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1. Lithium in Turnoff Stars in the Globular Cluster M5: A Quest for Primordial Lithium

   https://doi.org/10.3847/1538-4357/ace975  

   Boesgaard, Ann Merchant; Deliyannis, Constantine P. (August 2023, The Astrophysical Journal)

   Abstract The light element lithium is formed by nucleosynthesis during the Big Bang. Its abundance can help to define the parameters of the early Universe. To find this primordial value, it is necessary to determine Li abundances in the oldest stars because it is readily destroyed by nuclear reactions in stellar interiors. We have made high-resolution (∼45,000) spectroscopic observations of five identical unevolved turnoff stars in the 13 Gyr old globular cluster M5. In our analysis we find a range in Li abundance of a factor of 2; the spread is 5 times the individual error. The comparison of these results with those for turnoff stars from five other globular clusters reveals a similarly large range in Li. Lithium in M5 and the other clusters all have stars above the field star Li plateau, but none are as high as the predictions for primordial Li. The maximum values for Li are the same in all six clusters. Multiple generations of stars are found in many globular clusters; those later generations are expected to have formed from Li-depleted gas. Such second- and later-generation stars would have no Li. However, only one of the six clusters has a few unevolved stars with upper limits on the Li abundance. 

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2. 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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3. Primordial Lithium from Globular Cluster Turnoff Stars: M13 and M71

   https://doi.org/10.3847/1538-4357/ad5a94  

   Boesgaard, Ann_Merchant; Deliyannis, Constantine_P (September 2024, The Astrophysical Journal)

   Abstract During Big Bang nucleosynthesis (BBN) in the first 15 minutes of the Universe, some⁷Li was created along with isotopes of H and He. The determination of that primordial value of Li can help constrain the conditions at that time. The oldest stars with known ages can be found in globular clusters which have well-determined ages through stellar evolution models. High-resolution spectra of Li have been obtained with the Keck I Telescope and HIRES in several unevolved stars in the clusters M13 and M71 withVmagnitudes of 17.6–17.9. Abundances of Li have been determined with spectrum synthesis techniques and show a range of a factor of 4. We attribute that spread to differences in initial angular momentum resulting in different amounts of spin-down, related mixing, and destruction of Li. Our results are compared with similar results for main-sequence and turnoff stars in other globular clusters. The range in age of these clusters is 11.2–14.2 Gyr for an age span of 3 Gyr. These clusters range in [Fe/H] from −0.75 to −2.24 corresponding to a factor of 30 in metallicity. The maximum in the Li abundance for these unevolved stars in all eight clusters is the same corresponding to Li/H = 3.16 × 10⁻¹⁰, while the predicted Li abundance, based on the deuterium abundance and the BBN predictions, is 5.24 × 10⁻¹⁰. 

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4. A critique of the Spite Plateau and the astration of primordial lithium

   https://doi.org/10.1093/mnras/stad936  

   Norris, J. E.; Yong, D.; Frebel, A.; Ryan, S. G. (March 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate the distribution of the lithium abundances, A(Li), of metal-poor dwarf and subgiant stars within the limits 5500 K < Teff < 6700 K, −6.0 < [Fe/H] < −1.5, and log  g ≳ 3.5 (a superset of parameters first adopted by Spite and Spite), using literature data for some 200 stars. We address the problem of the several methods that yield Teff differences up to 350 K, and hence uncertainties of 0.3 dex in [Fe/H] and A(Li), by anchoring Teff to the infrared flux method. We seek to understand the behaviour of A(Li) as a function of [Fe/H] – small dispersion at highest [Fe/H], ‘meltdown’ at intermediate values (i.e. large spread in Li below the Spite Plateau), and extreme variations at lowest [Fe/H]. Decreasing A(Li) is accompanied by increasing dispersion. Insofar as [Fe/H] increases as the Universe ages, the behaviour of A(Li) reflects chaotic star formation involving destruction of primordial Li, which settles to the classic Spite Plateau, with A(Li) ∼ 2.3, by the time the Galactic halo reaches [Fe/H] ∼ −3.0. We consider three phases: (1) first star formation in C-rich environments ([C/Fe] > 2.3), with depleted Li; (2) silicates-dominated star formation and destruction of primordial Li during pre-main-sequence evolution; and (3) materials from these two phases co-existing and coalescing to form C-rich stars with A(Li) below the Spite Plateau, leading to a toy model with the potential to explain the ‘meltdown’. We comment on the results of Mucciarelli et al. on the Lower RGB, and the suggestion of Aguado et al. favouring a lower primordial lithium abundance than generally accepted. 

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5. Origin of the elements

   https://doi.org/10.1007/s00159-022-00146-x  

   Arcones, Almudena; Thielemann, Friedrich-Karl (December 2023, The Astronomy and Astrophysics Review)

   Abstract What is the origin of the oxygen we breathe, the hydrogen and oxygen (in form of water H 2 O) in rivers and oceans, the carbon in all organic compounds, the silicon in electronic hardware, the calcium in our bones, the iron in steel, silver and gold in jewels, the rare earths utilized, e.g. in magnets or lasers, lead or lithium in batteries, and also of naturally occurring uranium and plutonium? The answer lies in the skies. Astrophysical environments from the Big Bang to stars and stellar explosions are the cauldrons where all these elements are made. The papers by Burbidge (Rev Mod Phys 29:547–650, 1957) and Cameron (Publ Astron Soc Pac 69:201, 1957), as well as precursors by Bethe, von Weizsäcker, Hoyle, Gamow, and Suess and Urey provided a very basic understanding of the nucleosynthesis processes responsible for their production, combined with nuclear physics input and required environment conditions such as temperature, density and the overall neutron/proton ratio in seed material. Since then a steady stream of nuclear experiments and nuclear structure theory, astrophysical models of the early universe as well as stars and stellar explosions in single and binary stellar systems has led to a deeper understanding. This involved improvements in stellar models, the composition of stellar wind ejecta, the mechanism of core-collapse supernovae as final fate of massive stars, and the transition (as a function of initial stellar mass) from core-collapse supernovae to hypernovae and long duration gamma-ray bursts (accompanied by the formation of a black hole) in case of single star progenitors. Binary stellar systems give rise to nova explosions, X-ray bursts, type Ia supernovae, neutron star, and neutron star–black hole mergers. All of these events (possibly with the exception of X-ray bursts) eject material with an abundance composition unique to the specific event and lead over time to the evolution of elemental (and isotopic) abundances in the galactic gas and their imprint on the next generation of stars. In the present review, we want to give a modern overview of the nucleosynthesis processes involved, their astrophysical sites, and their impact on the evolution of galaxies. 

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