WIYN open cluster study: The old open cluster, NGC 188, and a re-evaluation of Lithium-richness among red giants

Sun, Qiwei; Deliyannis, Constantine P.; Twarog, Bruce A.; Anthony-Twarog, Barbara J.; Cummings, Jeffrey D.; Steinhauer, A.

doi:10.1093/mnras/stac1251

Cited by 15 publications

(13 citation statements)

References 112 publications

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“…It was also found that the Li abundances of a fraction of giant stars are higher than 1.5 dex, which is the traditional definition of a so-called Li-rich giant star. Although some recent researches show that this threshold is flawed (e.g., Chanamé et al 2022;Sun et al 2022), we still follow this threshold in this work. The percentage of Li-rich giant stars is around ∼1% (Brown et al 1989;Gao et al 2019;Martell et al 2021).…”

Section: Introductionmentioning

confidence: 81%

Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

Shi

et al. 2023

ApJ

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The study of Li phenomena in red clump (RC) stars can give us a deeper understanding of the structure and evolution of stars. Chanamé et al. explained the RC Li abundance distributions naturally using only standard post-main-sequence (MS) Li evolution models when the distribution of progenitor masses and the depletion of Li during the MS observed in MS stars were considered, thus neither an extra Li depletion nor Li creation mechanism is required. Nevertheless, it is interesting to consider the effects of mixing caused by some extra mechanisms. By constructing different models, we find that the mixing caused by internal gravity waves can explain the observed Li abundances of RC stars with low-mass progenitors. To explain this, we rely on the extra mixing induced by internal gravity waves that are excited at the bottom of the convective envelope during the red giant branch (RGB) stage. During the RGB stage, introducing internal gravity waves can improve the diffusion coefficient and strengthen the mixing effect. The effective enrichment of Li occurs during the late RGB stage and requires the diffusion coefficient of the H-burning shell to reach ∼108 cm2 s−1. Our models predict that the Li abundance decreases from ∼1.5 to ∼0.0 dex at the end of the core He-burning stage, thereby revealing ∼99% of the observed Li abundance distribution. Thermohaline mixing regulates the Li abundance of RGB stars, which combined with internal gravity waves can explain the Li abundances of most giants.

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Section: Introductionmentioning

confidence: 81%

Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

Shi

et al. 2023

ApJ

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“…Given that planet engulfment is not expected to decrease the carbon isotopic ratio of the star, this seems an unlikely explanation for the abundance of this particular giant. This Li-rich giant is an excellent example of why 12 C/ 13 C should definitely be considered when discussing the source of Li-enrichment and anomalous surface abundances in evolved stars (e.g., Sun et al 2022).…”

Section: Discussion: Extra Mixing In the Rgbmentioning

confidence: 99%

Evidence of extra mixing in field giants as traced by the lithium and carbon isotope ratio

Aguilera-Gómez

Jones

Chanamé

2023

A&A

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Context. Although not predicted by standard stellar evolution, the surface abundance of light elements, such as lithium (Li), carbon, and nitrogen, changes during the red giant branch (RGB) as a result of extra mixing. This is usually associated with thermohaline mixing acting after the RGB bump. Peculiar Li-enriched RGB stars might also be related to either enhanced mixing or pollution from external sources. Aims. We measure the Li abundance and carbon isotopic ratio 12C/13C in a sample of 166 field red giants with −0.3 ≤ [Fe/H] ≤ 0.2, targeted by the EXPRESS radial velocity program to analyze the effects of extra mixing. Methods. We measured the abundances with spectral synthesis using high-quality spectra. Multiple-epoch observations needed for exoplanet detection were used to decrease the effects of telluric contamination in 12C/13C measurements. Results. Due to the prevalence of upper limits, the Li abundance pattern is complicated to interpret, but the comparison between RGB and core He-burning giants shows effects of mixing consistent with thermohaline. The most Li-enriched giant in the sample, classified as a RGB star close to the RGB bump, has low 12C/13C. Given that the 12C/13C should not be affected by planet engulfment, this does not seem to be the source of the high Li. There is a decreasing correlation between mass and 12C/13C in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also show a correlation between 12C/13C and [Fe/H]. There is no evident impact of binarity either on Li or on 12C/13C. Conclusions. Our sample shows behavior consistent with additional mixing acting after the RGB bump. The 12C/13C adds new clues which can be used to describe extra mixing, and it could well be the best tool to study mixing in giants. Additional measurements of 12C/13C in field stars would greatly improve our ability to compare data with models and understand mixing mechanisms.

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“…The C 12 /C 13 should definitely be considered when discussing the source of Li-enrichment and anomalous surface abundances in evolved stars (e.g., Sun et al 2022).…”

Section: Discussion: Extra-mixing In the Rgbmentioning

confidence: 99%

Evidence of extra-mixing in field giants as traced by lithium and carbon isotope ratio

Aguilera-Gómez¹,

Jones²,

Chanamé³

2022

Preprint

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Context.Although not predicted by standard stellar evolution, it is known that the surface abundance of light elements, such as lithium, carbon, and nitrogen, changes during the red giant branch (RGB) as a result of extra-mixing. This is associated usually with thermohaline mixing acting after the RGB bump. Peculiar lithium-enriched RGB stars might also be related to either enhanced extra-mixing or pollution from external sources. Aims. We analyze the lithium (Li) abundance and carbon isotopic ratio C 12 /C 13 in a sample of 166 field red giants with −0.3 ≤[Fe/H]≤ 0.2, targeted by the EXPRESS radial velocity program to analyze the effects of extra-mixing. Methods. We measure the abundances with spectral synthesis using high-resolution and signal-to-noise spectra. Multiple-epoch observations needed for exoplanet detection are used to decrease the effects of telluric contamination in C 12 /C 13 measurements.Results. The Li abundance pattern is complicated to interpret, but the comparison between RGB and core-He burning giants shows the effects of extra-mixing consistent with thermohaline. The most Li-enriched giant in the sample was classified as a RGB star close to the luminosity function bump with low C 12 /C 13 . Given that the C 12 /C 13 should not be affected by external mechanisms, contamination by an external source, such as a planet, does not seem to be the source of the high Li. The carbon isotopic ratio presents new clues to describe the extra-mixing. There is a decreasing correlation between mass and C 12 /C 13 in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also shows a correlation between C 12 /C 13 and [Fe/H]. There is no evident impact of binarity either on Li or C 12 /C 13 . Conclusions. Our sample shows behavior that is consistent with additional mixing acting after the RGB bump. Lithium, which is heavily affected by rotational mixing and other processes, does not show a clear trend. Instead, the carbon isotope ratio could be the best tool to study mixing in red giants. Additional measurements of C 12 /C 13 in field stars would greatly improve our ability to compare with models and understand the mixing mechanisms.

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WIYN open cluster study: The old open cluster, NGC 188, and a re-evaluation of Lithium-richness among red giants

Cited by 15 publications

References 112 publications

Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

Evidence of extra mixing in field giants as traced by the lithium and carbon isotope ratio

Evidence of extra-mixing in field giants as traced by lithium and carbon isotope ratio

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