Flash‐Driven Convective Mixing in Low‐Mass, Metal‐deficient Asymptotic Giant Branch Stars: A New Paradigm for Lithium Enrichment and a Possible<i>s</i>‐Process

Iwamoto, Nobuyuki; Kajino, Toshitaka; Mathews, Grant J.; Fujimoto, Masayuki Y.; Aoki, Wako

doi:10.1086/380989

Cited by 94 publications

(112 citation statements)

References 43 publications

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“…One interesting signature of the PIE is the 7 Li production (Iwamoto et al 2004;Cristallo et al 2009). Unfortunately, observations only allow an upper limit determination of its abundance in these two stars.…”

Section: Discussionmentioning

confidence: 99%

S-process in extremely metal-poor, low-mass stars

Cruz¹,

Serenelli²,

Weiß³

2013

A&A

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Context. Extremely metal-poor (EMP), low-mass stars experience an ingestion of protons into the helium-rich layer during the core He-flash, resulting in the production of neutrons through the reactions 12 C(p, γ) 13 N(β) 13 C(α, n) 16 O. This is a potential site for the production of s-process elements in EMP stars, which does not occur in more metal-rich counterparts. The signatures of s-process elements in the two most iron deficient stars observed to date, HE1327-2326 & HE0107-5240, still await for an explanation. Aims. We investigate the possibility that low-mass EMP stars could be the source of s-process elements observed in extremely iron deficient stars, either as a result of self-enrichment or in a binary scenario as the consequence of a mass transfer episode. Methods. We present evolutionary and post-processing s-process calculations of a 1 M stellar model with metallicities of Z = 0, 10 −8 , and 10 −7 . We assess the sensitivity of nucleosynthesis results to uncertainties in the input physics of the stellar models with particular regard to the details of convective mixing during the core He-flash. Results. Our models provide the possibility of explaining the C, O, Sr, and Ba abundance for the star HE0107-5240 as the result of mass-transfer from a low-mass EMP star. The drawback of our model is that nitrogen would be overproduced and the 12 C/ 13 C abundance ratio would be underproduced in comparison to the observed values if mass would be transferred before the primary star enters the asymptotic giant branch phase. Conclusions. Our results show that low-mass EMP stars cannot be ruled out as companion stars that might have polluted HE1327-2326 and HE0107-5240 and produced the observed s-process pattern. However, more detailed studies of the core He-flash and the proton ingestion episode are needed to determine the robustness of our predictions.

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Section: Discussionmentioning

confidence: 99%

S-process in extremely metal-poor, low-mass stars

Cruz¹,

Serenelli²,

Weiß³

2013

A&A

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“…In models of low-mass AGB stars (1 M M 3 M ) of ½Fe/ H ¼ À2:7, Iwamoto et al (2004) find that an H-flash episode can occur subsequent to the first fully developed He shell flash. The convection produced by the He shell flash in this metal-poor star model can reach the bottom of the overlying H layer and bring protons down into the He intershell region, a region hot enough to induce an H flash.…”

Section: The Extraordinary Abundance Of Lithiummentioning

confidence: 92%

CS 22964−161: A Double‐Lined Carbon‐ ands‐Process–Enhanced Metal‐Poor Binary Star

Thompson¹,

Ivans²,

Bisterzo³

et al. 2008

ApJ

147

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A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low-metallicity Galactic halo object CS 22964À161. We have discovered that CS 22964À161 is a double-lined spectroscopic binary and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the ''Spite plateau.'' The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964À161 abundance set with nucleosynthesis predictions of AGB stars, discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system.

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“…Cassisi et al (1996) report the hydrogen mixing into helium convection during a helium shell flash for a star of M ¼ 0:8 M and log Z ¼ À10; despite the sufficiently low metallicity, their model star has skipped He-FDDM-R, presumably owing to the difference in the input physics affecting the evolution leading to the main He flash at the RGB tip. The progress of He-FDDM-A is investigated by a few other authors (Iwamoto et al 2004, a 4 schematically illustrates the progress of He-FDDM during core flash on the RGB tip (top) and He-FDDM during shell flashes on the AGB (bottom). The physical processes are quite similar, and yet they may differ according to the rate of hydrogen mixing when the helium convection first makes contact with the hydrogen-containing layer (Sweigart 1974).…”

Section: Evolutionary Paths To Surface Carbon Enhancementmentioning

confidence: 99%

“…Thus, we can well set the lower mass limit for the TDU to occur at $1.5 M , although this assumption has little to do with the following discussion. This implies that stars in the middle mass range between $1.5 and $3.5 M experience both, first He-FDDM and then the TDU (Iwamoto et al 2004;Straniero et al 2004). It is also known that in massive AGB stars, the temperature at the bottom of the convective envelope reaches high enough to convert carbon into nitrogen and even to reduce the C/O ratio in the envelope below unity (Boothroyd et al 1993), while the total abundance of CN elements increases as carbon is dredged up via TDU.…”

Section: Evolutionary Paths To Surface Carbon Enhancementmentioning

confidence: 99%

The Origin of Carbon Enhancement and the Initial Mass Function of Extremely Metal‐poor Stars in the Galactic Halo

Komiya

Suda

Minaguchi

et al. 2007

ApJ

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130

220

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It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars constitute a substantial proportion of the extremely metal-poor (EMP) stars of the Galactic halo, and a by far larger proportion than CH stars among Population II stars. We investigate their origin by taking into account an additional evolutionary path to the surface carbon enrichment, triggered by hydrogen engulfment by the helium flash convection, in EMP stars with ½Fe/H P À2:5. This process is distinct from the third dredge-up operating in more metal-rich stars and in EMP stars. In binary systems of EMP stars, the secondary stars become CEMP stars through mass transfer from the low-and intermediatemass primary stars that have developed the surface carbon enhancement. Our binary scenario can predict the variations in the abundances not only for carbon but also for nitrogen and s-process elements and can reasonably explain the observed properties such as the stellar distributions of the carbon abundances, the binary periods, and the evolutionary stages. Furthermore, from the observed frequencies of CEMP stars with and without s-process element enhancement, we demonstrate that the initial mass function of EMP stars needed gives the mean mass $ 10 M under the reasonable assumptions for the distributions of orbital separations and mass ratios of the binary components. This also indicates that the currently observed EMP stars were exclusively born as the secondary members of binaries, making up $10% of EMP binary systems, with mass $ 10 8 M in total; in addition to CEMP stars with white dwarf companions, a significant fraction of them have experienced supernova explosions of their companions. We discuss the implications of the present results for the formation of the Galactic halo.

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Flash‐Driven Convective Mixing in Low‐Mass, Metal‐deficient Asymptotic Giant Branch Stars: A New Paradigm for Lithium Enrichment and a Possibles‐Process

Cited by 94 publications

References 43 publications

S-process in extremely metal-poor, low-mass stars

S-process in extremely metal-poor, low-mass stars

CS 22964−161: A Double‐Lined Carbon‐ ands‐Process–Enhanced Metal‐Poor Binary Star

The Origin of Carbon Enhancement and the Initial Mass Function of Extremely Metal‐poor Stars in the Galactic Halo

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