Papers by Maurizio M. Busso
EPJ Web of Conferences, 2017
Noticeable improvements were recently introduced in the modelling of ncapture nucleosynthesis in ... more Noticeable improvements were recently introduced in the modelling of ncapture nucleosynthesis in the advanced evolutionary stages of giant stars (Asymptotic Giant Branch, or AGB, stars). Two such improvements are closely linked together and concern the introduction of non-parameterized, physical models for extended mixing processes and the adoption of accurate reaction rates for Hand He-burning reactions, including the one for the main neutron source 13 C(α,n) 16 O. These improvements profited of a longstanding collaboration between stellar physicists and C. Spitaleri's team and of his seminal work both as a leader in the Nuclear Astrophysics scenario and as a talentscout in the recruitment of young researchers in the field. We present an example of the innovative results that can be obtained thanks to the novelties introduced, by estimating the contributions from a nearby AGB star to the synthesis of short-lived (t 1/2 ≤ 10 Myr) radioactive nuclei which were alive in early Solar System condensates. We find that the scenario indicating an AGB star as the source of such radioactivities, discussed for many years by researchers in this field, appears now to be no longer viable, when the mentioned improvements of AGB models and nuclear parameters are considered.
The European Physical Journal A
EPJ Web of Conferences
Theory predicts that lifetimes of β-radionuclides can change dramatically as a function of their ... more Theory predicts that lifetimes of β-radionuclides can change dramatically as a function of their ionization state. Experiments performed in Storage Rings on highly ionized atom have proven nuclei can change their beta decay lifetime up to several orders of magnitude. The PANDORA (Plasmas for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry) experiment is now conceived to measure, for the first time, nuclear β-decay rates using magnetized laboratory plasma that can mimic selected stellar-like conditions in terms of the temperature of the environment. The main feature of the setup which is based on a plasma trap to create and sustain the plasma, a detector array for the measurement of the gamma-rays emitted by the daughter nuclei after the decay process and the diagnostic tools developed to online monitor the plasma will be presented. A short list of the physics cases we plan to investigate together with an evaluation of their feasibility will be also discussed.
Astronomy & Astrophysics, 2010
Context. In the framework of the study of the Galactic metallicity gradient and its time evolutio... more Context. In the framework of the study of the Galactic metallicity gradient and its time evolution, we present new high-resolution spectroscopic observations obtained with FLAMES and the fiber link to UVES at VLT of three open clusters (OCs) located within ∼7 kpc from the Galactic centre (GC): NGC 6192, NGC 6404, NGC 6583. We also present new orbit determination for all OCs with Galactocentric distances (R GC) ≤ 8 kpc and metallicity from high-resolution spectroscopy. Aims. We aim to investigate the slope of the inner disc metallicity gradient as traced by OCs and to discuss its implication for the chemical evolution of our Galaxy. Methods. We have derived memberships of a group of evolved stars for each clusters, obtaining a sample of 4, 4, and 2 member stars in NGC 6192, NGC 6404, and NGC 6583, respectively. Using standard LTE analysis we derived stellar parameters and abundance ratios for the iron-peak elements Fe, Ni, Cr, and for the α-elements Al, Mg, Si, Ti, Ca. We calculated the orbits of the OCs currently located within 8 kpc from the GC, and discuss their implication on the present-time radial location. Results. The average metallicities of the three clusters are all oversolar: [Fe/H] = +0.12±0.04 (NGC 6192), +0.11±0.04 (NGC 6404), +0.37 ± 0.03 (NGC 6583). They are in qualitative agreement with their Galactocentric distances, because they are all internal OCs, and expected to be more metal rich than the solar neighbourhood. The abundance ratios of the other elements over iron [X/Fe] are consistent with solar values. Conclusions. The clusters we have analysed confirm, together with other OC and Cepheid data, a steep gradient in the inner disc, a signature of an evolutionary rate that is different from the one in the outer disc.
EPJ Web of Conferences
I outline a few features of recent models for the formation of the neutron source 13C(α,n)16O in ... more I outline a few features of recent models for the formation of the neutron source 13C(α,n)16O in low mass stars (1 ≲ M/M⊙ ≲ 3, LMS ) ascendingfor the second time the Red Giant Branch, generally called Asymptotic Giant Branch, or AGB stars. I also briefly outline the nucleosynthesis results obtained trough them. The mentioned models consider the physical structure below the frequent downward extensions of the convective envelope into the He-intershell (the so-called third dredge-up or TDU episodes). There, the conditions are such that the occurrence of further mixing is strongly facilitated, due to the minimal temperature gradient. A way to induce proton mixing from the envelope (certainly not the only one) arises whenever the ambient magnetic fields expected for LMS promote the buoyancy of strongly magnetized flux tubes. I review some characteristics of the ensuing mixing episodes, mentioning how different hydrodynamical processes might yield similar effects, thus encouraging stella...
The European Physical Journal A
The European Physical Journal A
Frontiers in Astronomy and Space Sciences
We present the status of nucleosynthesis beyond Sr, using up-to-date nuclear inputs for both the ... more We present the status of nucleosynthesis beyond Sr, using up-to-date nuclear inputs for both the slow (s-process) and rapid (r-process) scenarios of neutron captures. It is now widely accepted that at least a crucial part of the r-process distribution is linked to neutron star merger (NSM) events. However, so far, we have found only a single direct observation of such a link, the kilonova GW170817. Its fast evolution could not provide strict constraints on the nucleosynthesis details, and in any case, there remain uncertainties in the local r-process abundance patterns, which are independent of the specific astrophysical site, being rooted in nuclear physics. We, therefore, estimate the contributions from the r-process to solar system (S.S.) abundances by adopting the largely site-independent waiting-point concept through a superposition of neutron density components normalized to the r-abundance peaks. Nuclear physics inputs for such calculations are understood only for the trans-F...
Astronomy and Astrophysics, 2007
Context. The stellar production of the light element lithium is still a matter of debate. Aims. W... more Context. The stellar production of the light element lithium is still a matter of debate. Aims. We report the detection of low-mass, Li-rich Asymptotic Giant Branch (AGB) stars located in the Galactic bulge. Methods. A homogeneous and well-selected sample of low mass, oxygen-rich AGB stars in the Galactic bulge has been searched for the absorption lines of Li. Using spectral synthesis techniques, we determine from high resolution UVES/VLT spectra the Li abundance in four out of 27 sample stars, and an upper limit for the remaining stars. Results. Two stars in our sample have a solar Li abundance or above; these stars seem to be a novelty, since they do not show any s-element enhancement. Two more stars have a Li abundance slightly below solar; these stars do show s-element enhancement in their spectra. Different scenarios which lead to an increased Li surface abundance in AGB stars are discussed. Conclusions. Of the different enrichment scenarios presented, Cool Bottom Processing (CBP) is the most likely one for the Li-rich objects identified here. Self-enrichment by Hot Bottom Burning (HBB) seems very unlikely as all Li-rich stars are below the HBB mass limit. Also, the ingestion of a low mass companion into the stars' envelope is unlikely because the associated additional effects are lacking. Mass transfer from a former massive binary companion is a possible scenario, if the companion produced little s-process elements. A simple theoretical estimation for the Li abundance due to CBP is presented and compared to the observed values.
The study of the evolutionary properties of Asymptotic Giant Branch stars still presents unresolv... more The study of the evolutionary properties of Asymptotic Giant Branch stars still presents unresolved topics. Progress in the theoretical understanding of their evolution is hampered by the difficulty to empirically explain key physical parameters like their luminosity, mass loss rate and chemical abundances. We are performing an analysis of Galactic AGB stars trying to find constraints for these parameters. Our aim is of extending this analysis to the AGB stars of the Magellanic Clouds and of the Dwarf Spheroidal Galaxies using also mid-infrared observations from the Antarctic telescope IRAIT. AGB sources from the Magellanic Clouds will be fundamental in our understanding of the AGB evolution because they are all at a well defined distance (differently from the Galactic AGBs). Moreover, these sources present different values of metallicity: this fact should permit us of examining in a better way their evolutionary properties comparing their behaviour with the one from Galactic sources.
The enrichment of Pb in the Galaxy is followed in the framework of a detailed model of Galactic c... more The enrichment of Pb in the Galaxy is followed in the framework of a detailed model of Galactic chemical evolution that already proved adequate to reproduce the chemical enrichment of O and of the elements from Ba to Eu. The stellar yields are computed through nucleosynthesis calculations in the Asymptotic Giant Branch (AGB) phase of low- and intermediate-mass stars, covering a wide range of metallicities. The physical parameters of the stellar structure were derived from full stellar evolutionary models previously computed. We show that low-mass AGB stars are the main producers of Pb in the Galaxy, with a complex dependence on metallicity and a maximum efficiency at [Fe/H] ~ -1. Our calculations succeed in reproducing the abundances of Pb isotopes in the solar system: the role attributed by the classical analysis of the s-process to the 'strong component', in order to explain more than 50% of solar 208Pb, is actually played by the high production of Pb in low-mass and low-m...
Context. Technetium (Tc) is an unstable element synthesised via the s-process in deep layers of a... more Context. Technetium (Tc) is an unstable element synthesised via the s-process in deep layers of asymptotic giant branch (AGB) stars, making it a reliable indicator of both recent s-process activity and third dredge-up. Aims. We searched for Tc in a sample of bright oxygen-rich asymptotic giant branch (AGB) stars located in the outer galactic bulge. We test theoretical predictions of the luminosity limit for the onset of third dredge-up. Methods. Using high-resolution optical spectra obtained with the UVES spectrograph at ESO's VLT, we searched for resonance lines of neutral Tc in the blue spectral region of our sample stars. These measurements allow us to improve the procedure of classification of stars with respect to their Tc content by using flux ratios. Synthetic spectra based on MARCS atmospheric models are presented and compared to the observed spectra around three lines of Tc. Bolometric magnitudes are calculated based on the near-infrared photometry of the objects. Resul...
The advection of thermonuclear ashes by magnetized domains emerging from near the H-shell was sug... more The advection of thermonuclear ashes by magnetized domains emerging from near the H-shell was suggested to explain AGB star abundances. Here we verify this idea quantitatively through exact MHD models. Starting with a simple 2D geometry and in an inertia frame, we study plasma equilibria avoiding the complications of numerical simulations. We show that, below the convective envelope of an AGB star, variable magnetic fields induce a natural expansion, permitted by the almost ideal MHD conditions, in which the radial velocity grows as the second power of the radius. We then study the convective envelope, where the complexity of macro-turbulence allows only for a schematic analytical treatment. Here the radial velocity depends on the square root of the radius. We then verify the robustness of our results with 3D calculations for the velocity, showing that, for both the studied regions, the solution previously found can be seen as a planar section of a more complex behavior, in which an...
We present the results of s-process nucleosynthesis calculations for AGB stars of different metal... more We present the results of s-process nucleosynthesis calculations for AGB stars of different metallicities and initial masses. The computations were based on previously published stellar evolutionary models that account for the III dredge up phenomenon occurring late on the AGB. Neutron production is driven by the 13C(alpha,n)16O reaction during the interpulse periods in a tiny layer in radiative equilibrium at the top of the He- and C-rich shell. The s-enriched material is subsequently mixed with the envelope by the III dredge up, and the envelope composition is computed after each thermal pulse. We follow the changes in the photospheric abundance of the Ba-peak elements (heavy s, or `hs') and that of the Zr-peak ones (light s, or `ls'), whose logarithmic ratio [hs/ls] has often been adopted as an indicator of the s-process efficiency. The theoretical predictions are compared with published abundances of s elements for Galactic AGB giants of classes MS, S, SC, post-AGB super...
We present new spectroscopic observations for a sample of C(N)-type red giants. These objects bel... more We present new spectroscopic observations for a sample of C(N)-type red giants. These objects belong to the class of Asymptotic Giant Branch stars, experiencing thermal instabilities in the He-burning shell (thermal pulses). Mixing episodes called third dredge-up enrich the photosphere with newly synthesized C12 in the He-rich zone, and this is the source of the high observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic abundance estimates confirm that, in agreement with the general understanding of the late evolutionary stages of low and intermediate mass stars, carbon enrichment is accompanied by the appearance of s-process elements in the photosphere. We discuss the details of the observations and of the derived abundances, focusing in particular on rubidium, a neutron-density sensitive element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The critical reaction branching at Kr85, which determines the relative enrichment of the stu...
It is known that there must be some weak form of transport (called cool bottom processing, or CBP... more It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low mass RGB and AGB stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra-mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer, and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 solar mass, in both the AGB and RGB phases. The field required for the AGB star in the processing zone is ...
AGB phases mark the end of the evolution for Low- and Intermediate-Mass Stars. Our understanding ... more AGB phases mark the end of the evolution for Low- and Intermediate-Mass Stars. Our understanding of the mechanisms through which they eject the envelope and our assessment of their contribution to the chemical evolution of Galaxies are hampered by poor knowledge of their Luminosities and mass loss rates,both for C-rich and for O-rich sources.We plan to establish criteria permitting a more quantitative determination of luminosities for the various types of AGB stars on the basis of IR fluxes.In this paper we concentrate on O-rich and s-element-rich MS, S stars and include a small sample of SC stars.We reanalyze the absolute bolometric magnitudes and colors of MS, S, SC stars on the basis of a sample of intrinsic and extrinsic long period variables.We derive bolometric corrections as a function of near- and mid-IR colors,adopting as references a group of stars for which the SED could be reconstructed in detail over a large wavelength range.We determine the absolute HR diagrams and com...
Universe
Theoretical predictions as well as experiments performed at storage rings have shown that the lif... more Theoretical predictions as well as experiments performed at storage rings have shown that the lifetimes of β-radionuclides can change significantly as a function of the ionization state. In this paper we describe an innovative approach, based on the use of a compact plasma trap to emulate selected stellar-like conditions. It has been proposed within the PANDORA project (Plasmas for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry) with the aim to measure, for the first time in plasma, nuclear β-decay rates of radionuclides involved in nuclear-astrophysics processes. To achieve this task, a compact magnetic plasma trap has been designed to reach the needed plasma densities, temperatures, and charge-states distributions. A multi-diagnostic setup will monitor, on-line, the plasma parameters, which will be correlated with the decay rate of the radionuclides. The latter will be measured through the detection of the γ-rays emitted by the excited daughter nuclei follo...
Abstract. We study the circulation of matter in AGB stars above the H-burning shell, which is kno... more Abstract. We study the circulation of matter in AGB stars above the H-burning shell, which is known to yield the appearance of p-rich isotopes like 13C, 17O and the unstable 26Al at the stellar surface. These nuclei were observed in presolar grains of AGB origin and in some cases (e.g. C isotopes) in spectroscopic observations of evolved stars. For the physical mechanism driving the mixing we consider magnetic flux tube buoyancy. Magnetic tubes are formed below the convective envelope and one can express the parameters of the required mixing phenomena in terms of the magnetic field intensity |−→B |. We show that the required values of |−→B | can span a considerable range. If the mixing arrives only at the innermost layers of the convective envelope, fields smaller than the solar ones are sufficient, according to our previous analysis. If instead magnetic fields have to appear at the surface, as is suggested by recent observations, than field intensities in the Megagauss range are ne...
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Papers by Maurizio M. Busso