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LJMU Research Online LJMU Research Online Casey, AR, Ruchti, G, Masseron, T, Randich, S, Gilmore, G, Lind, K, Kennedy, GM, Koposov, SE, Hourihane, A, Franciosini, E, Lewis, JR, Magrini, L, Morbidelli, L, Sacco, GG, Worley, CC, Feltzing, S, Jeffries, RD, Vallenari, A, Bensby, T, Bragaglia, A, Flaccomio, E, Francois, P, Korn, AJ, Lanzafame, A, Pancino, E, Recio-Blanco, A, Smiljanic, R, Carraro, G, Costado, MT, Damiani, F, Donati, P, Frasca, A, Jofre, P, Lardo, C, de Laverny, P, Monaco, L, Prisinzano, L, Sbordone, L, Sousa, SG, Tautvaisiene, G, Zaggia, S, Zwitter, T, Delgado Mena, E, Chorniy, Y, Martell, SL, Aguirre, VS, Miglio, A, Chiappini, C, Montalban, J, Morel, T and Valentini, M The Gaia-ESO Survey: revisiting the Li-rich giant problem http://researchonline.ljmu.ac.uk/id/eprint/4834/ Article Citation (please note it is advisable to refer to the publisher’s version if you intend to cite from this work) Casey, AR, Ruchti, G, Masseron, T, Randich, S, Gilmore, G, Lind, K, Kennedy, GM, Koposov, SE, Hourihane, A, Franciosini, E, Lewis, JR, Magrini, L, Morbidelli, L, Sacco, GG, Worley, CC, Feltzing, S, Jeffries, RD, Vallenari, A, Bensby, T, Bragaglia, A, Flaccomio, E, Francois, P, Korn, AJ, LJMU has developed LJMU Research Online for users to access the research output of the University more effectively. Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Users may download and/or print one copy of any article(s) in LJMU Research Online to facilitate their private study or for non-commercial research. You may not engage in further distribution of the material or use it for any profit-making activities or any commercial gain. http://researchonline.ljmu.ac.uk/ The version presented here may differ from the published version or from the version of the record. Please see the repository URL above for details on accessing the published version and note that access may require a subscription. For more information please contact [email protected] http://researchonline.ljmu.ac.uk/ Mon. Not. R. Astron. Soc. 000, 1{24 (2015) Printed 6 August 2016 (MN LATEX style file v2.2) The Gaia-ESO Survey: Revisiting the Li-rich giant problem A. R. Casey1?, G. Ruchti2, T. Masseron1, S. Randich3, G. Gilmore1, K. Lind4;5, G. M. Kennedy1, S. E. Koposov1, A. Hourihane1, E. Franciosini3, J. R. Lewis1, L. Magrini3, L. Morbidelli3, G. G. Sacco3, C. C. Worley1, S. Feltzing2, R. D. Jeffries6, A. Vallenari7, T. Bensby2, A. Bragaglia8, E. Flaccomio9, P. Francois10, A. J. Korn5, A. Lanzafame11, E. Pancino3;12, A. Recio-Blanco13, R. Smiljanic14, G. Carraro15, M. T. Costado16, F. Damiani9, P. Donati8;17, A. Frasca18, P. Jofr´e1, C. Lardo19, P. de Laverny13, L. Monaco20, L. Prisinzano9, L. Sbordone21;22, S. G. Sousa23, G. Tautvaiˇsien_e24, S. Zaggia7, T. Zwitter25, E. Delgado Mena23, Y. Chorniy24, S. L. Martell26, V. Silva Aguirre27, A. Miglio28, C. Chiappini29, J. Montalban30, T. Morel31, M. Valentini29 Affiliations can be found at the end of this Article. Accepted 2016 XX XX. Received 2016 February XX; in original form 2016 May XX ABSTRACT The discovery of lithium-rich giants contradicts expectations from canonical stellar evolution. Here we report on the serendipitous discovery of 20 Li-rich giants observed during the Gaia-ESO Survey, which includes the first nine Li-rich giant stars known towards the CoRoT fields. Most of our Li-rich giants have near-solar metallicities, and stellar parameters consistent with being before the luminosity bump. This is difficult to reconcile with deep mixing models proposed to explain lithium enrichment, because these models can only operate at later evolutionary stages: at or past the luminosity bump. In an effort to shed light on the Li-rich phenomenon, we highlight recent evidence of the tidal destruction of close-in hot Jupiters at the sub-giant phase. We note that when coupled with models of planet accretion, the observed destruction of hot Jupiters actually predicts the existence of Li-rich giant stars, and suggests Li- rich stars should be found early on the giant branch and occur more frequently with increasing metallicity. A comprehensive review of all known Li-rich giant stars reveals that this scenario is consistent with the data. However more evolved or metal-poor stars are less likely to host close-in giant planets, implying that their Li-rich origin requires an alternative explanation, likely related to mixing scenarios rather than arXiv:1603.03038v2 [astro-ph.SR] 1 Jul 2016 external phenomena. Key words: stars: abundances 1 INTRODUCTION exists a growing number of giant stars with lithium abun- dances that are near to, or exceed Big Bang nucleosynthesis Lithium is a fragile element which cannot be easily replen- predictions. Although rare, these stars constitute a funda- ished. Given its fragility, canonical stellar evolution models mental outstanding problem for stellar evolution. predict that a star's Li abundance should decrease as it as- cends the giant branch. Observations since Bonsack (1959) Stellar evolution theory suggests that the depth of the have repeatedly confirmed these predictions. However, Pop- convective envelope increases when a star leaves the main- ulation I stars show lithium abundances approximately ten sequence. In doing so, the star experiences first dredge-up: times higher than older Population II stars, implying some material from deep internal layers is mixed towards the sur- kind of Galactic lithium enrichment. More puzzlingly, there face (Iben 1967a,b). The inner material is hot enough that Li has been destroyed, therefore first dredge up dilutes the surface Li abundance. Consequently, stellar evolution theory ? E-mail: [email protected] predicts the observable Li abundance should be ∼1.5 dex c 2015 RAS 2 Casey et al. lower for evolved stars than their main-sequence counter- mixing can only be induced in intermediate-mass asymp- parts. (e.g., Iben 1967a; Lagarde et al. 2012) Stars on the totic giant branch (AGB) stars when the convective envelope upper red giant branch (RGB) may be even more depleted deepens at the base of the AGB. While these scenarios ex- in Li due to mixing occurring just after the RGB bump plain the necessary internal conditions required to produce (Sweigart & Mengel 1979; Charbonnel 1994, 1995). Other and transport Li to the photosphere, they do not speculate changes to surface abundances are also predicted: increases on the actual mechanism that drives the mixing (however see in 4He, 14N, 13C, and decreases in 12C (Iben 1964; Chanam´e Charbonnel & Zahn 2007). Palacios et al. (2006) have shown et al. 2005; Charbonnel 2006; Charbonnel & Lagarde 2010; that rotation alone is insufficient to produce the observed Li Karakas 2010; Lattanzio et al. 2015). Detailed observations abundances, implying that an additional mechanism is re- have repeatedly provided convincing evidence of these pre- quired to induce the extra mixing. dictions (e.g., Lambert et al. 1980; Spite & Spite 1982; Grat- Thermohaline mixing has been proposed as a mecha- ton et al. 2000; Lind et al. 2009b; Mucciarelli et al. 2012; nism to drive extra mixing at the bump in the giant branch Tautvaiˇsien_eet al. 2013). luminosity function. In addition to removing any existing The existence of Li-rich (A(Li) & 2) giant stars im- molecular weight gradient, an inversion in the molecular plies an additional mechanism that produces and/or pre- weight gradient is produced, which drives thermohaline mix- serves surface Li. This process may be internal or exter- ing (Eggleton et al. 2006). In contrast, Denissenkov & Van- nal. In the right conditions stars can produce Li inter- denBerg (2003) incorporate diffusion and shear-driven mix- nally through the Cameron-Fowler mechanism (Cameron ing to facilitate extra-mixing in low-mass RGB stars. Their & Fowler 1971): 3He(α; γ)7Be(e−; ν)7Li. The temperature prescription relies on main-sequence stars (as the precursors must be hot enough for 7Be to be produced, but 7Be must of upper RGB stars) to possess rapidly rotating radiative be quickly transported towards cooler regions so that fresh cores. Instead of encouraging interactions between different 7Li can be created without being immediately destroyed by mass shells (e.g., Charbonnel & Balachandran 2000), Denis- proton capture. The Cameron-Fowler mechanism can oper- senkov & VandenBerg (2003) require the specific angular ate in red giants in two different stages. During hot bottom momentum to be conserved in each shell during the star's burning (HBB), the bottom of the convective envelope is evolution. This situation would therefore permit a reservoir hot enough for 7Be production. The convection carries 7Be of angular momentum which could later induce deep mixing. to cooler regions where it can capture an electron to pro- Palacios et al. (2001) proposed that internal instabili- duce 7Li. In the absence of HBB, a radiative zone exists be- ties occurring near the luminosity bump were sufficient to tween the shell and the convective envelope. A mechanism is produce additional Li. Specifically, internally-produced 7Be then required to mix material down to the outer part of the could be transported to a nearby convective region where shell { where temperatures are high enough to produce 7Be 7Li is produced, but immediately destroyed by proton cap- { and then fresh 7Be must be mixed across the radiative ture. In effect, a thin burning layer of Li is created, where zone to the convective envelope. The mechanism for mix- 7Li(p; α)α becomes the dominant reaction, increasing the lo- ing through the radiative zone is under debate, but various cal temperature and the level of meridional circulation. The mechanisms are collectively referred to as `deep-mixing' or molecular weight gradient is eventually destroyed, allowing `extra-mixing'. Moreover, because the conditions required to for deep mixing to occur.
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