Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models

AbstractAsymptotic Giant Branch (AGB) stars play a fundamental role in s-process nucleosynthesis during their thermal pulsing phase. The theoretical predictions obtained by AGB models at different masses, s-process efficiencies, dilution factors and initial r-enrichment, are compared with spectroscopic observations of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s), collected from the literature. We discuss here five stars as example, CS 22880-074, CS 22942-019, CS 29526-110, HE 0202-2204 and LP 625-44. All these objects lie on the main sequence or on the giant phase, clearly before the thermally pulsing AGB stage. The hypothesis of mass transfer from an AGB companion, would explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are CEMP(s + r) objects, and are interpreted assuming that the molecular cloud, from which the binary system formed, was already enriched in r-process elements by SNII pollution. In several cases, the observed s-process distribution may be accounted for by AGB models of different initial masses with proper 13C-pocket efficiencies and dilution factors. Na (and Mg), produced via the neutron capture chain starting from 22Ne, may provide an indicator of the initial AGB mass.

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Discovery of technetium- and niobium-rich S stars: The case for bitrinsic stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037481 ◽

2020 ◽

Vol 635 ◽

pp. L6 ◽

Cited By ~ 1

Author(s):

S. Shetye ◽

S. Van Eck ◽

S. Goriely ◽

L. Siess ◽

A. Jorissen ◽

...

Keyword(s):

Mass Transfer ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Chemical Abundances ◽

Trace Evidence ◽

Distinctive Features ◽

The Third ◽

Process Pattern ◽

Process Elements ◽

Hr Diagram

Context. S stars are late-type giants with overabundances of s-process elements. They come in two flavors depending on the presence or lack of presence of technetium (Tc), an element without stable isotopes. Intrinsic S stars are Tc-rich and genuine asymptotic giant branch (AGB) stars, while extrinsic S stars owe their s-process over abundances to the pollution from a former AGB companion, which is now a white dwarf (WD). In addition to Tc, another distinctive feature between intrinsic and extrinsic S stars is the overabundance of niobium (Nb) in the latter class. Indeed, since the mass transfer occurred long ago, 93Zr had time to decay into the only stable isotope of Nb, 93Nb, causing its overabundance. Aims. We discuss the case of the S stars BD+79°156 and o1 Ori, whose specificity lies in sharing the distinctive features of both intrinsic and extrinsic S stars, namely the presence of Tc along with a Nb overabundance. Methods. We used high-resolution HERMES optical spectra, MARCS model atmospheres of S stars, Gaia DR2 parallaxes, and STAREVOL evolutionary tracks to determine the stellar parameters and chemical abundances of the two S stars, and to locate them in the Hertzsprung-Russell (HR) diagram. Results. BD+79°156 is the first clear case of a bitrinsic star, that is, a doubly s-process-enriched object, first through mass transfer in a binary system and then through internal nucleosynthesis that is responsible for the Tc-enrichment in BD+79°156, which must, therefore, have reached the AGB phase of its evolution. This hybrid nature of the s-process pattern in BD+79°156 is supported by its binary nature and its location in the HR diagram that is just beyond the onset of the third dredge-up on the AGB. The Tc-rich, binary S-star o1 Ori with a WD companion was another long-standing candidate for a similar hybrid s-process enrichment. However, the marginal overabundance of Nb derived in o1 Ori does not allow one to trace evidence of large amounts of pollution coming from the AGB progenitor of its current WD companion unambiguously. As a side product, the current study offers a new way of detecting binary AGB stars with WD companions by identifying their Tc-rich nature along with a Nb overabundance.

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Rubidium in Barium Stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3888 ◽

2020 ◽

Author(s):

M P Roriz ◽

M Lugaro ◽

C B Pereira ◽

N A Drake ◽

S Junqueira ◽

...

Keyword(s):

Neutron Source ◽

Slow Neutron ◽

Asymptotic Giant Branch ◽

Neutron Density ◽

Data Sets ◽

Agb Stars ◽

Branching Points ◽

Theoretical Predictions ◽

Low Mass ◽

Chemically Peculiar Stars

Abstract Barium (Ba) stars are chemically peculiar stars that display in their atmospheres the signature of the slow neutron-capture (the s-process) mechanism that occurs in asymptotic giant branch (AGB) stars, a main contributor to the cosmic abundances. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories providing strong constraints for the nucleosynthesis of the s-process in AGB stars. In particular, rubidium (Rb) is a key element for the s-process diagnostic because it is sensitive to the neutron density and therefore its abundance can reveal the main neutron source for the s-process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high resolution spectra (R = 48000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from AGB s-process nucleosynthesis models. The target Ba stars in this study display [Rb/Zr] <0, showing that Rb was not efficiently produced by the activation of branching points. Model predictions from the Monash and FRUITY data sets of low-mass (≲ 4 M⊙) AGB stars are able to cover the Rb abundances observed in the target Ba stars. These observations indicate that the 13C(α,n)16O reaction is the main neutron source of the s-process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companion for IR/OH massive AGB stars.

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Constraining nucleosynthesis in two CEMP progenitors using fluorine

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2610 ◽

2020 ◽

Vol 498 (3) ◽

pp. 3549-3559

Author(s):

Aldo Mura-Guzmán ◽

D Yong ◽

C Abate ◽

A Karakas ◽

C Kobayashi ◽

...

Keyword(s):

Neutron Capture ◽

Slow Neutron ◽

Asymptotic Giant Branch ◽

Capture Process ◽

Upper Limit ◽

Infrared Spectrometer ◽

Binary Evolution ◽

Vibration Rotation ◽

R Process ◽

Process Elements

ABSTRACT We present new fluorine abundance estimations in two carbon enhanced metal-poor (CEMP) stars, HE 1429−0551 and HE 1305+0007. HE 1429−0551 is also enriched in slow neutron-capture process (s-process) elements, a CEMP-s, and HE 1305+0007 is enhanced in both, slow and rapid neutron-capture process elements, a CEMP-s/r. The F abundances estimates are derived from the vibration–rotation transition of the HF molecule at 23358.6 Å using high-resolution infrared spectra obtained with the Immersion Grating Infrared Spectrometer (IGRINS) at the 4-m class Lowell Discovery Telescope. Our results include an F abundance measurement in HE 1429−0551 of A(F) = +3.93 ([F/Fe] = +1.90) at [Fe/H] = −2.53, and an F upper limit in HE 1305+0007 of A(F) < +3.28 ([F/Fe] < +1.00) at [Fe/H] = −2.28. Our new derived F abundance in HE 1429−0551 makes this object the most metal-poor star where F has been detected. We carefully compare these results with literature values and state-of-the-art CEMP-s model predictions including detailed asymptotic giant branch (AGB) nucleosynthesis and binary evolution. The modelled fluorine abundance for HE 1429−0551 is within reasonable agreement with our observed abundance, although is slightly higher than our observed value. For HE 1429−0551, our findings support the scenario via mass transfer by a primary companion during its thermally pulsing phase. Our estimated upper limit in HE 1305+0007, along with data from the literature, shows large discrepancies compared with AGB models. The discrepancy is principally due to the simultaneous s- and r-process element enhancements which the model struggles to reproduce.

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Neutron-capture elements in dwarf galaxies

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937075 ◽

2020 ◽

Vol 634 ◽

pp. A84 ◽

Cited By ~ 2

Author(s):

Á. Skúladóttir ◽

C. J. Hansen ◽

A. Choplin ◽

S. Salvadori ◽

M. Hampel ◽

...

Keyword(s):

Massive Star ◽

Milky Way ◽

Dwarf Galaxies ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Chemical Enrichment ◽

Theoretical Predictions ◽

Low Metallicity ◽

First Time ◽

Best Fit

The slow (s) and intermediate (i) neutron (n) capture processes occur both in asymptotic giant branch (AGB) stars, and in massive stars. To study the build-up of the s- and i-products at low metallicity, we investigate the abundances of Y, Ba, La, Nd, and Eu in 98 stars, at −2.4 < [Fe/H] < −0.9, in the Sculptor dwarf spheroidal galaxy. The chemical enrichment from AGB stars becomes apparent at [Fe/H] ≈ −2 in Sculptor, and causes [Y/Ba], [La/Ba], [Nd/Ba] and [Eu/Ba] to decrease with metallicity, reaching subsolar values at the highest [Fe/H] ≈ −1. To investigate individual nucleosynthetic sites, we compared three n-rich Sculptor stars with theoretical yields. One carbon-enhanced metal-poor (CEMP-no) star with high [Sr, Y, Zr] > +0.7 is best fit with a model of a rapidly-rotating massive star, the second (likely CH star) with the i-process, while the third has no satisfactory fit. For a more general understanding of the build-up of the heavy elements, we calculate for the first time the cumulative contribution of the s- and i-processes to the chemical enrichment in Sculptor, and compare with theoretical predictions. By correcting for the r-process, we derive [Y/Ba]s/i = −0.85 ± 0.16, [La/Ba]s/i = −0.49 ± 0.17, and [Nd/Ba]s/i = −0.48 ± 0.12, in the overall s- and/or i-process in Sculptor. These abundance ratios are within the range of those of CEMP stars in the Milky Way, which have either s- or i-process signatures. The low [Y/Ba]s/i and [La/Ba]s/i that we measure in Sculptor are inconsistent with them arising from the s-process only, but are more compatible with models of the i-process. Thus we conclude that both the s- and i-processes were important for the build-up of n-capture elements in the Sculptor dwarf spheroidal galaxy.

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Asymptotic Giant Branch Stars in the Magellanic Clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900200855 ◽

1991 ◽

Vol 148 ◽

pp. 363-364

Author(s):

Neill Reid ◽

J. R. Mould

Keyword(s):

Large Magellanic Cloud ◽

Magellanic Cloud ◽

Magellanic Clouds ◽

Asymptotic Giant Branch ◽

Red Giants ◽

Asymptotic Giant Branch Stars ◽

Agb Stars ◽

Satellite Systems ◽

Luminosity Functions ◽

Theoretical Predictions

Since the pioneering objective prism surveys by Westerlund (1960) and Blanco et al. (1980), the Magellanic Clouds have proved a fruitful site for exploring the evolution of AGB stars. We have used photometric techniques to extend the prism C-star surveys to M- and S-type AGB stars, constructing luminosity functions and obtaining spectra of individual stars for comparison with theoretical predictions. We have concentrated on the Large Magellanic Cloud (LMC), but we have recently obtained observations of luminous red giants in a region of the Small Magellanic Cloud (SMC). In this paper we compare the results from these studies of the two satellite systems.

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Unusual neutron-capture nucleosynthesis in a carbon-rich Galactic bulge star

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834241 ◽

2019 ◽

Vol 622 ◽

pp. A159 ◽

Cited By ~ 3

Author(s):

Andreas Koch ◽

Moritz Reichert ◽

Camilla Juul Hansen ◽

Melanie Hampel ◽

Richard J. Stancliffe ◽

...

Keyword(s):

Neutron Capture ◽

Galactic Halo ◽

Asymptotic Giant Branch ◽

Galactic Bulge ◽

Element Abundances ◽

Intermediate Neutron ◽

Low Mass ◽

Process Component ◽

Process Elements ◽

The Impact

Metal-poor stars in the Galactic halo often show strong enhancements in carbon and/or neutron-capture elements. However, the Galactic bulge is notable for its paucity of these carbon-enhanced metal-poor (CEMP) and/or CH-stars, with only two such objects known to date. This begs the question whether the processes that produced their abundance distribution were governed by a comparable nucleosynthesis in similar stellar sites as for their more numerous counterparts in the halo. Recently, two contenders of these classes of stars were discovered in the bulge, at [Fe/H] = −1.5 and −2.5 dex, both of which show enhancements in [C/Fe] of 0.4 and 1.4 dex (respectively), [Ba/Fe] in excess of 1.3 dex, and also elevated nitrogen. The more metal-poor of the stars can be well matched by standard s-process nucleosynthesis in low-mass asymptotic giant branch (AGB) polluters. The other star shows an abnormally high [Rb/Fe] ratio. Here, we further investigate the origin of the abundance peculiarities in the Rb-rich star by new, detailed measurements of heavy element abundances and by comparing the chemical element ratios of 36 species to several models of neutron-capture nucleosynthesis. The i-process with intermediate neutron densities between those of the slow (s-) and rapid (r)-neutron-capture processes has been previously found to provide good matches of CEMP stars with enhancements in both r- and s-process elements (class CEMP-r/s), rather than invoking a superposition of yields from the respective individual processes. However, the peculiar bulge star is incompatible with a pure i-process from a single ingestion event. Instead, it can, statistically, be better reproduced by more convoluted models accounting for two proton ingestion events, or by an i-process component in combination with s-process nucleosynthesis in low-to-intermediate mass (2–3 M⊙) AGB stars, indicating multiple polluters. Finally, we discuss the impact of mixing during stellar evolution on the observed abundance peculiarities.

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Heavy-Element Nucleosynthesis in AGB Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900002710 ◽

2000 ◽

Vol 177 ◽

pp. 443-448

Author(s):

Verne V. Smith

Keyword(s):

Neutron Source ◽

Heavy Element ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Process Elements

The production of certain neutron-rich elements heavier than iron occurs during He shell-burning on the asymptotic giant branch (AGB). These neutron captures occur at rather low neutron densities and, thus, the resulting heavy-element nucleosynthesis is characterisitic of the so-called s-process. Abundance analyses of the s-process elements in AGB stars can reveal details of the neutron densities and the stage of AGB evolution at which s-processing occurs, as well as the nature of the neutron source. These details derived from observations can constrain models of stars evolving along the AGB. Recent results concerning the nature of the s-process as a function of metallicity and the nature of the neutron source are reviewed.

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(Pre)-white dwarf stars as measuring tools for yields of AGB nucleosynthesis

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320000617 ◽

2019 ◽

Vol 15 (S357) ◽

pp. 158-161

Author(s):

Lisa Löbling

Keyword(s):

White Dwarf ◽

Neutron Capture ◽

Convection Zone ◽

Slow Neutron ◽

Asymptotic Giant Branch ◽

Heavy Elements ◽

Agb Stars ◽

Subsequent Evolution ◽

Dwarf Stars ◽

White Dwarf Stars

AbstractIn the helium-rich intershell region of asymptotic giant branch (AGB) stars, slow neutron-capture nucleosynthesis produces heavy elements beyond iron. If the stars experience a final-flash of the He-burning shell, a pulse-driven convection zone establishes, the stars become hydrogen-deficient and exhibit former intershell material at their surfaces. In their subsequent evolution towards the white-dwarf cooling sequence, but still at constant luminosity, a strong stellar wind prevents diffusion to wipe out the information about AGB yields. We present and interpret the analysis results of hydrogen-rich and -deficient post-AGB stars, discuss difficulties in their analysis and review the implications on the understanding of post-AGB evolution.

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The role of AGB stars in Galactic and cosmic chemical enrichment

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318007184 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 247-257

Author(s):

Chiaki Kobayashi ◽

Christopher J. Haynes ◽

Fiorenzo Vincenzo

Keyword(s):

Electron Capture ◽

Galaxy Formation ◽

Neutron Capture ◽

Slow Neutron ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Capture Process ◽

Chemical Enrichment ◽

Cno Abundances

AbstractThe role of asymptotic giant branch (AGB) stars in chemical enrichment is significant for producing 12,13C, 14N, F, 25,26Mg, 17O and slow neutron-capture process (s-process) elements. The contribution from super-AGB stars is negligible in classical, one-zone chemical evolution models, but the mass ranges can be constrained through the contribution from electron-capture supernovae and possibly hybrid C+O+Ne white dwarfs, if they explode as Type Iax supernovae. In addition to the recent s-process yields of AGB stars, we include various sites for rapid neutron-capture processes (r-processes) in our chemodynamical simulations of a Milky Way type galaxy. We find that neither electron-capture supernovae or neutrino-driven winds are able to adequately produce heavy neutron-capture elements such as Eu in quantities to match observations. Both neutron-star mergers (NSMs) and magneto-rotational supernovae (MRSNe) are able to produce these elements in sufficient quantities. Using the distribution in [Eu/(Fe, α)] – [Fe/H], we predict that NSMs alone are unable to explain the observed Eu abundances, but may be able to together with MRSNe. In order to discuss the role of long-lifetime sources such as NSMs and AGB stars at the early stages of galaxy formation, it is necessary to use a model that can treat inhomogeneous chemical enrichment, such as in our chemodynamical simulations. In our cosmological, chemodynamical simulations, we succeed in reproducing the observed N/O-O/H relations both for global properties of galaxies and for local inter-stellar medium within galaxies, without rotation of stars. We also predict the evolution of CNO abundances of disk galaxies, from which it will be possible to constrain the star formation histories.

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Heavy elements in planetary nebulae: A theorist's gold mine

Proceedings of the International Astronomical Union ◽

10.1017/s1743921312010824 ◽

2011 ◽

Vol 7 (S283) ◽

pp. 127-130

Author(s):

Amanda I. Karakas ◽

Maria Lugaro

Keyword(s):

Neutron Capture ◽

Planetary Nebulae ◽

Asymptotic Giant Branch ◽

Heavy Elements ◽

Gold Mine ◽

Agb Stars ◽

Capture Process ◽

Wide Range ◽

Model Predictions

AbstractObservations of planetary nebulae have revealed a wealth of information about the composition of heavy elements synthesized by the slow neutron capture process (the s process). In some of these nebulae the abundances of neutron-capture elements are enriched by factors of 10 to 30 times the solar value, indicating that these elements were produced in the progenitor star while it was on the asymptotic giant branch (AGB). In this proceedings we summarize results of our recent full s-process network predictions covering a wide range of progenitor masses and metallicities. We compare our model predictions to observations and show how this can provide important insights into nucleosynthesis processes occurring deep within AGB stars.

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