1,953 research outputs found
Optical Identification of ROSAT sources in M 67: activity in an old cluster
We present optical identification and high-resolution spectroscopy of ROSAT
sources in the field of the old open cluster M 67. For the first time it is
possible to analyze coronal and chromospheric activity of active stars in a
solar-age cluster, and to compare it with field stars. ROSAT observed the high
X-ray luminosity tail of the cluster sources. In agreement with what expected
from studies of field stars, most of the detected X-ray sources are binaries,
preferably with short periods and eccentric orbits. In addition, several of the
M 67 ROSAT sources have peculiar locations in the cluster colour-magnitude
diagram. This is most likely due to rather complex evolutionary histories,
involving the presence of mass transfer or large mass losses. The X-ray
luminosity of the sources does not scale with the stellar parameters in an
obvious way. In particular, no relationship is found between coronal emission
and stellar magnitude or binary period. The Ca II K chromospheric flux from
most of the counterparts is in excess to that of single stars in the cluster by
one order of magnitude. The X-ray luminosity of the sources in the old M 67 is
one order of magnitude lower than the most active active binaries in the field,
but comparable to that of the much younger binaries in the Hyades.Comment: 11 pages, 6 figures. Accepted for publication by Astronomy &
Astrophysic
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found that in a
log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components.
One is consistent with predictions of evolutionary models while the other is
chemically indistinguishable from the thick-disk stars. This is interpreted as
a difference in the star formation history of the two components and suggests
that the local halo is not a single uniform population where a clear
age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray
spallation in the ISM. Assuming that the cosmic-rays are globally transported
across the Galaxy, the beryllium production should be a widespread process and
its abundance should be roughly homogeneous in the early-Galaxy at a given
time. Thus, it could be useful as a tracer of time. In an investigation of the
use of Be as a cosmochronometer and of its evolution in the Galaxy, we found
evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate
into two components. One is consistent with predictions of evolutionary models
while the other is chemically indistinguishable from the thick-disk stars. This
is interpreted as a difference in the star formation history of the two
components and suggests that the local halo is not a single uniform population
where a clear age-metallicity relation can be defined. We also found evidence
that the star formation rate was lower in the outer regions of the thick disk,
pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 -
Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds
Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models
[abridged] Previous analyses of lithium abundances in main sequence and red
giant stars have revealed the action of mixing mechanisms other than convection
in stellar interiors. Beryllium abundances in stars with lithium abundance
determinations can offer valuable complementary information on the nature of
these mechanisms. Our aim is to derive beryllium abundances along the whole
evolutionary sequence of an open cluster, IC 4651. These Be abundances are used
together with previously determined Li abundances, in the same sample stars, to
investigate the mixing mechanisms in a range of stellar masses and evolutionary
stages. New beryllium abundances are determined from high-resolution, high
signal-to-noise UVES spectra using spectrum synthesis and model atmospheres.
The careful synthetic modelling of the Be lines region is used to calculate
reliable abundances in rapidly rotating stars. The observed behavior of Be and
Li is compared to theoretical predictions from stellar models including
rotation-induced mixing, internal gravity waves, atomic diffusion, and
thermohaline mixing. Beryllium is detected in all the main sequence and
turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip
stars, but was not detected in the red giants. Confirming previous results, we
find that the Li dip is also a Be dip, although the depletion of Be is more
modest than that of Li in the corresponding effective temperature range. For
post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung
gap than expected from classical predictions as does the Li dilution. A clear
dispersion in the Be abundances is also observed. Theoretical stellar models
including the hydrodynamical transport processes mentioned above are able to
reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio
Nanostructured Materials for Energy Storage and Conversion
The conversion and storage of renewable energy sources is an urgent challenge that we need to tackle to transition from a fossil fuel-based economy to a low-carbon society [...]
Design of nanomaterials for hydrogen storage
The interaction of hydrogen with solids and the mechanisms of hydride formation experience significant changes in nanomaterials due to a number of structural features. This review aims at illustrating the design principles that have recently inspired the development of new nanomaterials for hydrogen storage. After a general discussion about the influence of nanomaterials' microstructure on their hydrogen sorption properties, several scientific cases and hot topics are illustrated surveying various classes of materials. These include bulk-like nanomaterials processed by mechanochemical routes, thin films and multilayers, nano-objects with composite architectures such as core-shell or composite nanoparticles, and nanoparticles on porous or graphene-like supports. Finally, selected examples of recent in situ studies of metal-hydride transformation mechanisms using microscopy and spectroscopy techniques are highlighted
Li - O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas
Elemental correlations and anti-correlations are known to be present in
globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its
fragility Li is destroyed at the temperature at which the CNO cycling occurs,
and this makes Li a crucial study for the nature of the contaminating stars. We
observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES
at the VLT to investigate the presence and the extent of a Li-O correlation.
This correlation is expected if there is a simple pollution scenario. Li (670.8
nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to
a very narrow region of the color-magnitude diagram, so they have similar
stellar parameters (Teff, log g). We find that O and Li correlate, with a high
statistical significance that confirms the early results for this cluster. At
first glance this is what is expected if a simple pollution of pristine gas
with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship,
however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one
is the value predicted for a pure contamination model. We confirm an extended
Li-O correlation in non evolved stars of NGC 6752. At the same time the
characteristic of the correlation shows that a simple pollution scenario is not
sufficient to explain the observations. Within this scenario the contaminant
gas must have been enriched in Li. This would rule out massive stars as main
polluters, and favor the hypothesis that the polluting gas was enriched by
intermediate or high-mass AGB stars, unless the former can be shown to be able
to produce Li. According to our observations, the fraction of polluting gas
contained in the stars observed is a considerable fraction of the stellar mass
of the cluster.Comment: 8pages, 2 figures, accepted by A&A Lette
Measuring helium abundance difference in giants of NGC 2808
Multiple populations have been detected in several globular clusters (GC)
that do not display a spread in metallicity. Unusual features of their CMD can
be interpreted in terms of differences in the Helium content of the stars
belonging to the sub-populations. Differences in He abundance have never been
directly observed. We attempt to measure these differences in two giant stars
of NGC 2808 with very similar parameters but different Na and O abundances,
hence that presumably belong to different sub-populations, by directly
comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the
upper chromosphere. Our detailed models derive the chromospheric structure
using the Ca II and H, and simulate the corresponding He I 10830 line
profiles. We show that, at a given value of He abundance, the He I 10830
equivalent width cannot significantly change without a corresponding much
larger change in the Ca II lines. We have used the VLT-CRIRES to obtain
high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain
spectra of the Ca II and H lines of our target stars. The two target
stars have very similar Ca II and H lines, but different appearances in
the He region. One line, blueshifted by 17 km s with respect to the He
10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas
the Na-poor star spectrum is consistent with a non-detection. The difference in
the spectra is consistent and most closely explained by an He abundance
difference between the two stars of 0.17.We provide direct
evidence of a significant He line strength difference in giant stars of NGC
2808 belonging to different sub-populations, which had been previously detected
by other photometric and spectroscopic means.Comment: Accepted for publication in A&
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