Low-mass low-metallicity AGB stars as an efficient <i>i</i>-process site explaining CEMP-rs stars

Karinkuzhi, D.; Eck, S. Van; Goriely, Stéphane; Siess, Lionel; Jorissen, A.; Merle, Thibault; Escorza, A.; Masseron, T.

doi:10.1051/0004-6361/202038891

Cited by 40 publications

(54 citation statements)

References 92 publications

(199 reference statements)

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“…In Figure 8, we show again our predicted [Ce/Fe] vs. [Ce/Y], but including m3z1m3bigpoc, m2z1m3-bigpoc, m2z2m3-bigpoc, and the observed abundances from different objects enriched with s-process elements from AGB stars: 10 post-AGBs [54,55], CEMP-s [50] and Ba stars [49,[56][57][58]. Names, references and [Fe/H] for each star are given in Table 4.…”

Section: Postprocessing Nucleosynthesis Calculationsmentioning

confidence: 53%

“…The same difference is visible when comparing m3z1m3 with m31m3-bigpoc in the lower panel, since in this case, the typical 13 C-pocket also differs by a factor of three between the two models. Surface abundances of the Ba star HD 123396 [49] and CEMP-s star HD 26 [50] for two first-peak elements (Y and Zr), two second-peak elements (Ce and Nd), and Pb are also plotted for comparison. The two Pb abundances are only from HD 26, with the higher one being determined including non-LTE correction.…”

Section: Postprocessing Nucleosynthesis Calculationsmentioning

confidence: 99%

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Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

et al. 2021

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The slow neutron-capture process (s-process) efficiency in low-mass AGB stars (1.5 < M/M⊙ < 3) critically depends on how mixing processes in stellar interiors are handled, which is still affected by considerable uncertainties. In this work, we compute the evolution and nucleosynthesis of low-mass AGB stars at low metallicities using the MESA stellar evolution code. The combined data set includes models with initial masses Mini/M⊙=2 and 3 for initial metallicities Z=0.001 and 0.002. The nucleosynthesis was calculated for all relevant isotopes by post-processing with the NuGrid mppnp code. Using these models, we show the impact of the uncertainties affecting the main mixing processes on heavy element nucleosynthesis, such as convection and mixing at convective boundaries. We finally compare our theoretical predictions with observed surface abundances on low-metallicity stars. We find that mixing at the interface between the He-intershell and the CO-core has a critical impact on the s-process at low metallicities, and its importance is comparable to convective boundary mixing processes under the convective envelope, which determine the formation and size of the 13C-pocket. Additionally, our results indicate that models with very low to no mixing below the He-intershell during thermal pulses, and with a 13C-pocket size of at least ∼3 × 10−4 M⊙, are strongly favored in reproducing observations. Online access to complete yield data tables is also provided.

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Section: Postprocessing Nucleosynthesis Calculationsmentioning

confidence: 53%

Section: Postprocessing Nucleosynthesis Calculationsmentioning

confidence: 99%

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

et al. 2021

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“…The solar abundances were taken from Asplund et al (2009). We used the line lists assembled in the framework of the Gaia-ESO survey (Heiter et al 2015;Heiter 2020) and presented in Karinkuzhi et al (2018Karinkuzhi et al ( , 2021; hence, we do not list them here. The abundances were derived under the LTE assumption, but a posteriori NLTE corrections have been added whenever available, as we discuss below.…”

Section: Abundance Analysismentioning

confidence: 99%

“…For the Sr I line at 4811.877 Å (not used by Karinkuzhi et al 2018Karinkuzhi et al , 2021, a log g f of 0.190 has been used (García & Campos 1988). For the 4607.327 Sr I line, Hansen et al (2013) advocate the value of 0.283 for its log g f (from Parkinson et al 1976) because it allows to match the solar Sr abundance.…”

Section: Light S-process Elements: Sr Y and Zrmentioning

confidence: 99%

Sr and Ba abundances: Comparing machine-learning with star-by-star analyses

Karinkuzhi

Eck

Jorissen

et al. 2021

A&A

Self Cite

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Context. A new large sample of 895 s-process-rich candidates out of 454 180 giant stars surveyed by LAMOST at a low spectral resolution (R ~ 1800) has been reported by Norfolk et al. (2019, MNRAS, 490, 2219; hereafter N19). Aims. This study is aimed at confirming the s-process enrichment at the higher resolution (R ~ 86 000) offered by the HERMES-Mercator spectrograph for the 15 brightest targets of the N19 sample, which consists of 13 Sr-only stars and two Ba-only stars (designating stars with only the Sr or only Ba lines strengthened). Methods. Abundances were derived for elements Li, C (including the 12C/13C isotopic ratio), N, O, Na, Mg, Fe, Rb, Sr, Y, Zr, Nb, Ba, La, and Ce, using the TURBOSPECTRUM radiative transfer LTE code with MARCS model atmospheres. Binarity has been tested by comparing the Gaia DR2 radial velocity (epoch 2015.5) with the HERMES velocity obtained 1600–1800 days (about 4.5 yr) later. Results. Among the 15 programme stars, 4 show no s-process overabundances ([X/Fe] < 0.2 dex), 8 show mild s-process overabundances (at least three heavy elements with 0.2 < [X∕Fe] < 0.8), and 3 have strong overabundances (at least three heavy elements with [X/Fe] ≥ 0.8). Among the 13 stars classified as Sr-only by the previous investigation, 4 have no s-process overabundances, 8 are mild barium stars, and 1 is a strong barium star. The two Ba-only stars turn out to be both strong barium stars. Especially noteworthy is the fact that these two are actually dwarf barium stars. Two among the three strong barium stars show clear evidence in support of their binary character, as expected for objects produced through mass-transfer. The results for the no s-process and mild barium stars are more surprising; namely, among the no-s stars, there are two binaries out of four, whereas only one out the eight diagnosed mild barium stars shows a clear signature of radial-velocity variations. Conclusions. Blending effects and saturated lines have to be considered very carefully when using machine-learning techniques, especially when applied to low-resolution spectra. Among the Sr-only stars from the previous study sample, about 60% (8/13) of them can be expected to be true mild barium stars and about 8% to be strong barium stars; this fraction is likely close to 100% for the N19 Ba-only stars (2/2). Therefore, we recommend to limit the sample to N19 Ba-only stars when one needs an unpolluted sample of mass-transfer (i.e., extrinsic) objects.

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“…Theoretical studies by [11,22,23] have shown that the observed abundances of CEMP stars are best explained with a neutron-capture process that occurs with neutron densities similar to that proposed in the i-process. Furthermore, recent studies by [24] have shown that to explain the abundances observed in objects such as CEMP-rs stars, which collectively represent characteristic of extrinsic stars such as CEMP-s, CH, barium, and extrinsic S stars, the i-process plays a crucial role. It is considered that CEMP-rs stars can be explained as being polluted by a low-mass, low-metallicity thermally pulsing AGB companion experiencing i-process nucleosynthesis after proton ingestion during its first convective thermal pulses.…”

Section: Introductionmentioning

confidence: 99%

The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars

Kamath

Winckel

2021

Universe

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Lead (Pb) is predicted to have large over-abundances with respect to other s-process elements in Asymptotic Giant Branch (AGB) stars, especially of low metallicities. However, our previous abundance studies of s-process enriched post-Asymptotic Giant Branch (post-AGB) stars in the Galaxy and the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. For the subset of post-AGB stars with low metallicities the determined upper limits based on detailed chemical abundance studies are much lower than what is predicted. Recent theoretical studies have pointed to the occurrence of the i-process to explain the observed chemical patterns, especially of Pb. A major development, in the observational context, is the release of the GAIA EDR3 parallaxes of the post-AGBs in the Galaxy, which has opened the gateway to systematically studying the sample of stars as a function of current luminosities (which can be linked to their initial masses). In this paper, we succinctly review the Pb discrepancy in post-AGB stars and present the latest observational and theoretical developments in this research landscape.

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Low-mass low-metallicity AGB stars as an efficient i-process site explaining CEMP-rs stars

Cited by 40 publications

References 92 publications

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

Mixing Uncertainties in Low-Metallicity AGB Stars: The Impact on Stellar Structure and Nucleosynthesis

Sr and Ba abundances: Comparing machine-learning with star-by-star analyses

The Missing Lead: Developments in the Lead (Pb) Discrepancy in Intrinsically s-Process Enriched Single Post-AGB Stars

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