Horizons: nuclear astrophysics in the 2020s and beyond

Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated.We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.National Science Foundation (NSF) PHY1430152 OISE-1927130ExtreMeMatter Institute EMMI at the GSI Helmholtzzentrumfur Schwerionenforschung in DarmstadtEuropean Cooperation in Science and Technology (COST) CA1611

On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios

Support from the Centre National d’Etudes Spatiales (CNES), France. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Support of ESO, OCA, and CNES is acknowledged for the AMBRE project. Some of the calculations have been performed with the high-performance computing facility SIGAMM, hosted by OCA. CA acknowledges partial support by project PGC2018-095317-B-C21 financed by the MCIN/AEI FEDER ‘Una manera de hacer Europa’, and by project PID2021-123110NB-I00 financed by MCIN/AEI /10.13039/501100011033/FEDER, UEUsing a semi-analytical model of the evolution of the Milky Way, we show how secular evolution can create distinct overdensities in the phase space of various properties (e.g. age versus metallicity or abundance ratios versus age) corresponding to the thin and thick discs. In particular, we show how key properties of the Solar vicinity can be obtained by secular evolution, with no need for external or special events, like galaxy mergers or paucity in star formation. This concerns the long established double-branch behaviour of [alpha/Fe] versus metallicity and the recently found non-monotonic evolution of the stellar abundance gradient, evaluated at the birth radii of stars. We extend the discussion to other abundance ratios and we suggest a classification scheme, based on the nature of the corresponding yields (primary versus secondary or odd elements) and on the lifetimes of their sources (short-lived versus long-lived ones). The latter property is critical in determining the single- or double- branch behaviour of an elementary abundance ratio in the Solar neighbourhood. We underline the high diagnostic potential of this finding, which can help to separate clearly elements with sources evolving on different time-scales and help determining the site of e.g. the r-process(es). We define the ‘abundance distance’ between the thin and thick disc sequences as an important element for such a separation. We also show how the inside-out evolution of the Milky Way disc leads rather to a single-branch behaviour in other disc regions.Centre National d’Etudes Spatiales (CNES)Gaia Data Processing and Analysis Consortium DPACEuropean Space Agency ESACentre National d’Etudes Spatiales PGC2018-095317-B-C21 CNESMinisterio de Ciencia e Innovación MICINNAgencia Estatal de Investigación PID2021-123110NB-I00 AE

A detailed analysis of the Gl 486 planetary system

Context. The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R-circle plus and 3.0 M-circle plus. It is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. Aims. To prepare for future studies, we aim to thoroughly characterise the planetary system with new accurate and precise data collected with state-of-the-art photometers from space and spectrometers and interferometers from the ground. Methods. We collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X at the 8.1 m Gemini North telescope and CARMENES at the 3.5 m Calar Alto telescope, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed near-infrared interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes (AstroLAB, LCOGT, OSN, TJO). This extraordinary and rich data set was the input for our comprehensive analysis. Results. From interferometry, we measure a limb-darkened disc angular size of the star Gl 486 at theta(LDD) = 0.390 +/- 0.018 mas. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius R-* = 0.339 +/- 0.015 R-circle plus. We also measure a stellar rotation period at P-rot = 49.9 +/- 5.5 days, an upper limit to its XUV (5-920 A) flux informed by new Hubble/STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Moreover, we imposed restrictive constraints on the presence of additional components, either stellar or sub-stellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R-p = 1.343(-0.062)(+0.063) R-circle plus and M-p = 3.00(-0.12)(+0.13) M-circle plus, with relative uncertainties of the planet radius and mass of 4.7% and 4.2%, respectively. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope (Webb) observations.The David & Lucile Packard FoundationHeising-Simons FoundationGemini ObservatoryUniversity of ChicagoMax Planck SocietyConsejo Superior de Investigaciones Cientificas (CSIC)Spanish GovernmentEuropean Commission FICTS-2011-02 ICTS-2017-07-CAHA-4 CAHA16-CE-3978German Research Foundation (DFG) FOR2544National Science Foundation (NSF) AST-1636624 AST-2034336 2108465 DGE 1746045European Research Council (ERC) 639889National Aeronautics & Space Administration (NASA) XRP NNX16AD43GNational Science Foundation (NSF) AST 1909165Wise Observatory, Tel-Aviv University, Israel TAU2021A-015Agencia Estatal de Investigacion of the Ministerio de Ciencia, Innovacion y Universidades and the ERDF PID2019-109522GB-C5[1:4] PID2019-107061GBC64 PID2019-110689RB-100 PGC2018-095317-B-C21 PGC2018-102108-BI00Centre of Excellence "Severo Ochoa" CEX2019-000920-SCentre of Excellence "Maria de Maeztu" CEX2019-000920-SInstituto de Astrofisica de Andalucia SEV-2017-0709Centro de Astrobiologia MDM2017-0737German Research Foundation (DFG)European Commission FOR2544 (KU 3625/2-1)Germany's Excellence Strategy to the Excellence Cluster ORIGINS EXC-2094 -390783311European Research Council (ERC)European Commission 639889Bulgarian National Science Fund through VIHREN-2021 KP-06-DB/5Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung/Fonds national suisse de la recherche scientifique PZ00P2_174028United Kingdom Science Technology and Facilities Council 630008203Princeton UniversityUniversidad La Laguna through the Margarita Salas Fellowship from the Spanish Ministerio de UniversidadesEU Next Generation funds UNI/551/2021Generalitat de Catalunya (CERCA programme

The carbon star mystery: 40 years later

In 1981 Icko Iben Jr published a paper entitled “The carbon star mystery: why do the lowmass ones become such, and where have all the high mass ones gone?”, where he discussed the discrepancy between the theoretical expectation and its observational counterpart about the luminosity function of AGB carbon stars. After more than 40 years, our understanding of this longstanding problem is greatly improved, also thanks to more refined stellar models and a growing amount of observational constraints. In this paper we review the state of the art of these studies and we briefly illustrate the future perspectives.Istituto Nazionale di Astrofisica within the CRUI-CARE AgreementPGC2018-095317-B-C21 MCIN /AEI FEDERPID2021-123110NBI00 financed by MCIN/AEI /10.13039/501100011033/FEDER, U

Production of s-process elements in asymptotic giant branch stars as revealed by Gaia/GSP-Spec abundances

Context. The recent parameterisation by the GSP-Spec module of Gaia/Radial Velocity Spectrometer stellar spectra has produced an homogeneous catalogue of about 174 000 asymptotic giant branch (AGB) stars. Among the 13 chemical elements presented in this Gaia third data release, the abundance of two of them (cerium and neodymium) have been estimated in most of these AGB stars. These two species are formed by slow neutron captures (s-process) in the interior of low- and intermediate-mass stars. They belong to the family of second-peak s-process elements. Aims. We study the content and production rate of Ce and Nd in AGB stars, using the atmospheric parameters and chemical abundances derived by the GSP-Spec module. Methods. We defined a working sample of 19 544 AGB stars with high-quality Ce and/or Nd abundances, selected by applying a specific combination of the GSP-Spec quality flags. We compared these abundances with the yield production predicted by AGB evolutionary models. Results. We first confirmed that the majority of the working sample is composed of AGB stars by estimating their absolute magnitude in the K-band and their properties in a Gaia-2MASS diagram. We also checked that these stars are oxygen-rich AGB stars, as assumed during the GSP-Spec parameterisation. We found a good correlation between the Ce and Nd abundances, confirming the high quality of the derived abundances and that these species indeed belong to the same s-process family. We also found higher Ce and Nd abundances for more evolved AGB stars of similar metallicity, illustrating the successive mixing episodes enriching the AGB star surface in s-process elements formed deeper in their stellar interior. We then compared the observed Ce and Nd abundances with the FRUITY and Monash AGB yields and found that the higher Ce and Nd abundances cannot be explained by AGB stars of masses higher than 5 M⊙ . In contrast, the yields predicted by both models for AGB stars with an initial mass between ~1.5 and ~2.5 M⊙ and metallicities between ~-0.5 and ~0.0 dex are fully compatible with the observed GSP-Spec abundances. Conclusions. This work based on the largest catalogue of high-quality second-peak s-element abundances in oxygen-rich AGB stars allows evolutionary models to be constrained and confirms the fundamental role played by low- and intermediate-mass stars in the enrichment of the Universe in these chemical species.European Union’s Horizon 2020 research and innovation program under SPACEH2020 grant agreement number 101004214 (EXPLORE project)Centre National d’Études Spatiales (CNES)Project PID2021-123110NB-I00 financed by MCIN/AEI/10.13039/501100011033/FEDER, UEFunding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement

Sub-stellar engulfment by a main-sequence star: Where is the lithium?

Context. Since the discovery of exoplanetary systems, questions have been raised as to the sub-stellar companions that can survive encounters with their host star, and how this interaction may affect the internal structure and evolution of the hosting star, and particularly its surface chemical composition. Aims. We study whether the engulfment of a brown dwarf (BD) by a solar-like main-sequence (MS) star can significantly alter the structure of the star and the Li content on its surface. Methods. We performed 3D smoothed particle hydrodynamics simulations of the engulfment of a BD with masses 0.01 and 0.019 M⊙, on an MS star of 1 M⊙ and solar composition, in three different scenarios: a head-on collision, a grazing collision with an impact parameter η = 0.5 R⊙, and a merger. We studied the dynamics of the interaction in detail, and the relevance of the type of interaction and the mass of the BD on the final fate of the sub-stellar object and the host star in terms of mass loss of the system, angular momentum transfer, and changes in the Li abundance on the surface of the host star. Results. In all the studied scenarios, most of the BD mass is diluted in the denser region of the MS star. Only in the merger scenario a significant fraction (∼40%) of the BD material would remain in the outer layers. We find a clear increase in the surface rotational velocity of the host star after the interaction, ranging between 25 km s−1 (grazing collision) to 50 km s−1 (merger). We also find a significant mass loss from the system (in the range 10−4 − 10−3 M⊙) due to the engulfment, which in the case of the merger may form a circumstellar disk-like structure. Assuming that neither the depth of the convective envelope of the host star nor its mass content are modified during the interaction, a small change in the surface Li abundance in the head-on and grazing collisions is found. However, in the merger we find large Li enhancements, by factors of 20 − 30, depending on the BD mass. Some of these features could be detected observationally in the host star, provided they remained for a long enough time. Conclusions. In our 3D simulations, a sizable fraction of the BD survives long enough to be mixed with the inner core of the MS star. This is at odds with previous suggestions based on 1D simulations. In some cases the final surface rotational velocity is very high, coupled with enough mass loss that may form a circumstellar disk. Merger scenarios tend to dilute considerably more BD material on the surface of the MS star, which could be detected as a Li-enhancement. The dynamic of the simulated scenarios suggests the development of asymmetries in the structure of the host star that can only be tackled with 3D codes, including the long-term evolution of the system.European Research Council (FP7) under ERC Advanced Grant Agreement No. 321263 – FISHSwiss Platform for Advanced Scientific Computing (PASC) projects DIAPHANE and SPH-EXASKACH consortium through funding by SERISpanish projects PGC2018-095317-BC21, PID2021-123110NB-100 financed by the MCIN/AEI FEDERMINECO Spanish project PID2020-117252GB- 100