s-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

Palmerini, S.; Trippella, O.; Busso, M.; Vescovi, D.; Petrelli, Maurizio; Zucchini, Azzurra; Frondini, Francesco

doi:10.1016/j.gca.2017.05.030

Cited by 34 publications

(30 citation statements)

References 76 publications

(147 reference statements)

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“…In comparison, model calculations by adopting variable 13 C-pockets, especially variable 13 C mass fractions, predict a wide range of s-process end-members. Therefore, previous arguments that heavy-element isotopic data of MS SiC grains require a wide range of 13 C mass fractions (e.g., Lugaro et al 2003) and/or large variations in the mass loss rate (Palmerini et al 2018) do not seem to hold true any more, especially because the highprecision Ni isotope data fall along straight lines with limited scatter in Figs. 6b and 6c.…”

Section: Resultsmentioning

confidence: 91%

“…We further tested the effects of variable mass extensions and 13 C mass fractions for the Trippella 13 C-pocket based on the consideration that the derivation of the magneticbuoyancy-induced 13 C-pocket is based on the chosen magnetic field (and thus buoyancy velocity) and dynamic viscosity (Palmerini et al 2018), and that variations in both parameters can lead to different amounts of 13 C formed in the 13 C-pocket. Note that we do not argue against the robustness of the results of Nucci & Busso (2014) by considering these variations, as the relative 13 C profile within the pocket, which was derived by those authors from detailed three-dimensional magnetohydrodynamic simulations, indeed provides better match to the Sr-Ba and Ni isotope data than the Exponential and Flat 13 C profiles and thus remains the same in these tests.…”

Section: Resultsmentioning

confidence: 99%

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New Constraints on the Major Neutron Source in Low-mass AGB Stars

Liu

Gallino

Cristallo

et al. 2018

ApJ

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We compare updated Torino postprocessing asymptotic giant branch (AGB) nucleosynthesis model calculations with isotopic compositions of mainstream SiC dust grains from low-mass AGB stars. Based on the data-model comparison, we provide new constraints on the major neutron source, 13 C(α,n) 16 O in the He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba grain data can only be consistently explained by the Torino model calculations that adopt the recently proposed magnetic-buoyancy-induced 13 C-pocket. This observation provides strong support to the suggestion of deep mixing of H into the Heintershell at low 13 C concentrations as a result of efficient transport of H through magnetic tubes. The Astrophysical Journal 2The main s-process 1 occurs in thermally pulsing asymptotic giant branch (AGB) stars, the final evolutionary stage of low-to-intermediate mass stars. Figure 1 illustrates the evolution of a 3 M ¤ , 1.5 Z ¤ 2 AGB star predicted by FRUITY model calculations (Cristallo et al. 2011). The star can be divided into three main regions: an innermost zone corresponding to the partially degenerate CO core, inside the magenta line; an external zone corresponding to the H-rich convective envelope, outside the black line; and a thin region (10 −2 −10 −3 M⊙) between the magenta and black lines, called the He-intershell. The red and blue lines show where maximum energy release from H-burning and within the He-intershell takes place, respectively. The blue line shows a plateau during each interpulse period, where the main energy release in the He-intershell is from the 13 C(α,n) 16 O reaction. The base of the H-rich envelope is radiative between the black and red lines. Hydrogen burning is activated for most of the time, on the top of the growing inert He-intershell. The He-intershell is heated and compressed until the temperature and density at its bottom are high enough to trigger thermonuclear runaways by He-burning (Thermal Pulse, TP, Iben & Renzini 1983). Because of the surplus of energy locally released, the whole He-intershell becomes unstable and convects. As a consequence of the energy release and expansion of the He-shell, the overlying layers, including the H-shell, are pushed to larger radii (e.g., numbered TPs in Fig. 1). At the quenching of a TP, the He-shell returns to radiative conditions; conversion of a significant fraction of the 4 He in the He-shell into 12 C is responsible for the jumps in size of the CO core (magenta line) after each TP. After a few hundred years, if the H-shell has been sufficiently lifted by the TP, the convective envelope penetrates the underlying region and brings newly synthesized materials to the surface. This process is called third dredge-up (TDU). Consequently, the convective envelope becomes more and more C-rich. As soon as the temperature becomes sufficiently high, H burning is reactivated in the H-shell. The time elapsed from the maximum penetration of the convective envelope and the hydrogen reignition is a few thousand years. An AGB star s...

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Section: Resultsmentioning

confidence: 91%

Section: Resultsmentioning

confidence: 99%

New Constraints on the Major Neutron Source in Low-mass AGB Stars

Liu

Gallino

Cristallo

et al. 2018

ApJ

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“…This last needs to be corrected for deriving a pure (solar) r-process component. This can be done using the r-residuals (1-s) from the models quoted in the next Section, in particular from Palmerini et al (2018). With respect to previous computations, these models yield lower estimates for the Dy r /Ba r ratio in the Sun, in the range 2.7 to 5, with an average value depending on the Initial Mass Function adopted for the weighting, on mass loss rates etc.…”

Section: Observed Constraints and R-process Sourcesmentioning

confidence: 99%

“…Again, we use the suffix "p" to indicate the values of the parameters pertaining to the layer from which buoyancy (on average) starts. For more details on the solution and its stellar applications see Nucci & Busso (2014); Trippella et al (2016); Palmerini et al (2017Palmerini et al ( , 2018.…”

Section: The Reference Models For Intermediate Mass Starsmentioning

confidence: 99%

On the Origin of Early Solar System Radioactivities: Problems with the Asymptotic Giant Branch and Massive Star Scenarios

Vescovi

Busso

Palmerini

et al. 2018

ApJ

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Recent improvements in stellar models for intermediate-mass and massive stars are recalled, together with their expectations for the synthesis of radioactive nuclei of lifetime τ 25 Myr, in order to re-examine the origins of now extinct radioactivities, which were alive in the solar nebula. The Galactic inheritance broadly explains most 2 of them, especially if r-process nuclei are produced by neutron star merging according to recent models. Instead, 26 Al, 41 Ca , 135 Cs and possibly 60 Fe require nucleosynthesis events close to the solar formation. We outline the persisting difficulties to account for these nuclei by Intermediate Mass Stars (2 M/M 7 − 8). Models of their final stages now predict the ubiquitous formation of a 13 C reservoir as a neutron capture source; hence, even in presence of 26 Al production from Deep Mixing or Hot Bottom Burning, the ratio 26 Al/ 107 Pd remains incompatible with measured data, with a large excess in 107 Pd. This is shown for two recent approaches to Deep Mixing. Even a late contamination by a Massive Star meets problems. In fact, inhomogeneous addition of Supernova debris predicts non-measured excesses on stable isotopes. Revisions invok-ing specific low-mass supernovae and/or the sequential contamination of the pre-solar molecular cloud might be affected by similar problems, although our conclusions here are weakened by our schematic approach to the addition of SN ejecta. The limited parameter space remaining to be explored for solving this puzzle is discussed.

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“…[30]. As an example of the further results that can be obtained in this scenario, we consider here the possible contributions of an intermediate-mass star to the production of the short-lived (τ ≤ 20 Myr) radioactivities (SLR) that were found to be alive in the first condensates of the Early Solar System (ESS).…”

Section: Introductionmentioning

confidence: 99%

Neutron-captures in Low Mass Stars and the Early Solar System Record of Short-lived Radioactivities

Busso¹,

Vescovi²,

Trippella³

et al. 2017

EPJ Web Conf.

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Abstract. 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 H-and 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.

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s-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

Cited by 34 publications

References 76 publications

New Constraints on the Major Neutron Source in Low-mass AGB Stars

New Constraints on the Major Neutron Source in Low-mass AGB Stars

On the Origin of Early Solar System Radioactivities: Problems with the Asymptotic Giant Branch and Massive Star Scenarios

Neutron-captures in Low Mass Stars and the Early Solar System Record of Short-lived Radioactivities

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