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Theoretical Predictions of Surface Light Element Abundances in Protostellar and Pre-Main Sequence Phase

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Theoretical Predictions of Surface Light Element Abundances in Protostellar and Pre-Main Sequence Phase Theoretical Predictions of Surface Light Element Abundances in Protostellar and Pre-Main Sequence Phase E Tognelli, S Degl’innocenti, P Prada Moroni, L Lamia, R Pizzone, A Tumino, C Spitaleri, A Chiavassa To cite this version: E Tognelli, S Degl’innocenti, P Prada Moroni, L Lamia, R Pizzone, et al.. Theoretical Predictions of Surface Light Element Abundances in Protostellar and Pre-Main Sequence Phase. Frontiers in Astronomy and Space Sciences, Frontiers Media, 2021, 8, 10.3389/fspas.2021.604872. hal-03226035 HAL Id: hal-03226035 https://hal.archives-ouvertes.fr/hal-03226035 Submitted on 13 May 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. REVIEW published: 13 May 2021 doi: 10.3389/fspas.2021.604872 Theoretical Predictions of Surface Light Element Abundances in Protostellar and Pre-Main Sequence Phase E. Tognelli 1,2*, S. Degl’Innocenti 1,3, P. G. Prada Moroni 1,3, L. Lamia 4,5,6, R. G. Pizzone 5, A. Tumino 5,7, C. Spitaleri 4,5 and A. Chiavassa 8 1Universitá di Pisa, Dipartimento di Fisica “Enrico Fermi”, Pisa, Italy, 2INAF, Osservatorio Astronomico d’Abruzzo, Teramo, Italy, 3INFN, Sezione di Pisa, Pisa, Italy, 4Dipartimento di Fisica e Astronomia, Università di Catania, Catania, Italy, 5INFN - Laboratori Nazionali del Sud, Catania, Italy, 6CSFNSM-Centro Siciliano di Fisica Nucleare e Struttura della Materia, Catania, Italy, 7Dipartimento di Ingegneria e Architettura, Universitá di Enna, Enna, Italy, 8Université Coteˆ d’Azur, Observatoire de la Coteˆ d’Azur, CNRS, Nice, France Theoretical prediction of surface stellar abundances of light elements–lithium, beryllium, and boron–represents one of the most interesting open problems in astrophysics. As well 7 Edited by: known, several measurements of Li abundances in stellar atmospheres point out a Paul Denis Stevenson, disagreement between predictions and observations in different stellar evolutionary University of Surrey, United Kingdom phases, rising doubts about the capability of present stellar models to precisely Reviewed by: Maria Lugaro, reproduce stellar envelope characteristics. The problem takes different aspects in the Hungarian Academy of Sciences various evolutionary phases; the present analysis is restricted to protostellar and pre-Main (MTA), Hungary Sequence phases. Light elements are burned at relatively low temperatures (T from ≈ 2to Oscar Straniero, Astronomical Observatory of Abruzzo ≈ 5 million degrees) and thus in the early evolutionary stages of a star they are gradually (INAF), Italy destroyed at different depths of stellar interior mainly by (p, α) burning reactions, in *Correspondence: dependence on the stellar mass. Their surface abundances are strongly influenced by E. Tognelli [email protected] the nuclear cross sections, as well as by the extension toward the stellar interior of the convective envelope and by the temperature at its bottom, which depend on the Specialty section: characteristics of the star (mass and chemical composition) as well as on the energy This article was submitted to transport in the convective stellar envelope. In recent years, a great effort has been made to Nuclear Physics, a section of the journal improve the precision of light element burning cross sections. However, theoretical Frontiers in Astronomy and predictions surface light element abundance are challenging because they are also Space Sciences influenced by the uncertainties in the input physics adopted in the calculations as well Received: 10 September 2020 fi Accepted: 02 February 2021 as the ef ciency of several standard and non-standard physical processes active in young Published: 13 May 2021 stars (i.e. diffusion, radiative levitation, magnetic fields, rotation). Moreover, it is still not Citation: completely clear how much the previous protostellar evolution affects the pre-Main ’ Tognelli E, Degl Innocenti S, Sequence characteristics and thus the light element depletion. This paper presents the Prada Moroni PG, Lamia L, Pizzone RG, Tumino A, Spitaleri C and state-of-the-art of theoretical predictions for protostars and pre-Main Sequence stars and Chiavassa A (2021) Theoretical their light element surface abundances, discussing the role of (p, α) nuclear reaction rates Predictions of Surface Light Element Abundances in Protostellar and Pre- and other input physics on the stellar evolution and on the temporal evolution of the Main Sequence Phase. predicted surface abundances. Front. Astron. Space Sci. 8:604872. doi: 10.3389/fspas.2021.604872 Keywords: Nuclear reactions, nucleosynthesis, abundances, stars:evolution, stars:pre-main sequence Frontiers in Astronomy and Space Sciences | www.frontiersin.org1 May 2021 | Volume 8 | Article 604872 Tognelli et al. Surface Abundances in Young Stars 1 INTRODUCTION characteristics (and thus light element abundances) is still an open problem. The inclusion of the protostellar accretion phase Light elements – lithium, beryllium and boron (hereafter Li, Be in evolutionary codes produces stars in the early pre-MS phase and B) – are burned at relatively low temperatures (T from ≈ 2to different from what expected in standard non accreting models, ≈ 5 million degrees) easy to reach in stellar interiors at the in which stars essentially contract along the Hayashi track. This bottom of the convective envelope, even during the early pre- eventually results in differences between standard and accreting Main Sequence (pre-MS) evolution. Therefore, surface light models still visible during the whole pre-MS or the MS phase, elements are depleted if the mixing processes become efficient with effects on both the structure and chemical composition of enough to bring them down to the destruction region. This the stellar models (Baraffe et al., 2009; Baraffe and Chabrier, 2010; property makes such elements very good tracers of the mixing Tognelli et al., 2013; Kunitomo et al., 2018; Tognelli et al., 2020). efficiency in stellar envelopes whose theoretical treatment is still a Light element burning cross sections are fundamental difficult task in stellar physics. Due to the different burning ingredients in the predictions of the time behavior of light temperatures, the comparison between theory and observation element stellar surface abundances. In recent years new values for Li, Be and B, if possible, would be very useful to constrain for (p,α) reaction rates have been proposed, mainly estimated theoretical models and in particular the extension of the using the Trojan Horse Method, greatly improving the precision convective envelope. The most of the observations concern the of these quantities. abundance of 7Li because, in most stars, surface 6Li is completely The present review summarizes the state-of-the-art of destroyed during the pre-MS phase and Be and B isotopic theoretical predictions for protostars and pre-MS stars and measurements are very problematic (e.g., Cunha, 2010; Kaufer, their light element surface abundances, in the light of recent 2010; Delgado Mena et al., 2012). improvements in the adopted input physics, updated reaction A huge amount of data for surface 7Li abundances are available rates and description of the formation and evolution of pre- both for disk, thick disk and halo field stars and for open clusters; MS stars. however, the well known discrepancy between predictions and The paper is structured as it follows. In Section 2 we observations of this quantity in clusters or in the Sun (the so- qualitatively show the location of observed young pre-MS stars called “lithium-problem”) is still an open question (see e.g., in the HR diagram and we compared it to the predictions of Charbonnel et al., 2000; Jeffries, 2000; Piau and Turck-Chièze, standard non accreting models. In Section 3 we give a brief 2002; Xiong and Deng, 2002; Sestito et al., 2003; Jeffries, 2006; overview of the main characteristics and evolutionary stages of a Talon, 2008). pre-MS model without protostellar accretion. In Section 4 we The theoretical prediction of surface light element abundances introduce the protostellar accretion phase, discussing the is complex because they are sensitive to both the input physics differences between spherical and disc accretion, along with (i.e., equation of state, reaction rates, opacity, etc...) and the main parameters the determine the structure of an chemical element abundances (i.e., initial abundance of accreting protostar. In Section 5 we analyze the burning of deuterium, helium, metals, etc...) adopted in stellar models, light elements (Li, Be and B) in pre-MS stars and the together with the assumed efficiency of microscopic diffusion predicted surface abundances during the pre-MS for stellar and radiative acceleration (see e.g., Piau and Turck-Chièze, 2002; models of different masses without or with protostellar Richard et al., 2002; Burke et al., 2004; Richard et al., 2005; accretion phase. In Section 6 we review the impact of updated Tognelli et al., 2012; Tognelli et al., 2015). The situation is even cross sections for the burning of light elements and their impact more complicated because surface
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