Abstract
Due to their production sites, as well as to how they are processed and destroyed in stars, the light elements are excellent tools to investigate a number of crucial issues in modern astrophysics: from stellar structure and non-standard processes at work in stellar interiors to age dating of stars; from pre-main sequence evolution to the star formation histories of young clusters and associations and to multiple populations in globular clusters; from Big Bang nucleosynthesis to the formation and chemical enrichment history of the Milky Way Galaxy and its populations, just to cite some relevant examples. In this paper, we focus on lithium, beryllium, and boron (LiBeB) and on carbon, nitrogen, and oxygen (CNO). LiBeB are rare elements, with negligible abundances with respect to hydrogen; on the contrary, CNO are among the most abundant elements in the Universe, after H and He. Pioneering observations of light-element surface abundances in stars started almost 70 years ago and huge progress has been achieved since then. Indeed, for different reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun; however, the advent of state-of-the-art ground- and space-based instrumentation has allowed the determination of high-quality abundances in stars of different type, belonging to different Galactic populations, from metal-poor halo stars to young stars in the solar vicinity and from massive stars to cool dwarfs and giants. Noticeably, the recent large spectroscopic surveys performed with multifiber spectrographs have yielded detailed and homogeneous information on the abundances of Li and CNO for statistically significant samples of stars; this has allowed us to obtain new results and insights and, at the same time, raise new questions and challenges. A complete understanding of the light-element patterns and evolution in the Universe has not been still achieved. Perspectives for further progress will open up soon thanks to the new generation instrumentation that is under development and will come online in the coming years.
Highlights
The most abundant isotope of lithium, 7Li1, is the heaviest element created during Big Bang nucleosynthesis (BBN) and its primordial abundance can be used to probe the standard model of cosmology (e.g., Wagoner, 1973, Steigman, 2006, and references therein)
Light Elements interstellar medium (Reeves et al, 1970; Meneguzzi et al, 1971) that can happen in two different channels, namely, a direct process in which protons and α-particles in Galactic cosmic rays (GCRs) collide with CNO nuclei in the ISM or an inverse process where CNO nuclei instead collide with protons and α-particle in the ISM
We conclude this section by noting that the three topics discussed above are tightly linked; a complete understanding of mixing mechanisms and Li depletion in stars of different mass and metallicity would allow a final answer to be put on the cosmological Li problem and, at the same time, the secure determination of the initial value of Li in the early Galaxy, which is critical in models of Galactic evolution
Summary
The most abundant isotope of lithium, 7Li1, is the heaviest element created during Big Bang nucleosynthesis (BBN) and its primordial abundance can be used to probe the standard model of cosmology (e.g., Wagoner, 1973, Steigman, 2006, and references therein). They are crucial for several astrophysical fields, including, e.g., the formation of planetary systems and astrobiology (e.g., Lodders and Fegley, 2002; Suárez-Andrés et al, 2016), stellar structure and evolution (e.g., Salaris and Cassisi, 2005; Busso et al, 2007; Charbonnel and Zahn, 2007; Lattanzio et al, 2015; Lagarde et al, 2019), stellar nucleosynthesis (e.g., van den Hoek and Groenewegen, 1997; Meynet and Maeder, 2002), and Galactic chemical evolution (e.g., Chiappini et al, 2003; Vincenzo et al, 2016) Their origins are still debated and the role in their production in stars with different masses, metallicities, and rotational velocities is still not definitively settled. The latter include the multiple populations of globular clusters (e.g., Gratton et al, 2004, Gratton et al, 2012; Renzini 2008; D’Orazi et al, 2015; Milone et al, 2017; D’Antona et al, 2019; Milone et al, 2020a, Milone et al, 2020b); the abundances of C, N, and O from emission-line spectra of H II regions and planetary nebulae (e.g., Toribio San Cipriano et al, 2016; Toribio San Cipriano et al, 2017; Esteban et al, 2018, Esteban et al, 2019, Esteban et al, 2020; Stanghellini and Haywood, 2018); post-main sequence Li evolution (e.g., Charbonnel et al, 2020; Deepak and Reddy, 2020; Kumar and Reddy, 2020; and references therein), Li-rich giant stars and their nature (e.g., Casey et al, 2016, Casey et al, 2019; Gao et al, 2019; Gonçalves et al, 2020; Jorissen et al, 2020; Martell et al, 2020; Sanna et al, 2020; Wheeler et al, 2020; Yan et al, 2020); boron abundances in massive stars (e.g., Proffitt et al, 2016, and references therein); light elements in the context of exoplanet hosting stars (e.g., Delgado Mena et al, 2012; Delgado Mena et al, 2014); the lithium depletion boundary (see, e.g., Stauffer, 2000; Lodieu, 2020; Martín et al, 2020; an references therein); 6Li/7Li isotopic ratio and its implications for Li nucleosynthesis (e.g., González Hernández et al, 2019, and references therein)
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