1,720,967 research outputs found
MHD as a driver for mixing in AGB stars
No full textWe present analytical exact 2D and 3DMHDcomputations for the layers of an AGB star known to be affected by deep mixing phenomena, in order to verify previous suggestions that magnetic buoyancy may provide a sound explaination for the isotopic changes observed in AGB stars and in presolar grains. The structure of the relevant layers is similar to a polytrope of index 3 (a bubble of radiation), containing little mass. Due to this, the material is close to be unstable for expansion. Addition of any extra engine under the form of a magnetic dynamo generating toroidal structures unstable for buoyancy yields plasma phenomena that closely resemble those of the solar wind, in which almost ideal, non-resistive MHD allows for an easy analytical integration of the model equations. The results show that a further expansion occurs for magnetized domains (flux tubes). These last form close to thermonuclear shells and transport outward nucleosynthesis products with a velocity v ∼ r2, faster than for diffusion but slower than for convection, adequate to give a physical interpretation to extra-mixing processes in evolved stars
A deep mixing solution to the aluminum and oxygen isotope puzzles in pre-solar grains
No full textWe present here the application of a model for a mass circulation mechanism in between the H-burning shell and the base of the convective envelope of low-mass asymptotic giant branch (AGB) stars, aimed at studying the isotopic composition of those pre-solar grains showing the most extreme levels of 18O depletion and high concentration of 26Mg from the decay of 26Al. The mixing scheme we present is based on a previously suggested magnetic-buoyancy process, already shown to account adequately for the formation of the main neutron source for slow neutron captures in AGB stars. We find that this scenario is also capable of reproducing for the first time the extreme values of the 17O/16O, 18O/16O, and 26Al/27Al isotopic ratios found in the mentioned oxide grains, including the highest amounts of 26Al measured there
MHD as a driver for mixing in AGB stars
No full textWe present analytical exact 2D and 3DMHDcomputations for the layers of an AGB star known to be affected by deep mixing phenomena, in order to verify previous suggestions that magnetic buoyancy may provide a sound explaination for the isotopic changes observed in AGB stars and in presolar grains. The structure of the relevant layers is similar to a polytrope of index 3 (a bubble of radiation), containing little mass. Due to this, the material is close to be unstable for expansion. Addition of any extra engine under the form of a magnetic dynamo generating toroidal structures unstable for buoyancy yields plasma phenomena that closely resemble those of the solar wind, in which almost ideal, non-resistive MHD allows for an easy analytical integration of the model equations. The results show that a further expansion occurs for magnetized domains (flux tubes). These last form close to thermonuclear shells and transport outward nucleosynthesis products with a velocity v ∼ r2, faster than for diffusion but slower than for convection, adequate to give a physical interpretation to extra-mixing processes in evolved stars
The solar Lithium problem: is the explanation due solely to mixing or also to the e−-capture decay rate of <sup>7</sup>Be?
Full text linkThe nucleosynthesis of 7Li is one of the most crucial problems in nuclear as- trophysics, as its observations in several sites are hard to be explained. Concerning the Sun, the most common interpretations of the low Li abundance invoke either burning in early stages or non-convective mixing below the envelope. Here we apply a diffusive mechanism of mixing, together with a recent estimate of the rate for e−-captures on 7Be, to establish whether the solar Li destruction should be attributed to purely pre-Main Se- quence (MS) nuclear processes or if the coupling of mixing and nucleosynthesis on the MS can account for it. Our preliminary results indicate that, whether Li survives the pre- MS phase, the changes of the 7Be e−-capture rate do not affect its production/destruction. The low Li abundance should then depend only on diffusion processes from the bottom of the convective envelope to the lowerlying tachocline zone. We suggest that, if diffusive processes occurred over the age of the Sun, they required diffusive mass transfers of a few 10−13 M⊙/yr to explain the Li drop. This is a high estimate: future works will tell us if it is realistic or not. In this second case, pre-MS burning would remain the only alternative
S-PROCESSING in AGB STARS REVISITED. II. ENHANCED 13C PRODUCTION THROUGH MHD-INDUCED MIXING
No full textSlow neutron captures are responsible for the production of about 50% of elements heavier than iron, mainly occurring during the asymptotic giant branch phase of low-mass stars (1 lsim; M/Mo ≲3), where the main neutron source is the 13C(α, n)16O reaction. This last reaction is activated from locally produced 13C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt to describe a physical mechanism for the formation of the 13C reservoir, studying the mass circulation induced by magnetic buoyancy without adding new free parameters to those already involved in stellar modeling. Our approach represents the application to the stellar layers relevant for s-processing of recent exact analytical 2D and 3D models for magneto-hydrodynamic processes at the base of convective envelopes in evolved stars in order to promote downflows of envelope material for mass conservation during the occurrence of a dredge-up phenomenon. We find that the proton penetration is characterized by small concentrations, but is extended over a large fractional mass of the He-layers, thus producing 13C reservoirs of several 10-3 Mo. The ensuing 13C-enriched zone has an almost flat profile, while only a limited production of 14N occurs. In order to verify the effects of our new findings we show how the abundances of the main s-component nuclei can be accounted for in solar proportions and how our large 13C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects
s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Sourve in the 13C-pocket
No full textNeutrons for -processing at are mainly provided by \ctanb in AGB stars, requiring some proton penetration below the envelope so far assumed to be of small mass ( \ms). However, models with rotation suggested that there excessive N would hamper -processing. On the other hand, -element abundances in Galaxies require \ct-rich layers more extended in mass. We present new calculations for clarifying the above issues, aiming at understanding if the solar composition can help in fixing the extension of the \ctb ``pocket''. We show: i) that mixing ``from bottom to top'' (like in magnetic buoyancy or other forced mechanisms) might in principle form a \ctb reservoir much larger than assumed so far; ii) that stellar models at a suitable metallicity, using a similarly extended pocket would reproduce the main -component as accurately as before; iii) that with the extended pocket the previously envisaged contributions from an unknown nucleosynthesis process () would no longer be required. The new scheme also fulfils the requirements of C-star luminosities. Consisting of a few large neutron exposures, it would imply a large production of nuclei below ; in particular, Sr would be fully synthesized by AGB stars, while Sr, Y and Zr would be contributed more efficiently by the new model. We suggest some tests, which would probably say a final word on the real extension of the \ctb pocket
Can the Main Component of the s-Process in AGB Stars Constrain the C-13-Pocket Formation?
No full textThe s-process main component is mainly produced in low-mass AGB stars by the C-13(alpha, n)O-16 reaction, requiring proton injection from the envelope. The C-13 pocket was typically assumed to involve a small mass (= 4 x 10(-3) M-circle dot). We speculated that mixing driven by magnetic buoyancy (as in Maiorca et al. 2012; Trippella et al. 2014, or other forced mechanisms "from bottom to top") can form a C-13 reservoir larger than assumed so far, covering most of the He-rich layers. We present new calculations (Trippella et al. 2014) aimed at understanding if the solar composition helps to constrain the C-13-pocket extension. Stellar models at a fixed metallicity, based on a large C-13 reservoir, reproduce the main s-component as accurately as before and don't require any nuclear contribution from an unknown nucleosynthesis processes (LEPP). These models also avoid problems of mixing at the envelope border and meet requirements from C-star luminosities. A large production of nuclei below A = 90 is expected, so that (86)'Sr-87 may be fully synthesized by AGB stars, while Sr-88, Y-89 and Zr-94 are contributed more efficiently than before
UPDATED THM ASTROPHYSICAL FACTOR OF THE REACTION AND INFLUENCE OF NEW DIRECT DATA AT ASTROPHYSICAL ENERGIES
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