2,153 research outputs found
s-Processing in the Galactic Disk. I. Super-Solar Abundances of Y, Zr, La, Ce in Young Open Clusters
In a recent study, based on homogeneous barium abundance measurements in open
clusters, a trend of increasing [Ba/Fe] ratios for decreasing cluster age was
reported. We present here further abundance determinations, relative to four
other elements hav- ing important s-process contributions, with the aim of
investigating whether the growth found for [Ba/Fe] is or not indicative of a
general property, shared also by the other heavy elements formed by slow
neutron captures. In particular, we derived abundances for yttrium, zirconium,
lanthanum and cerium, using equivalent widths measurements and the MOOG code.
Our sample includes 19 open clusters of different ages, for which the spectra
were obtained at the ESO VLT telescope, using the UVES spectrometer. The growth
previously suggested for Ba is confirmed for all the elements analyzed in our
study. This fact implies significant changes in our views of the Galactic
chemical evolution for elements beyond iron. Our results necessarily require
that very low-mass AGB stars (M < 1.5M\odot) produce larger amounts of
s-process elements (hence acti- vate the 13 C-neutron source more effectively)
than previously expected. Their role in producing neutron-rich elements in the
Galactic disk has been so far underestimated and their evolution and
neutron-capture nucleosynthesis should now be reconsidered.Comment: ApJ accepte
The effects of a revised Be e-capture rate on solar neutrino fluxes
The electron-capture rate on Be is the main production channel for Li
in several astrophysical environments. Theoretical evaluations have to account
for not only the nuclear interaction, but also the processes in the plasma
where Be ions and electrons interact. In the past decades several estimates
were presented, pointing out that the theoretical uncertainty in the rate is in
general of few percents. In the framework of fundamental solar physics, we
consider here a recent evaluation for the Be+e rate, not used up to now
in the estimate of neutrino fluxes. We analysed the effects of the new
assumptions on Standard Solar Models (SSMs) and compared the results obtained
by adopting the revised Be+e rate to those obtained by the one reported
in a widely used compilation of reaction rates (ADE11). We found that new SSMs
yield a maximum difference in the efficiency of the Be channel of about
-4\% with respect to what is obtained with the previously adopted rate. This
fact affects the production of neutrinos from B, increasing the relative
flux up to a maximum of 2.7\%. Negligible variations are found for the physical
and chemical properties of the computed solar models. The agreement with the
SNO measurements of the neutral current component of the B neutrino flux is
improved.Comment: 7 pages, 3 figures, 4 tables. Accepted for the publication on A&
A physics-based life prediction methodology for thermal barrier coating systems
A novel mechanistic approach is proposed for the prediction of the life of
thermal barrier coating (TBC) systems. The life prediction methodology is based
on a criterion linked directly to the dominant failure mechanism. It relies on
a statistical treatment of the TBC's morphological characteristics,
non-destructive stress measurements and on a continuum mechanics framework to
quantify the stresses that promote the nucleation and growth of microcracks
within the TBC. The last of these accounts for the effects of TBC constituents'
elasto-visco-plastic properties, the stiffening of the ceramic due to sintering
and the oxidation at the interface between the thermally insulating yttria
stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic
approach is used to investigate the effects on TBC life of the properties and
morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond
coat and the thermally grown oxide. Its calibration is based on TBC damage
inferred from non-destructive fluorescence measurements using
piezo-spectroscopy and on the numerically predicted local TBC stresses
responsible for the initiation of such damage. The potential applicability of
the methodology to other types of TBC coatings and thermal loading conditions
is also discussed
Low-mass lithium-rich AGB stars in the Galactic bulge: evidence for Cool Bottom Processing?
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.Comment: 5 pages, 3 figures, accepted by A&A Letter
Al production from magnetically induced extramixing in AGB Stars
We discuss nucleosynthesis results obtained following the recent suggestion
that extramixing phenomena in red giants might be driven by magnetic buoyancy.
We explore for this model the production of the short-lived radioactive isotope
Al and of stable light nuclei, considering both the case of the general
buoyancy of flux tubes and that of the intermittent release of magnetized
unstable structures. We show that abundant Al can be produced, up to,
and above, the highest levels measured in presolar grains. This level would be
also sufficient to explain the early solar system Al as coming from a
nearby AGB star of low mass. The case of fast-moving instabilities is the most
efficient, reaching almost the same effectiveness as hot bottom burning (HBB)
Discovery of Blue Hook Stars in the Massive Globular Cluster M54
We present BV photometry centered on the globular cluster M54 (NGC 6715). The
color-magnitude diagram clearly shows a blue horizontal branch extending
anomalously beyond the zero age horizontal branch theoretical models. These
kinds of horizontal branch stars (also called ``blue hook'' stars), which go
beyond the lower limit of the envelope mass of canonical horizontal branch hot
stars, have so far been known to exist in only a few globular clusters: NGC
2808, Omega Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like
M54, are among the most luminous in our Galaxy, indicating a possible
correlation between the existence of these types of horizontal branch stars and
the total mass of the cluster. A gap in the observed horizontal branch of M54
around T(eff)= 27000 K could be interpreted within the late helium flash
theoretical scenario, a possible explanation for the origin of those stars.Comment: 10 pages, 2 figures, accepted for publication in the Astrophysical
Journa
The peculiar horizontal branch morphology of the Galactic globular clusters NGC 6388 and NGC 6441: new insights from UV observations
Context. In this paper we present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441.
Aims. We investigate the properties of their anomalous horizontal branches in different photometric planes in order to shed light on the nature of the physical mechanism(s) responsible for the existence of an extended blue tail and of a slope in the horizontal branch, visible in all the color-magnitude diagrams.
Methods. New photometric data have been collected and carefully reduced. Empirical data have been compared with updated stellar models of low-mass, metal-rich, He-burning structures, transformed to the observational plane with appropriate model atmospheres.
Results. We have obtained the first UV color-magnitude diagrams for NGC 6388 and NGC 6441. These diagrams confirm previous results, obtained in optical bands, about the presence of a sizeable stellar population of extremely hot horizontal branch stars. At least in NGC 6388, we find a clear indication that at the hot end of the horizontal branch the distribution of stars forms a hook-like feature, closely resembling those observed in NGC 2808 and Omega Cen. We briefly review the theoretical scenarios that have been suggested for interpreting this observational feature. We also investigate the tilted horizontal branch morphology and provide further evidence that supports early suggestions that this feature cannot be interpreted as an effect of differential reddening. We show that a possible solution of the puzzle is to assume that a small fraction - ranging between 10-20% - of the stellar population in the two clusters is strongly helium-enriched (Y ~ 0.40 in NGC 6388 and Y ~ 0.35 in NGC 6441). The occurrence of a spread in the He abundance between the canonical value (Y ~ 0.26) and the quoted upper limits can significantly help in explaining the "whole" morphology of the horizontal branch and the pulsational properties of the variable stars in the target clusters
On the Origin of the Early Solar System Radioactivities. Problems with the AGB and Massive Star Scenarios
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 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 of them, especially if -process
nuclei are produced by neutron star merging according to recent models.
Instead, Al, Ca, Cs and possibly 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). Models of their final stages now
predict the ubiquitous formation of a C reservoir as a neutron capture
source; hence, even in presence of Al production from Deep Mixing or Hot
Bottom Burning, the ratio Al/Pd remains incompatible with
measured data, with a large excess in 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 invoking 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.Comment: Accepted for publication on Ap
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