Context. We reassess the problem of the production and evolution of the light elements Li, Be and B and of their isotopes in the Milky Way in the light of new observational and theoretical ...developments. Aims. The main novelty is the introduction of a new scheme for the origin of Galactic cosmic rays (GCR), which for the first time enables a self-consistent calculation of their composition during galactic evolution. Methods. The scheme accounts for key features of the present-day GCR source composition, it is based on the wind yields of the Geneva models of rotating, mass-losing stars and it is fully coupled to a detailed galactic chemical evolution code. Results. We find that the adopted GCR source composition accounts naturally for the observations of primary Be and helps understanding why Be follows Fe more closely than O. We find that GCR produce ~70% of the solar 11B/10B isotopic ratio; the remaining 30% of 11B presumably result from ν-nucleosynthesis in massive star explosions. We find that GCR and primordial nucleosynthesis can produce at most ~30% of solar Li. At least half of the solar Li has to originate in low-mass stellar sources (red giants, asymptotic giant branch stars, or novae), but the required average yields of those sources are found to be much higher than obtained in current models of stellar nucleosynthesis. We also present radial profiles of LiBeB elemental and isotopic abundances in the Milky Way disc. We argue that the shape of those profiles – and the late evolution of LiBeB in general – reveals important features of the production of those light elements through primary and secondary processes.
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ABSTRACT
In evaluating the number of technological civilizations N in the Galaxy through the Drake formula, emphasis is mostly put on the astrophysical and biotechnological factors describing the ...emergence of a civilization and much less on its the lifetime, which is intimately related to its demise. It is argued here that this factor is in fact the most important regarding the practical implications of the Drake formula, because it determines the maximal extent of the ‘sphere of influence’ of any technological civilization. The Fermi paradox is studied in the terms of a simplified version of the Drake formula, through Monte Carlo simulations of N civilizations expanding in the Galaxy during their space faring lifetime L. In the framework of that scheme, the probability of ‘direct contact’ is determined as the fraction of the Galactic volume occupied collectively by the ‘spheres of influence’ of N civilizations. The results of the analysis are used to determine regions in the parameter space where the Fermi paradox holds. It is argued that in a large region of the diagram the corresponding parameters suggest rather a ‘weak’ Fermi paradox. Future research may reveal whether a ‘strong’ paradox holds in some part of the parameter space. Finally, it is argued that the value of N is not bound by N = 1 from below, contrary to what is usually assumed, but it may have a statistical interpretation.
Abstract
We present a comprehensive study of the abundance evolution of the elements from H to U in the Milky Way halo and local disc. We use a consistent chemical evolution model, ...metallicity-dependent isotopic yields from low and intermediate mass stars and yields from massive stars which include, for the first time, the combined effect of metallicity, mass loss, and rotation for a large grid of stellar masses and for all stages of stellar evolution. The yields of massive stars are weighted by a metallicity-dependent function of the rotational velocities, constrained by observations as to obtain a primary-like 14N behaviour at low metallicity and to avoid overproduction of s-elements at intermediate metallicities. We show that the Solar system isotopic composition can be reproduced to better than a factor of 2 for isotopes up to the Fe-peak, and at the 10 per cent level for most pure s-isotopes, both light ones (resulting from the weak s-process in rotating massive stars) and the heavy ones (resulting from the main s-process in low and intermediate mass stars). We conclude that the light element primary process (LEPP), invoked to explain the apparent abundance deficiency of the s-elements with A < 100, is not necessary. We also reproduce the evolution of the heavy to light s-elements abundance ratio (hs/ls) – recently observed in unevolved thin disc stars – as a result of the contribution of rotating massive stars at sub-solar metallicities. We find that those stars produce primary F and dominate its solar abundance and we confirm their role in the observed primary behaviour of N. In contrast, we show that their action is insufficient to explain the small observed values of $\rm ^{12}C/^{13}C$ in halo red giants, which is rather due to internal processes in those stars.
Abstract
The decomposition of the Solar system abundances of heavy isotopes into their s- and r- components plays a key role in our understanding of the corresponding nuclear processes and the ...physics and evolution of their astrophysical sites. We present a new method for determining the s- and r- components of the Solar system abundances, fully consistent with our current understanding of stellar nucleosynthesis and galactic chemical evolution. The method is based on a study of the evolution of the solar neighborhood with a state-of-the-art 1-zone model, using recent yields of low and intermediate mass stars as well as of massive rotating stars. We compare our results with previous studies and we provide tables with the isotopic and elemental contributions of the s- and r-processes to the Solar system composition.
Context. The composition of Galactic cosmic rays (GCR) presents strong similarities to the standard (cosmic) composition, but also noticeable differences, the most important of which is the high ...isotopic ratio of 22Ne/20Ne, which is ~5 times higher in GCR than in the Sun. This ratio provides key information on the GCR origin. Aims. We investigate the idea that GCR are accelerated by the forward shocks of supernova explosions, while they run through the pre-supernova winds of the massive stars and through the interstellar medium. Methods. We use detailed wind and core yields of rotating and non-rotating models of massive stars with mass loss, as well as simple models for the properties of the forward shock and of the circumstellar medium. Results. We find that the observed GCR 22Ne/20Ne ratio can be explained if GCR are accelerated only during the early Sedov phase, for shock velocities >1600 km s-1. The acceleration efficiency is found to be of the order of 10-6–10-5, i.e. a few particles out of a million encountered by the shock escape the SN at GCR energies. We also show quantitatively that the widely publicized idea that GCR are accelerated in superbubbles fails to account for the high 22Ne/20Ne ratio in GCR.
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We study the role of radial migration of stars on the chemical evolution of the Milky Way disk. We are interested in the impact of that process on the local properties of the disk and on the ...morphological properties of the resulting thick and thin disks. We use a model with several new or up-dated ingredients: atomic and molecular gas phases, star formation that depends on molecular gas, yields from a recent homogeneous grid and observationally inferred SNIa rates. Our model reproduces current values of most of the main global observables of the MW disk and bulge, and also the observed "stacked" evolution of MW-type galaxies. The thick disk extends up to ~11 kpc and has a scale length of 1.8 kpc, which is considerably shorter than the thin disk, because of the inside-out formation scheme. We also show how, in this framework, current and forthcoming spectroscopic observations can constrain the nucleosynthesis yields of massive stars for the metallicity range of 0.1 Z to 2-3 Z.
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Aims. We study the role of the radial motions of stars and gas on the evolution of abundance profiles in the Milky Way disk. We investigate, in a parametrized way, the impact of radial flows of gas ...and radial migration of stars induced mainly by the Galactic bar and its iteraction with the spiral arms. Methods. We use a model with several new or up-dated ingredients (atomic and molecular gas phases, star formation depending on molecular gas, recent sets of metallicity-dependent stellar yields from H to Ni, observationally inferred SNIa rates), which reproduces most global and local observables of the Milky Way well. Results. We obtain abundance profiles flattening both in the inner disk (because of radial flows) and in the outer disk (because of the adopted star formation law). The gas abundance profiles flatten with time, but the corresponding stellar profiles appear to be steeper for younger stars, because of radial migration. We find a correlation between the stellar abundance profiles and O/Fe, which is a proxy for stellar age. Our final abundance profiles are in overall agreement with observations, but slightly steeper (by 0.01-0.02 dex kpc super(-1)) for elements above S. We find an interesting "odd-even effect" in the behaviour of the abundance profiles (steeper slopes for odd elements) for all sets of stellar yields; however, this behaviour does not appear in observations, suggesting that the effect is, perhaps, overestimated in current stellar nucleosynthesis calculations.
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We study radial migration and chemical evolution in a bar-dominated disc galaxy, by analysing the results of a fully self-consistent, high-resolution N-body+smoothed particle hydrodynamics (SPH) ...simulation. We find different behaviours for gas and star particles. Gas within corotation is driven in the central regions by the bar, where it forms a pseudo-bulge (discy-bulge), but it undergoes negligible radial displacement outside the bar region. Stars undergo substantial radial migration at all times, caused first by transient spiral arms and later by the bar. Despite the important amount of radial migration occurring in our model, its impact on the chemical properties is limited. The reason is the relatively flat abundance profile, due to the rapid early evolution of the whole disc. We show that the implications of radial migration on chemical evolution can be studied to a good accuracy by post-processing the results of the N-body+SPH calculation with a simple chemical evolution model having detailed chemistry and a parametrized description of radial migration. We find that radial migration impacts on chemical evolution both directly (by moving around the long-lived agents of nucleosynthesis, like e.g. SNIa or asymptotic giant branch stars, and thus altering the abundance profiles of the gas) and indirectly (by moving around the long-lived tracers of chemical evolution and thus affecting stellar metallicity profiles, local age-metallicity relations and metallicity distributions of stars, etc.).
Context. The chemical evolution of lithium in the Milky Way represents a major problem in modern astrophysics. Indeed, lithium is, on the one hand, easily destroyed in stellar interiors, and, on the ...other hand, produced at some specific stellar evolutionary stages that are still not well constrained. Aims. The goal of this paper is to investigate the lithium stellar content of Milky Way stars in order to put constraints on the lithium chemical enrichment in our Galaxy, in particular in both the thin and thick discs. Methods. Thanks to high-resolution spectra from the ESO archive and high quality atmospheric parameters, we were able to build a massive and homogeneous catalogue of lithium abundances for 7300 stars derived with an automatic method coupling, a synthetic spectra grid, and a Gauss-Newton algorithm. We validated these lithium abundances with literature values, including those of the Gaia benchmark stars. Results. In terms of lithium galactic evolution, we show that the interstellar lithium abundance increases with metallicity by 1 dex from M/H = −1 dex to + 0.0 dex. Moreover, we find that this lithium ISM abundance decreases by about 0.5 dex at super-solar metalllicity. Based on a chemical separation, we also observed that the stellar lithium content in the thick disc increases rather slightly with metallicity, while the thin disc shows a steeper increase. The lithium abundance distribution of α-rich, metal-rich stars has a peak at ALi ~ 3 dex. Conclusions. We conclude that the thick disc stars suffered of a low lithium chemical enrichment, showing lithium abundances rather close to the Spite plateau while the thin disc stars clearly show an increasing lithium chemical enrichment with the metallicity, probably thanks to the contribution of low-mass stars.
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Aims.We propose the Wind of Fast Rotating Massive Stars scenario to explain the origin of the abundance anomalies observed in globular clusters. Methods.We compute and present models of fast rotating ...stars with initial masses between 20 and 120 $M_\odot$ for an initial metallicity Z = 0.0005 (${\rm Fe/H}\simeq-1.5$). We discuss the nucleosynthesis in the H-burning core of these objects and present the chemical composition of their ejecta. We consider the impact of uncertainties in the relevant nuclear reaction rates. Results.Fast rotating stars reach critical velocity at the beginning of their evolution and remain near the critical limit during the rest of the main sequence and part of the He-burning phase. As a consequence they lose large amounts of material through a mechanical wind which probably leads to the formation of a slow outflowing disk. The material in this slow wind is enriched in H-burning products and presents abundance patterns similar to the chemical anomalies observed in globular cluster stars. In particular, the C, N, O, Na and Li variations are well reproduced by our model. However the rate of the $\rm{}^{24}\kern-0.6ptMg$$(p,\gamma)$ has to be increased by a factor 1000 around 50 $\times$ 106 K in order to reproduce the amplitude of the observed Mg-Al anticorrelation. We discuss how the long-lived low-mass stars currently observed in globular clusters could have formed out of the slow wind material ejected by massive stars.
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