15,020 research outputs found
Evolution and chemical yields of AGB stars: effects of low-temperature opacities
The studies focused on the Thermally-Pulsing Asymptotic Giant Branch phase
experienced by low- and intermediate-mass stars are extremely important in many
astrophysical contexts. In particular, a detailed computation of their chemical
yields is essential for several issues, ranging from the chemical evolution of
galaxies, to the mechanisms behind the formation of globular clusters. Among
all the uncertainties affecting the theoretical modelling of this phase, and
described in the literature, it remains to be fully clarified which results are
severely affected by the use of inadequate low-temperature opacities, that are
in most cases calculated on the basis of the original chemical composition of
the stars, and do not consider the changes in the surface chemistry due to the
occurrence of the third dredge-up and hot-bottom burning. Our investigation is
aimed at investigating this point. By means of full evolutionary models
including new set of molecular opacities computed specifically with the AESOPUS
tool, we highlight which stellar models, among those present in the literature,
need a substantial revision, mainly in relation to the predicted chemical
yields. The interplay among convection, hot bottom burning and the
low-temperature opacity treatment is also discussedComment: 6 pages, 2 figure
A model for the Globular Cluster extreme anomalies
In spite of the efforts made in the latest years, still there is no
comprehensive explanation for the chemical anomalies of globular cluster stars.
Among these, the most striking is oxygen depletion, which reaches values down
to [O/Fe]~-0.4 in most clusters, but in M13 it goes down to less than [O/Fe]~ -
1. In this work we suggest that the anomalies are due to the super position of
two different events: 1) PRIMORDIAL SELF-ENRICHMENT: this is asked to explain
the oxygen depletion down to a minimum value [O/Fe]~ -0.4; 2) EXTRA MIXING IN A
FRACTION OF THE STARS ALREADY BORN WITH ANOMALOUS COMPOSITION: these objects,
starting with already low [O/Fe], will reduce the oxygen abundance down to the
most extreme values. Contrary to other models that invoke extra mixing to
explain the chemical anomalies, we suggest that it is active only if there is a
fraction of the stars in which the primordial composition is not only oxygen
depleted, but also extremely helium rich (Y~ 0.4), as found in a few GCs from
their main sequence multiplicity. We propose that the rotational evolution (and
an associated extra mixing) of extremely helium rich stars may be affected by
the fact that they develop a very small or non existent molecular weight
barrier during the evolution. We show that extra mixing in these stars, having
initial chemistry that has already been CNO processed, affects mainly the
oxygen abundance, and to a much smaller extent if affects the sodium abundance.
The model also predicts a large fluorine depletion concomitant with the oxygen
depletion, and a further enhancement of the surface helium abundance, which
reaches values close to Y=0.5 in the computed models. We stress that, in this
tentative explanation, those stars that are primordially O--depleted, but ARE
NOT extremely helium rich do not suffer deep extra mixing.Comment: 12 pages, 8 figures and 5 tables, accepted for publication in MNRA
Modelling the closest double degenerate system RXJ0806.3+1527 and its decreasing period
In the hypothesis that the 5.4m binary RXJ0806.3+1527 consists of a low mass
helium white dwarf (donor) transferring mass towards its more massive white
dwarf companion (primary), we consider as possible donors white dwarfs which
are the result of common envelope evolution occurring when the helium core mass
of the progenitor giant was still very small (~ 0.2Msun), so that they are
surrounded by a quite massive hydrogen envelope (~1/100Msun or larger), and
live for a very long time supported by proton--proton burning. Mass transfer
from such low mass white dwarfs very probably starts during the hydrogen
burning stage, and the donor structure will remain dominated by the burning
shell until it loses all the hydrogen envelope and begins transferring helium.
We model mass transfer from these low mass white dwarfs, and show that the
radius of the donor decreases while they shed the hydrogen envelope. This
radius behavior, which is due to the fact that the white dwarf is not fully
degenerate, has two important consequences on the evolution of the binary: 1)
the orbital period decreases, with a timescale consistent with the period
decrease of the binary RXJ0806.3+1527; 2) the mass transfer rate is a factor of
about 10 smaller than from a fully degenerate white dwarf, easing the problem
connected with the small X-ray luminosity of this object. The possibility that
such evolution describes the system RXJ0806.3+1527 is also consistent with the
possible presence of hydrogen in the optical spectrum of the star, whose
confirmation would become a test of the model.Comment: 13 pages, 4 figures, accepted for publication on ApJ, main journa
Nanoscopic processes of Current Induced Switching in thin tunnel junctions
In magnetic nanostructures one usually uses a magnetic field to commute
between two resistance (R) states. A less common but technologically more
interesting alternative to achieve R-switching is to use an electrical current,
preferably of low intensity. Such Current Induced Switching (CIS) was recently
observed in thin magnetic tunnel junctions, and attributed to electromigration
of atoms into/out of the insulator. Here we study the Current Induced
Switching, electrical resistance, and magnetoresistance of thin
MnIr/CoFe/AlO/CoFe tunnel junctions. The CIS effect at room temperature
amounts to 6.9% R-change between the high and low states and is attributed to
nanostructural rearrangements of metallic ions in the electrode/barrier
interfaces. After switching to the low R-state some electro-migrated ions
return to their initial sites through two different energy channels. A low
(high) energy barrier of 0.13 eV (0.85 eV) was estimated. Ionic
electromigration then occurs through two microscopic processes associated with
different types of ions sites/defects. Measurements under an external magnetic
field showed an additional intermediate R-state due to the simultaneous
conjugation of the MR (magnetic) and CIS (structural) effects.Comment: 6 pages, 4 figure
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