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The Evolution of Lithium: Implications of a Universal Spite Plateau

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The Evolution of Lithium: Implications of a Universal Spite Plateau MNRAS 000,1–7 (2021) Preprint 26 April 2021 Compiled using MNRAS LATEX style file v3.0 The evolution of Lithium: implications of a universal Spite plateau Francesca Matteucci,1,2,3¢ Marta Molero,1 David S. Aguado4 and Donatella Romano5 1Department of Physics, University of Trieste, Italy 2Italian National Institute for Astrophysics (I.N.A.F.) Trieste, Italy 3Italian National Institute for Nuclear Physics (I.N.F.N.), Trieste, Italy 4Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 5Italian National Institute for Astrophysics (I.N.A.F.) Bologna, Italy Accepted XXX. Received YYY; in original form ZZZ ABSTRACT The cosmological 7Li problem consists in explaining why the primordial Li abundance, as predicted by the standard Big Bang nucleosynthesis theory with constraints from WMAP and Planck, is a factor of 3 larger than the Li abundance measured in the stars of the Spite plateau defined by old, warm dwarf stars of the Milky Way halo. Several explanations have been proposed to explain this difference, including various Li depletion processes as well as non standard Big Bang nucleosynthesis, but the main question remains unanswered. In this paper, we present detailed chemical evolution models for dwarf spheroidal and ultra faint galaxies, compute the galactic evolution of 7Li abundance in these objects and compare it with observations of similar objects. In our models, Li is mainly produced by novae and cosmic rays and to a minor extent by low and intermediate mass stars. We adopt the yield combination which best fits the Li abundances in the Milky Way stars. It is evident that the observations of dwarf objects define a Spite plateau, identical to that observed in the Milky Way, thus suggesting that the Spite plateau could be a universal feature and its meaning should be discussed. The predictions of our models for dwarf galaxies, are obtained by assuming as Li primordial abundance either the one detected in the atmospheres of the oldest halo stars (Spite plateau; A(Li) ∼ 2.2 dex), or the one from cosmological observations (WMAP; A(Li) ∼ 2.66 dex). Finally, we discuss the implications of the universality of the Spite plateau results. Key words: stars:abundances – galaxies:abundances – cosmology:primordial nucleosynthesis 1 INTRODUCTION lack of Li destruction. The upper envelope of the data in the Figure 3 of Rebolo et al.(1988) shows a plateau for [Fe/H] < -1.0 dex, The study of the evolution of the abundance of 7Li is very important followed by a steep rise of the Li abundance in disk stars, reaching for cosmology because its primordial abundance, together with the the highest value in TTauri stars, where Li is not yet destroyed, abundances of He and D, allows us to impose constraints on the and meteorites, and this value is a factor of ten higher than the Spite baryonic density of the Universe (Ω ). The study of the evolution of 1 plateau. An alternative interpretation of this plot is that the primordial 7Li abundance has started with Spite & Spite(1982), Spite & Spite Li abundance is that observed in meteorites and the Li in halo stars is (1986), Bonifacio & Molaro(1997) and Bonifacio et al.(2007), who the result of its destruction during stellar evolution (see Mathews et al. identified a clear plateau for the Li abundance in dwarf halo stars. 1990. However, in this case a non-standard Big Bang theory would Rebolo et al.(1988) showed, for the first time, the relation of the be required. Before the WMAP results, the theoretical primordial abundance of Li versus metallicity ([Fe/H]) for stars in the solar abundances of all light elements were in agreement with the observed neighbourhood, and suggested that the upper envelope of such a arXiv:2104.11504v1 [astro-ph.GA] 23 Apr 2021 values (e. g. Ryan 2000), thus suggesting a value for the baryon to plot indicates the evolution of the Li abundance during the Galactic photon ratio ([) and therefore for Ω . The WMAP results (Spergel lifetime. This is because the large spread in Li abundances, visible 1 et al. 2003), later confirmed by Planck (Coc et al. 2014), suggested especially in disk stars, is the result of different levels of Li destruction directly a precise value for [ and therefore a value for the primordial in stars in different evolutionary stages. This element, in fact, is easily Li abundance. Such a value (A(Li)= 2.66-2.73 dex) is higher by a destroyed by nuclear reactions in stars. Such a spread is not evident, factor of 3 than the one suggested by the Spite plateau (A(Li)∼ 2.2 instead, in the Li abundances of halo stars, and the interpretation of dex), thus creating what is called the Cosmological Lithium Problem: this fact has been that Li in halo stars represents the primordial Li why halo stars show an almost constant Li abundance lower than the abundance, because the plateau value was, at that time, in agreement primordial one? Before discussing the various proposed solutions with Big Bang nucleosynthesis (BBN) theory. This fact and the to the Li problem, we remind how 7Li is produced in stars: there small observed spread in halo stars were interpreted as due to the is practically only one channel for Li production and in particular lower convection occurring in low metallicity stars with consequent there should be a site where the reaction 3퐻4 ¸ U !7 퐵4 ¸ W can occur, but 7Be should be rapidly transported by convection in regions of lower temperatures where it decays into 7Li, and this is known ¢ E-mail: [email protected] (KTS) © 2021 The Authors 2 F. Matteucci et al. as the Cameron & Fowler(1971) mechanism. The proposed stellar metal-rich stars with higher Li abundances are likely to be negligible. sources of 7Li are: red giant (RG), asymptotic giant branch stars The common feature of the Spite plateau among dwarf galaxies and (AGB), novae and supernovae core-collapse. D’Antona & Matteucci Galactic halo is here discussed in the light of cosmology and galaxy (1991) computed the Li evolution in the solar vicinity by means of a formation and evolution. detailed chemical evolution model and concluded that RG, AGB and The paper is organized as follows: in Section 2 we describe the novae should all contribute to Li production, and in particular that observational data, in Section 3 the chemical evolution models and only novae, exploding with long time delays (being binary systems of in Section 4 the results. Finally, in Section 5 we present a discussion low mass stars), could reproduce the steep rise of the Li abundance and conclusions. at [Fe/H] ¡ -1.0 dex, whereas AGB stars could not and RGs give only a negligible contribution. Later on, other papers included all or part of above mentioned sources in Galactic models (see Romano 2 OBSERVATIONAL DATA et al. 1999, 2001; Travaglio et al. 2001; Prantzos et al. 2017). The Li production by the neutrino ¹aº−process (Woosley et al. 1990) The observed Li abundances we adopted in this paper are the same was included by Matteucci et al.(1995) as one of the Li sources, shown in Figure A1 of Aguado et al.(2021). These data include but this contribution is also negligible. The a-process occurs in the stars from the Galactic halo, Sculptor, S2 and Slygr streams, the shell above the collapsing core in a pre-supernova. The high flux of globular cluster M54, Omega Centauri and Gaia-Enceladus. From the neutrinos evaporates neutrons and protons from heavy nuclei with theoretical point of view we have modeled several dSphs: Sculptor, the result of producing Li. Fornax, Carina, Draco, Ursa Minor, Sextans and Sagittarius, plus Various explanations have been put forward to explain the Spite two UfDs: Reticulum II and Bootes I. For all these galaxies we have plateau Li abundance lower than the primordial one. They are: tur- adopted the metallicity ([Fe/H]) distribution functions (MDF) from bulent mixing (Richard et al. 2005), gravity waves in stellar interiors SAGA database. All these objects are on average metal-poor as the (Charbonnel & Talon 2005), pre-galactic Li processing by massive stars in the Galactic halo, and their [Fe/H] extends from -5.0 to -1.0 Population III stars (Piau et al. 2006), tachocline mixing induced by dex. rotation (Spiegel & Zahn 1992; Piau 2008), mass dependence of Li In Figure1 we show a compilation of A(Li) values as functions depletion (Meléndez et al. 2010), pre-Main Sequence depletion plus of the effective temperature ()eff) for stars of the Galactic halo, S2, accretion (Fu et al. 2015; Molaro et al. 2012) and finally, non-standard Slygr, M54 and Omega Cen. We include in the plot only stars with Big Bang nucleosynthesis (Jedamzik et al. 2006; Hisano et al. 2009). log(g)¡3.7, in order to be sure that no diffusion with lower layers What remains difficult to explain is how such processes can deplete could affect the Li values. In the same plot we show the metallicities Li at the same level in all halo stars. Moreover, even the Milky Way of the stars: as one can see, the Li Spite plateau is evident and Spite-plateau itself has been challenged in the last years, since very is neither affected by )eff nor metallicity. In the Figure it is also metal-poor stars have been found with Li abundances lower than the indicated the primordial Li value as suggested by BBN. Spite-plateau (e.g. Sbordone et al. 2010; Bonifacio et al. 2015a), al- though in Aguado et al.(2019) is presented a star with [Fe/H] <-6.0 dex with a Li abundance lying on the Spite-plateau.
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