arXiv subjects

J. C. Lattanzio

Publications and source records attributed to J. C. Lattanzio.

At least 19 recordsLinked to original sources

arXiv2020

Nucleosynthetic yields of Z=$10^{-5}$ intermediate-mass stars

Abridged: Observed abundances of extremely metal-poor (EMP) stars in the Halo hold clues for the understanding of the ancient universe. Interpreting these clues requires theoretical stellar models at the low-Z regime. We provide the nucleosynthetic yields of intermediate-mass Z=$10^{-5}$ stars between 3 and 7.5 $M_{sun}$, and quantify the effects of the uncertain wind rates. We expect these yields can be eventually used to assess the contribution to the chemical inventory of the early universe, and to help interpret abundances of selected C-enhanced EMP stars. By comparing our models and other existing in the literature, we explore evolutionary and nucleosynthetic trends with wind prescriptions and with initial metallicity. We compare our results to observations of CEMP-s stars belonging to the Halo. The yields of intermediate-mass EMP stars reflect the effects of very deep second dredge-up (for the most massive models), superimposed with the combined signatures of hot-bottom burning and third dredge-up. We confirm the reported trend that models with initial metallicity Z$_{ini}$ <= 0.001 give positive yields of $^{12}C, ^{15}N, ^{16}O$, and $^{26}Mg$. The $^{20}Ne, ^{21}Ne$, and $^{24}Mg$ yields, which were reported to be negative at Z$_{ini}$ = 0.0001, become positive for Z=$10^{-5}$. The results using two different prescriptions for mass-loss rates differ widely in terms of the duration of the thermally-pulsing (Super) AGB phase, overall efficiency of the third dredge-up episode, and nucleosynthetic yields. The most efficient of the standard wind rates frequently used in the literature seems to favour agreement between our yield results and observational data. Regardless of the wind prescription, all our models become N-enhanced EMP stars.

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arXiv2017

Discovery of a Metal-poor, Luminous Post-AGB Star that Failed the Third Dredge-up

Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is a A-type ($T_{\rm eff}$ = 8250 $\pm$ 250K) luminous (8200 $\pm$ 700 $\rm L_{\odot}$), metal-poor ($\textrm{[Fe/H]}$ = $- 1.18 \pm$ 0.10), low-mass (M$_{\rm initial}$ $\approx$ 1.5 $-$ 2.0 $\rm M_{\odot}$) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of [C/Fe] $<$ 0.50) nor enrichment of $s$-process elements. We derived an oxygen abundance of [O/Fe] = 0.29 $\pm$ 0.1. For Fe and O, we took into account the effects of non-local thermodynamic equilibrium (NLTE). We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB and be carbon-enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogues which are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any third dredge-up episodes.

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arXiv2017

NGC 6752 AGB Stars Revisited: I. Improved AGB temperatures remove apparent overionisation of Fe I

A recent study reported a strong apparent depression of Fe I, relative to Fe II, in the AGB stars of NGC 6752. This depression is much greater than that expected from the neglect of non-local thermodynamic equilibrium effects, in particular the dominant effect of overionisation. Here we attempt to reproduce the apparent Fe discrepancy, and investigate differences in reported sodium abundances. We compare in detail the methods and results of the recent study with those of an earlier study of NGC 6752 AGB stars. Iron and sodium abundances are derived using Fe I, Fe II, and Na I lines. Various uncertainties are explored. We reproduce the large Fe I depression found by the recent study, using different observational data and computational tools. Further investigation shows that the degree of the apparent Fe I depression is strongly dependent on the adopted stellar effective temperature. To minimise uncertainties in Fe I we derive temperatures for each star individually using the infrared flux method (IRFM). We find that the $T_{\rm{eff}}$ scales used by both the previous studies are cooler, by up to 100 K; such underestimated temperatures amplify the apparent Fe I depression. Our IRFM temperatures result in negligible apparent depression, consistent with theory. We also re-derived sodium abundances and, remarkably, found them to be unaffected by the new temperature scale. [Na/H] in the AGB stars is consistent between all studies. Since Fe is constant, it follows that [Na/Fe] is also consistent between studies, apart from any systematic offsets in Fe. We recommend the use of $(V-K)$ relations for AGB stars. We plan to investigate the effect of the improved temperature scale on other elements, and re-evaluate the subpopulation distributions on the AGB, in the next paper of this series. [abridged]

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arXiv2017

Overshoot Inwards from the Bottom of the Intershell Convective Zone in (S)AGB stars

We estimate the extent of overshooting inwards from the bottom of the intershell convective zone in thermal pulses in (S)AGB stars. We find that the buoyancy is so strong that any overshooting should be negligible. The temperature inversion at the bottom of the convective zone adds to the stability of the region. Any mixing that occurs in this region is highly unlikely to be due to convective overshooting, and so must be due to another process.

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arXiv2017

Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements

The production of the elements heavier than iron via slow neutron captures (the s process) is a main feature of the contribution of asymptotic giant branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos. However, our understanding of the main neutron source, the 13C(alpha,n)16O reaction, is still incomplete. It is commonly assumed that in AGB stars mixing beyond convective borders drives the formation of 13C pockets. However, there is no agreement on the nature of such mixing and free parameters are present. By means of a parametric model we investigate the impact of different mixing functions on the final s-process abundances in low-mass AGB models. Typically, changing the shape of the mixing function or the mass extent of the region affected by the mixing produce the same results. Variations in the relative abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are generally within +/-0.2 dex, similar to the observational error bars. We conclude that other stellar uncertainties - the effect of rotation and of overshoot into the C-O core - play a more important role than the details of the mixing function. The exception is at low metallicity, where the Pb abundance is significantly affected. In relation to the composition observed in stardust SiC grains from AGB stars, the models are relatively close to the data only when assuming the most extreme variation in the mixing profile.

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arXiv2015

Towards 21st Century Stellar Models: Star Clusters, Supercomputing, and Asteroseismology

Stellar models provide a vital basis for many aspects of astronomy and astrophysics. Recent advances in observational astronomy -- through asteroseismology, precision photometry, high-resolution spectroscopy, and large-scale surveys -- are placing stellar models under greater quantitative scrutiny than ever. The model limitations are being exposed and the next generation of stellar models is needed as soon as possible. The current uncertainties in the models propagate to the later phases of stellar evolution, hindering our understanding of stellar populations and chemical evolution. Here we give a brief overview of the evolution, importance, and substantial uncertainties of core helium burning stars in particular and then briefly discuss a range of methods, both theoretical and observational, that we are using to advance the modelling.

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arXiv2015

Lithium abundances in globular cluster giants: NGC 1904, NGC 2808, and NGC 362

The presence of multiple populations in globular clusters has been well established thanks to high-resolution spectroscopy. It is widely accepted that distinct populations are a consequence of different stellar generations: intra-cluster pollution episodes are required to produce the peculiar chemistry observed in almost all clusters. Unfortunately, the progenitors responsible have left an ambiguous signature and their nature remains unresolved. To constrain the candidate polluters, we have measured lithium and aluminium abundances in more than 180 giants across three systems: NGC~1904, NGC~2808, and NGC~362. The present investigation along with our previous analysis of M12 and M5 affords us the largest database of simultaneous determinations of Li and Al abundances. Our results indicate that Li production has occurred in each of the three clusters. In NGC~362 we detected an M12-like behaviour, with first and second-generation stars sharing very similar Li abundances favouring a progenitor that is able to produce Li, such as AGB stars. Multiple progenitor types are possible in NGC~1904 and NGC~2808, as they possess both an intermediate population comparable in lithium to the first generation stars and also an extreme population, that is enriched in Al but depleted in Li. A simple dilution model fails in reproducing this complex pattern. Finally, the internal Li variation seems to suggest that the production efficiency of this element is a function of the cluster's mass and metallicity - low-mass or relatively metal-rich clusters are more adept at producing Li.

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arXiv2013

The Asymptotic Giant Branches of GCs: Selective Entry Only

The handful of available observations of AGB stars in Galactic Globular Clusters suggest that the GC AGB populations are dominated by cyanogen-weak stars. This contrasts strongly with the distributions in the RGB (and other) populations, which generally show a 50:50 bimodality in CN band strength. If it is true that the AGB populations show very different distributions then it presents a serious problem for low mass stellar evolution theory, since such a surface abundance change going from the RGB to AGB is not predicted by stellar models. However this is only a tentative conclusion, since it is based on very small AGB sample sizes. To test whether this problem really exists we have carried out an observational campaign specifically targeting AGB stars in GCs. We have obtained medium resolution spectra for about 250 AGB stars across 9 Galactic GCs using the multi-object spectrograph on the AAT (2df/AAOmega). We present some of the preliminary findings of the study for the second parameter trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there is a deficiency of stars with strong CN bands on the AGB. To confirm that this phenomenon is robust and not just confined to CN band strengths and their vagaries, we have made observations using FLAMES/VLT to measure elemental abundances for NGC 6752.We present some initial results from this study also. Our sodium abundance results show conclusively that only a subset of stars in GCs experience the AGB phase of evolution. This is the first direct, concrete confirmation of the phenomenon.

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arXiv2012

Cyanogen in NGC 1851 RGB and AGB Stars: Quadrimodal Distributions

The Galactic globular cluster NGC 1851 has raised much interest since HST photometry revealed that it hosts a double subgiant branch. Here we report on our homogeneous study into the cyanogen (CN) bandstrengths in the RGB population (17 stars) and AGB population (21 stars) using AAOmega/2dF spectra with R $\sim 3000$. We discover that NGC 1851 hosts a quadrimodal distribution of CN bandstrengths in its RGB and AGB populations. This result supports the merger formation scenario proposed for this cluster, such that the CN quadrimodality could be explained by the superposition of two `normal' bimodal populations. A small sample overlap with an abundance catalogue allowed us to tentatively explore the relationship between our CN populations and a range of elemental abundances. We found a striking correlation between CN and [O/Na]. We also found that the four CN peaks may be paired -- the two CN-weaker populations being associated with low Ba and the two CN-stronger populations with high Ba. If true then s-process abundances would be a good diagnostic for disentangling the two original clusters in the merger scenario. More observations are needed to confirm the quadrimodality, and also the relationship between the subpopulations. We also report CN results for NGC 288 as a comparison. Our relatively large samples of AGB stars show that both clusters have a bias towards CN-weak AGB populations.

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arXiv2010

Observational Constraints for Thermohaline Mixing

We provide a brief review of thermohaline physics and why it is a candidate extra mixing mechanism during the red giant branch (RGB). We discuss how thermohaline mixing (also called $δ$ $μ$ mixing) during the RGB due to helium-3 burning, is more complicated than the operation of thermohaline mixing in other stellar contexts (such as following accretion from a binary companion). We try to use observations of carbon depletion in globular clusters to help constrain the formalism and the diffusion coefficient or mixing velocity that should be used in stellar models. We are able to match the spread of carbon depletion for metal poor field giants but are unable to do so for cluster giants, which may show evidence of mixing prior to even the first dredge-up event.

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arXiv2010

The case of the disappearing CN-strong AGB stars in Galactic globular clusters -- Preliminary Results

A previously reported literature search suggested that the AGB stars in Galactic globular clusters may be showing different distributions of CN-strong and CN-weak stars as compared to their RGB stars. In most cases the second giant branches of GCs appeared to be deficient in stars with strong CN bands. However the sample sizes of AGB stars at that time were too small to give a definitive picture. Thus an observing campaign targeting GC AGB stars was proposed. We now have medium resolution spectral observations of about 250 GC AGB stars across 9 globular clusters, obtained with the 2dF/AAOmega instrument on the Anglo-Australian Telescope. In this paper we report some preliminary results regarding the distributions of CN-strong and CN-weak stars on the two giant branches of a selection of globular clusters. We find that some GCs show a total lack of CN-strong stars on the AGB, whilst some show a reduction in CN-strong stars as compared to the RGB. Standard stellar evolution does not predict this change in surface abundance between the two giant branches. We discuss some possible causes of this unexpected phenomenon.

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arXiv2009

MONTAGE: AGB nucleosynthesis with full s-process calculations

We present MONTAGE, a post-processing nucleosynthesis code that combines a traditional network for isotopes lighter than calcium with a rapid algorithm for calculating the s-process nucleosynthesis of the heavier isotopes. The separation of those parts of the network where only neutron-capture and beta-decay reactions are significant provides a substantial advantage in computational efficiency. We present the yields for a complete set of s-process isotopes for a 3 Mo, Z = 0.02 stellar model, as a demonstration of the utility of the approach. Future work will include a large grid of models suitable for use in calculations of Galactic chemical evolution.

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arXiv2009

Evolution and Nucleosynthesis of Extremely Metal Poor & Metal-Free Low- and Intermediate-Mass Stars I: Stellar Yield Tables and the CEMPs

[Abridged] We calculate the structural evolution and nucleosynthesis of a grid of models covering the metallicity range: -6.5 < [Fe/H] < -3.0 (plus Z=0), and mass range: 0.85 < M < 3.0 Msun, amounting to 20 stars in total. In this paper, the first of a series describing and analysing this large data set, we present the resulting stellar yields. Many of the models experience violent nuclear burning episodes not seen at higher metallicities. We refer to these events as `Dual Flashes'. These events have also been reported by previous studies. Some of the material processed by the Dual Flashes is dredged up causing significant surface pollution with a distinct chemical composition. We also analyse the yields in terms of C and N, comparing them to the observed CEMP abundances. At the lowest metallicities ([Fe/H] < -4.0) we find the yields to contain ~1 to 2 dex too much carbon, in agreement with all previous studies. At higher metallicities ([Fe/H] = -3.0), where the observed data set is much larger, all our models produce yields with [C/Fe] values consistent with those observed in the most C-rich CEMPs. However it is only the low-mass models that undergo the Dual Shell Flash (which occurs at the start of the TPAGB) that can best reproduce the C and N observations. Normal Third Dredge-Up can not reproduce the observations because at these metallicities intermediate mass models (M > 2 Msun) suffer HBB which converts the C to N thus lowering [C/N] well below the observations, whilst if TDU were to occur in the low-mass (M < 1 Msun) models (we do not find it to occur in our models), the yields would be expected to be C-rich only, which is at odds with the `dual pollution' of C and N generally observed in the CEMPs.

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arXiv2008

Fluorine in carbon-enhanced metal-poor stars: a binary scenario

A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = -2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars. We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars. The abundances observed in HE 1305+0132 can be explained if this star accreted 3-11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2 Msun of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of Z ~ 0.0001 and mass ~ 2 Msun. Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the slow neutron-capture process.

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arXiv2007

Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning

High dispersion near-infrared spectra have been taken of seven highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and SMC in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible.

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arXiv2007

Origin of Abundance Inhomogeneity in Globular Clusters

We numerically investigate abundance properties of the Galactic globular clusters (GCs) by adopting a new ``external pollution'' scenario. In this framework, GCs are assumed to originate in forming low-mass dwarfs embedded in dark matter subhalos at very high redshifts (z) and thus be chemically influenced by field AGB stars of the dwarfs during early GC formation processes. In this external pollution scenario, the ratio of the total mass of infalling gas to that of AGB ejecta during GC formation in a dwarf (s) and the time scale of gas infall (sigma_I) are the most important key parameters that can determine abundance properties of GCs. We mainly investigate the abundance inhomogeneity among light elements (e.g., C, N, O, Na, and Al) of stars in GCs by using the latest stellar yield models of metal-poor AGB stars with and without third dredge-up. Our principal results for the models with no third dredge-up, which are more consistent with observations, are as follows. Both [N/Fe] and [C/Fe] can be diverse among stars within a GC owing to chemical pollution from field AGB stars. [N/Fe] distributions in some GCs can clearly show bimodality whereas [C/Fe] is monomodal in most models. [N/Fe] distributions depend on s such that models with smaller $s$ (i.e., larger mass fraction of AGB ejecta used for GC formation) show the [N/Fe] bimodality more clearly. N-rich, C-poor stars in GCs also have higher He abundances owing to pollution from massive AGB stars with He-rich ejecta. The number fraction of He-rich stars (Y >0.30) is higher for the models with smaller s and shorter sigma_I for 3 < s <24 and 10^5 < sigma_I < 10^7 yr.Our models are in strong disagreement with the observed O-Na anticorrelation.

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arXiv2006

The Destruction of 3He by Rayleigh-Taylor Instability on the First Giant Branch

Low-mass stars, ~1-2 solar masses, near the Main Sequence are efficient at producing 3He, which they mix into the convective envelope on the giant branch and distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the observed cosmic abundance of 3He with the predictions of Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell. In this zone the burning of the 3He left behind by the retreating convective envelope is predominantly by the reaction 3He + 3He -> 4He + 2p, a reaction which, untypically for stellar nuclear reactions, {\it lowers} the mean molecular weight, leading to a local minimum. This local minimum leads to Rayleigh-Taylor instability, and turbulent motion is generated which will continue ultimately up into the normal convective envelope. Consequently material from the envelope is dragged down sufficiently close to the burning shell that the 3He in it is progressively destroyed. Thus we are able to remove the threat that 3He production in low-mass stars poses to the Big Bang nucleosynthesis of 3He. Some slow mixing mechanism has long been suspected, that connects the convective envelope of a red giant to the burning shell. It appears to be necessary to account for progressive changes in the 12C/13C and 14N/12C ratios on the First Giant Branch. We suggest that these phenomena are also due to the Rayleigh-Taylor-unstable character of the 3He-burning region.

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arXiv2006

On the asymptotic giant branch star origin of peculiar spinel grain OC2

Microscopic presolar grains extracted from primitive meteorites have extremely anomalous isotopic compositions revealing the stellar origin of these grains. The composition of presolar spinel grain OC2 is different from that of all other presolar spinel grains. Large excesses of the heavy Mg isotopes are present and thus an origin from an intermediate-mass (IM) asymptotic giant branch (AGB) star was previously proposed for this grain. We discuss the isotopic compositions of presolar spinel grain OC2 and compare them to theoretical predictions. We show that the isotopic composition of O, Mg and Al in OC2 could be the signature of an AGB star of IM and metallicity close to solar experiencing hot bottom burning, or of an AGB star of low mass (LM) and low metallicity suffering very efficient cool bottom processing. Large measurement uncertainty in the Fe isotopic composition prevents us from discriminating which model better represents the parent star of OC2. However, the Cr isotopic composition of the grain favors an origin in an IM-AGB star of metallicity close to solar. Our IM-AGB models produce a self-consistent solution to match the composition of OC2 within the uncertainties related to reaction rates. Within this solution we predict that the 16O(p,g)17F and the 17O(p,a)14N reaction rates should be close to their lower and upper limits, respectively. By finding more grains like OC2 and by precisely measuring their Fe and Cr isotopic compositions, it may be possible in the future to derive constraints on massive AGB models from the study of presolar grains.

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