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1. Enhancement of lithium in red clump stars by the additional energy loss induced by new physics

   https://doi.org/10.1093/mnras/stab595  

   Mori, Kanji; Kusakabe, Motohiko; Balantekin, A Baha; Kajino, Toshitaka; Famiano, Michael A (March 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Since 7Li is easily destroyed in low temperatures, the surface lithium abundance decreases as stars evolve. This is supported by the lithium depletion observed in the atmosphere of most red giants. However, recent studies show that almost all of red clump stars have high lithium abundances A(Li) > −0.9, which are not predicted by the standard theory of the low-mass stellar evolution. In order to reconcile the discrepancy between the observations and the model, we consider additional energy loss channels that may come from physics beyond the Standard Model. A(Li) slightly increases near the tip of the red giant branch even in the standard model with thermohaline mixing because of the 7Be production by the Cameron–Fowler mechanism, but the resultant 7Li abundance is much lower than the observed values. We find that the production of 7Be becomes more active if there are additional energy loss channels, because themohaline mixing becomes more efficient and a heavier helium core is formed. 

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2. Detection of 7Be ii in the Small Magellanic Cloud

   https://doi.org/10.1093/mnras/stab3761  

   Izzo, Luca; Molaro, Paolo; Cescutti, Gabriele; Aydi, Elias; Selvelli, Pierluigi; Harvey, Eamonn; Agnello, Adriano; Bonifacio, Piercarlo; Della Valle, Massimo; Guido, Ernesto; et al (December 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We analyse high-resolution spectra of two classical novae that exploded in the Small Magellanic Cloud (SMC). 7Be ii resonance transitions are detected in both ASASSN-19qv and ASASSN-20ni novae. This is the first detection outside the Galaxy and confirms that thermo-nuclear runaway reactions, leading to the 7Be formation, are effective also in the low-metallicity regime, characteristic of the SMC. Derived yields are of N(7Be = 7Li)/N(H)  = (5.3 ± 0.2) × 10−6 which are a factor 4 lower than the typical values of the Galaxy. Inspection of two historical novae in the Large Magellanic Cloud observed with IUE in 1991 and 1992 showed also the possible presence of 7Be and similar yields. For an ejecta of MH, ej = 10−5 M⊙, the amount of 7Li produced is of $$M_{^7 Li} = (3.7 \pm 0.6) \times 10^{-10}$$ M⊙ per nova event. Detailed chemical evolutionary model for the SMC shows that novae could have made an amount of lithium in the SMC corresponding to a fractional abundance of A(Li) ≈ 2.6. Therefore, it is argued that a comparison with the abundance of Li in the SMC, as measured by its interstellar medium, could effectively constrain the amount of the initial abundance of primordial Li, which is currently controversial. 

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3. Mass Matters: No Evidence for Ubiquitous Lithium Production in Low-mass Clump Giants

   https://doi.org/10.3847/1538-4357/ac70c8  

   Chanamé, Julio; Pinsonneault, Marc H.; Aguilera-Gómez, Claudia; Zinn, Joel C. (July 2022, The Astrophysical Journal)

   Abstract Known sources of lithium (Li) in the universe include the Big Bang, novae, asymptotic giant branch stars, and cosmic-ray spallation. During their longer-lived evolutionary phases, stars are not expected to add to the Li budget of the Galaxy, but to largely deplete it. In this context, recent analyses of Li data from GALAH and LAMOST for field red clump (RC) stars have concluded that there is the need for a new production channel of Li, ubiquitous among low-mass stars, and that would be triggered on the upper red giant branch (RGB) or at helium ignition. This is distinct from the Li-rich giant problem and reflects bulk RC star properties. We provide an analysis of the GALAH Li data that accounts for the distribution of progenitor masses of field RC stars observed today. Such progenitors are different than today’s field RGB stars. Using standard post-main-sequence stellar evolution, we show that the distribution of Li among field RC giants as observed by GALAH is consistent with standard model predictions, and does not require new Li production mechanisms. Our model predicts a large fraction of very low Li abundances from low-mass progenitors, with higher abundances from higher mass ones. Moreover, there should be a large number of upper limits for RC giants, and higher abundances should correspond to higher masses. The most recent GALAH data indeed confirm the presence of large numbers of upper limits, and a much lower mean Li abundance in RC stars, in concordance with our interpretation. 

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4. Enhanced extra mixing in low-mass stars approaching the RGB tip and the problem of Li-rich red-clump stars

   https://doi.org/10.1093/mnras/stae2407  

   Denissenkov, Pavel_A; Blouin, Simon; Herwig, Falk; Stott, Jacob; Woodward, Paul_R (October 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT A few per cent of red giants are enriched in lithium with $$A(\mathrm{Li}) \gt 1.5$$. Their evolutionary status has remained uncertain because these Li-rich giants can be placed both on the red giant branch (RGB) near the bump luminosity and in the red clump (RC) region. However, thanks to asteroseismology, it has been found that most of them are actually RC stars. Starting at the bump luminosity, RGB progenitors of the RC stars experience extra mixing in the radiative zone separating the H-burning shell from the convective envelope followed by a series of convective He-shell flashes at the RGB tip, known as the He-core flash. The He-core flash was proposed to cause fast extra mixing in the stars at the RGB tip that is needed for the Cameron–Fowler mechanism to produce Li. We propose that the RGB stars are getting enriched in Li by the RGB extra mixing that is getting enhanced and begins to produce Li, instead of destroying it, when the stars are approaching the RGB tip. After a discussion of several mechanisms of the RGB extra mixing, including the joint operation of rotation-driven meridional circulation and turbulent diffusion, the azimuthal magnetorotational instability (AMRI), thermohaline convection, buoyancy of magnetic flux tubes, and internal gravity waves, and based on results of (magneto-) hydrodynamics simulations and asteroseismology observations, we are inclined to conclude that it is the mechanism of the AMRI or magnetically enhanced thermohaline convection, that is most likely to support our hypothesis. 

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5. The 3-Helium Problem

   Balser, D.; Bania, T. (January 2020, American Astronomical Society meeting #235)

   The light element 3He is produced in copious amounts during the first three minutes after the Big Bang. The 3He abundance is then modified primarily by nucleosynthesis in stars, whereby low-mass stars (< 2 solar masses) are expected to produce 3He due to the astration of deuterium. The higher temperatues in more massive stars fuse 3He completely into 4He thus destroying 3He. Measurements of 3He are made via observations of the hyperfine transition of 3He+ at 3.46 cm. Observations of 3He+ in HII regions located throughout the Milky Way disk reveal very little variation in the 3He/H abundance ratio — the "3He Plateau", indicating that the net effect of 3He production in stars is negligible. This is in contrast to much higher 3He/H abundance ratios found in some planetary nebula (PNe). This discrepancy is known as the "3He Problem." One solution to this problem is that thermohaline mixing occurs just above the hydrogen-burning shell to process 3-Helium: 3He(3He, 2p)4He. Thermohaline mixing is a double-diffusive instability that occurs in oceans and is also called thermohaline convection. We discuss how more accurate observations of the 3He/H abundance ratio can constrain stellar evolution models that include thermohaline mixing. 

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