteorites, which have retained the exotic isotopic signatures of their parent stars and display enormous anomalies (up to four orders of magnitude) with respect to the composition of the solar system. The large majority of stardust grains originated from the winds of asymptotic giant branch (AGB) stars. These are stars of low mass (less massive than roughly 8M) at the end of their evolution, which burn H and He in shells located above a C-O degenerate core. The burning shells are separated by a He-rich intershell, and a convective envelope forms the outer layer of the star. The H- and He-burning shells are activated alternately. The mass of the He intershell increases and results in a massive increase in the He-burning rate for a short time, producing a `thermal pulse' (TP). The star expands and cools and H and He burning ceases. The envelope sinks into the intershell and carries material to the stellar surface by the `third dredge-up' (TDU). This cycle is repeated many times, depending on the initial stellar mass. Very dense winds erode the envelope of an AGB star down to a thin H-rich layer. The star becomes a post-AGB star and evolves at constant luminosity towards hotter temperatures. It may become a planetary nebula (PN) with a planetary nebula nucleus (PNN) at its centre, and will then spend the remainder of its life cooling as a white dwarf (WD). Specific nucleosynthesis processes like the slow (s) neutron-capture process occur in AGB stars and their signatures are imprinted in the stardust grains. This process is responsible for roughly half of the cosmic abundances of the elements heavier than Fe (e.g., Kr, Hf, W and Pb) and operates in AGB stars via two neutron sources: the 13C(α,n)16O and the 22Ne(α,n)25Mg reactions, with the 13C(α,n)16O reaction being the main neutron source. This reaction requires protons to be mixed down from the convective envelope into the He intershell for a sufficient amount of 13C to be produced for the s-process. This 13C-rich region is known as the 13C pocket. This thesis aims to investigate which are the AGB parent stars of stardust grains in terms of their range of masses and metallicities, allowing us to understand which type of stars contributed to the inventory of stardust in the early solar system. We also use the composition of the grains to constrain our models of evolution and nucleosynthesis in AGB stars to understand mixing processes in stars and to estimate nuclear reaction rates. We run stellar nucleosynthesis models based on computed stellar structures of low masses from 1M to 4M, and metallicities of 0.0001, 0.01, 0.014 and 0.02. We then compare model predictions of nuclear abundances to the composition of the grains. We compared the composition of the winds coming from the post-AGB and PNN phases of low mass and solar metallicity AGB models to stardust oxide and silicate grains to test the hypothesis that some oxide grains originated from post-AGB stars and PNN. We find that overall the models do not match most of the grains, unless some of the reaction rates used are different than currently assumed. We tested different proton profiles, which determine the size of the 13C pocket formed during nucleosynthesis calculations. We found that abundances produced in models less massive than 1.8M are not affected by the choice of proton profile. On the other hand, stellar models more massive than 1.8M are sensitive to the choice of proton profile, and this affects their abundances. We studied the rate of the 13C(α,n)16O neutron source and its effect on heavy element production. We investigated a number of evaluations of this rate, however, we cannot conclude which evaluation is the most accurate. We studied the W and Hf isotopic compositions predicted in AGB stars and compared them to stardust grains. We found that there are no stellar models in our range of masses or metallicities that match all of the grain data, specifically the 186W/184W ratios in the models is lower than observed. Finally, we analysed the Kr isotopic compositions in AGB stars using the new determination of the 85Kr(n,γ)86Kr neutron-capture rate from Raut et al. (2013). We investigated whether the Kr isotopes measured in large stardust SiC grains can be explained by the composition of the fast stellar winds of the PNN phase. We compared the composition of this material to SiC grains, and found that models less massive than 1.8M that experience 13C ingestion during the TP may be the source of the largest SiC grains.

Context: The stellar production of the light element lithium is still a matter of debate. Aims: We report the detection of low-mass, Li-rich Asymptotic Giant Branch (AGB) stars located in the Galactic bulge. Methods: A homogeneous and well-selected sample of low mass, oxygen-rich AGB stars in the Galactic bulge has been searched for the absorption lines of Li. Using spectral synthesis techniques, we determine from high resolution UVES/VLT spectra the Li abundance in four out of 27 sample stars, and an upper limit for the remaining stars. Results: Two stars in our sample have a solar Li abundance or above; these stars seem to be a novelty, since they do not show any s-element enhancement. Two more stars have a Li abundance slightly below solar; these stars do show s-element enhancement in their spectra. Different scenarios which lead to an increased Li surface abundance in AGB stars are discussed. Conclusions: Of the different enrichment scenarios presented, Cool Bottom Processing (CBP) is the most likely one for the Li-rich objects identified here. Self-enrichment by Hot Bottom Burning (HBB) seems very unlikely as all Li-rich stars are below the HBB mass limit. Also, the ingestion of a low mass companion into the stars' envelope is unlikely because the associated additional effects are lacking. Mass transfer from a former massive binary companion is a possible scenario, if the companion produced little s-process elements. A simple theoretical estimation for the Li abundance due to CBP is presented and compared to the observed values.

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.

We report the discovery of two hibonite grains (CaAl_(12)O_(19)) whose isotopic compositions show that they formed in the winds of red giant and asymptotic giant branch (AGB) stars. While hibonite is the second major phase (after corundum, Al_2O_3) expected to condense from stellar ejecta with C/O < 1, it has not previously been found. One circumstellar hibonite grain is highly enriched in ^(17)O and slightly depleted in ^(18)O relative to the solar composition and has large excesses in ^(26)Mg and ^(41)K, decay products of ^(26)Al and ^(41)Ca. The inferred initial values (^(26)Al/^(27)Al)0 ≈ 5 × 10^(-3) and (^(41)Ca/^(40)Ca)0 ≈ 1.5 × 10^(-4) of this grain are consistent with models of nucleosynthesis in an AGB star. The other hibonite is enriched in ^(17)O, strongly depleted in ^(18)O, shows no evidence of ^(41)Ca and formed with (^(26)Al/^(27)Al)0 ≈ 2 × 10^(-2). The low ^(18)O/^(16)O and very high (^(26)Al/^(27)Al)_0 may indicate substantial proton exposure during cool bottom processing in a low-mass parent star. The low upper limit on ^(41)Ca/^(40)Ca (≤ 3.2 × 10^(-5)) implies that little or no He-shell material had been dredged into the envelope when this grain formed. We also report isotopic compositions for 12 new circumstellar corundum grains. The compositions of 11 of these grains are consistent with current models for red giant and AGB stars. One corundum grain has extremely high ^(17)O/^(16)O and near-solar ^(18)O/^(16)O and may have formed in a star that was initially enriched in ^(17)O and ^(18)O.

view Abstract Citations (122) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Low-Mass Stars. II. The Core Mass--Luminosity Relations for Low-Mass Stars Boothroyd, Arnold I. ; Sackmann, I. -Juliana Abstract It was investigated whether the core mass-luminosity (Mc-L) relation that had been established in the literature for intermediate-mass stars (3 Msun < M < 9 Msun) can be extended to low-mass stars (0.8 Msun< Msun < 3 Msun), where many of the observations take place. Stars were evolved from the main sequence up the red giant branch, through the helium core flash and the horizontal branch phase, up to the asymptotic giant branch where helium shell flashes were followed. Two types of Mc-L relations were obtained, one for the red giant branch (when a single hydrogen-burning shell surrounds a degenerate helium core), and another one for the asymptotic giant branch (when two burning shells, of helium and hydrogen respectively, surround a degenerate carbon-oxygen core). Detailed calculations were carried out for a metal-poor case (Z = 0.001) for stars of initial masses 1.0 Msun, 1.2 Msun, 2.0 Msun, and 3.0 Msun and for a metal-rich case (Z= 0.02) for stars of initial masses 1.2 Msun and 3.0 Msun. The latest nuclear reaction rates were used, as well as the latest opacities (including some molecular opacities) and mass loss via a Reimers-type wind. The dependence of the relation on chemical composition was investigated. For the red giant branch, the relation for the metal- rich case (Z = 0.02, µ ≍ 0.624) was L = (6.86 Mc)7 for 0.3 Msun < Msun< 0.45 Msun, where all units are in solar units; the composition dependence was L ∝ µ 7(ZCNO)1/12 (µ is the envelope mean molecular weight, including free electrons). For the asymptotic giant branch, the Mc-L relation for the metal-rich case (Z = 0.02, µ ≍ 0.618) was L = 52,000 (Mc - 0.456) for 0.52 Msun < Mc < 0.7 Msun; the composition dependence was L ∝ µ 3(ZCNO)1/25. The Mc-L relation obtained for the low-mass asymptotic giant branch stars drops less steeply than would be expected from the previous higher Mc work; the difference is large at low core masses. Because of luminosity variations over the flash cycle, observers will see stars that do not lie on the Mc-L relation; the probability and extent of these deviations was described. The fortunate circumstance was established that, for the Mc relation of low-mass stars, (i) changes in the hydrogen-burning reaction rate have only a very minor effect; (ii) uncertainties in the convective mixing length have negligible effect; and (iii) there is no evidence that the star's total mass has any appreciable effect. Publication: The Astrophysical Journal Pub Date: May 1988 DOI: 10.1086/166322 Bibcode: 1988ApJ...328..641B Keywords: MASS-LUMINOSITY RELATION; STARS: EVOLUTION; STARS: INTERIORS; STARS: LATE-TYPE full text sources ADS | Related Materials (3) Part 1: 1988ApJ...328..632B Part 3: 1988ApJ...328..653B Part 4: 1988ApJ...328..671B

NanoSIMS isotopic analysis of small presolar grains: Search for Si 3N 4 grains from AGB stars and Al and Ti isotopic compositions of rare presolar SiC grains

The photospheres of low-mass red giants show CNO isotopic abundances that are\nnot satisfactorily accounted for by canonical stellar models. The same is true\nfor the measurements of these isotopes and of the $^{26}$Al/$^{27}$Al ratio in\npresolar grains of circumstellar origin. Non-convective mixing, occurring\nduring both Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stages is\nthe explanation commonly invoked to account for the above evidence. Recently,\nthe need for such mixing phenomena on the AGB was questioned, and chemical\nanomalies usually attributed to them were suggested to be formed in earlier\nphases. We have therefore re-calculated extra-mixing effects in low mass stars\nfor both the RGB and AGB stages, in order to verify the above claims. Our\nresults contradict them; we actually confirm that slow transport below the\nconvective envelope occurs also on the AGB. This is required primarily by the\noxygen isotopic mix and the $^{26}$Al content of presolar oxide grains. Other\npieces of evidence exist, in particular from the isotopic ratios of carbon\nstars of type N, or C(N), in the Galaxy and in the LMC, as well as of SiC\ngrains of AGB origin. We further show that, when extra-mixing occurs in the RGB\nphases of population I stars above about 1.2 $M_{\\odot}$, this consumes $^3$He\nin the envelope, probably preventing the occurrence of thermohaline diffusion\non the AGB. Therefore, we argue that other extra-mixing mechanisms should be\nactive in those final evolutionary phases.\n