Search for: All records

ABSTRACT Using 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. 

ABSTRACT The cosmic production of the short-lived radioactive nuclide 26Al is crucial for our understanding of the evolution of stars and galaxies. However, simulations of the stellar sites producing 26Al are still weakened by significant nuclear uncertainties. We re-evaluate the 26Al(n, p)26Mg, and 26Al(n, α)23Na ground state reactivities from 0.01 GK to 10 GK, based on the recent n_TOF measurement combined with theoretical predictions and a previous measurement at higher energies, and test their impact on stellar nucleosynthesis. We computed the nucleosynthesis of low- and high-mass stars using the Monash nucleosynthesis code, the NuGrid mppnp code, and the FUNS stellar evolutionary code. Our low-mass stellar models cover the 2–3 M⊙ mass range with metallicities between Z = 0.01 and 0.02, their predicted 26Al/27Al ratios are compared to 62 meteoritic SiC grains. For high-mass stars, we test our reactivities on two 15 M⊙ models with Z = 0.006 and 0.02. The new reactivities allow low-mass AGB stars to reproduce the full range of 26Al/27Al ratios measured in SiC grains. The final 26Al abundance in high-mass stars, at the point of highest production, varies by a factor of 2.4 when adopting the upper, or lower limit of our rates. However, stellar uncertainties still play an important role in both mass regimes. The new reactivities visibly impact both low- and high-mass stars nucleosynthesis and allow a general improvement in the comparison between stardust SiC grains and low-mass star models. Concerning explosive nucleosynthesis, an improvement of the current uncertainties between T9∼0.3 and 2.5 is needed for future studies. 

ABSTRACT The joint detection of the gravitational wave GW170817, of the short γ-ray burst GRB170817A and of the kilonova AT2017gfo, generated by the the binary neutron star (NS) merger observed on 2017 August 17, is a milestone in multimessenger astronomy and provides new constraints on the NS equation of state. We perform Bayesian inference and model selection on AT2017gfo using semi-analytical, multicomponents models that also account for non-spherical ejecta. Observational data favour anisotropic geometries to spherically symmetric profiles, with a log-Bayes’ factor of ∼104, and favour multicomponent models against single-component ones. The best-fitting model is an anisotropic three-component composed of dynamical ejecta plus neutrino and viscous winds. Using the dynamical ejecta parameters inferred from the best-fitting model and numerical–relativity relations connecting the ejecta properties to the binary properties, we constrain the binary mass ratio to q < 1.54 and the reduced tidal parameter to $$120\lt \tilde{\Lambda }\lt 1110$$. Finally, we combine the predictions from AT2017gfo with those from GW170817, constraining the radius of a NS of 1.4 M⊙ to 12.2 ± 0.5 km (1σ level). This prediction could be further strengthened by improving kilonova models with numerical-relativity information. 

Abstract Radioactive nuclei with lifetimes on the order of millions of years can reveal the formation history of the Sun and active nucleosynthesis occurring at the time and place of its birth^1,2. Among such nuclei whose decay signatures are found in the oldest meteorites,²⁰⁵Pb is a powerful example, as it is produced exclusively by slow neutron captures (thesprocess), with most being synthesized in asymptotic giant branch (AGB) stars^3–5. However, making accurate abundance predictions for²⁰⁵Pb has so far been impossible because the weak decay rates of²⁰⁵Pb and²⁰⁵Tl are very uncertain at stellar temperatures^6,7. To constrain these decay rates, we measured for the first time the bound-state β⁻decay of fully ionized²⁰⁵Tl⁸¹⁺, an exotic decay mode that only occurs in highly charged ions. The measured half-life is 4.7 times longer than the previous theoretical estimate⁸and our 10% experimental uncertainty has eliminated the main nuclear-physics limitation. With new, experimentally backed decay rates, we used AGB stellar models to calculate²⁰⁵Pb yields. Propagating those yields with basic galactic chemical evolution (GCE) and comparing with the²⁰⁵Pb/²⁰⁴Pb ratio from meteorites^9–11, we determined the isolation time of solar material inside its parent molecular cloud. We find positive isolation times that are consistent with the others-process short-lived radioactive nuclei found in the early Solar System. Our results reaffirm the site of the Sun’s birth as a long-lived, giant molecular cloud and support the use of the²⁰⁵Pb–²⁰⁵Tl decay system as a chronometer in the early Solar System. 

Abstract We study the production of very light elements (Z< 20) in the dynamical and spiral-wave wind ejecta of binary neutron star mergers by combining detailed nucleosynthesis calculations with the outcome of numerical relativity merger simulations. All our models are targeted to GW170817 and include neutrino radiation. We explore different finite-temperature, composition-dependent nuclear equations of state, and binary mass ratios, and find that hydrogen and helium are the most abundant light elements. For both elements, the decay of free neutrons is the driving nuclear reaction. In particular, ∼0.5–2 × 10⁻⁶M_⊙of hydrogen are produced in the fast expanding tail of the dynamical ejecta, while ∼1.5–11 × 10⁻⁶M_⊙of helium are synthesized in the bulk of the dynamical ejecta, usually in association with heavyr-process elements. By computing synthetic spectra, we find that the possibility of detecting hydrogen and helium features in kilonova spectra is very unlikely for fiducial masses and luminosities, even when including nonlocal thermodynamic equilibrium effects. The latter could be crucial to observe helium lines a few days after merger for faint kilonovae or for luminous kilonovae ejecting large masses of helium. Finally, we compute the amount of strontium synthesized in the dynamical and spiral-wave wind ejecta, and find that it is consistent with (or even larger than, in the case of a long-lived remnant) the one required to explain early spectral features in the kilonova of GW170817.