2,134 research outputs found
Lithium abundances in extremely metal-poor turn-off stars
We discuss the current status of the sample of Lithium abundances in
extremely metal poor (EMP) turn-off (TO) stars collected by our group, and
compare it with the available literature results. In the last years, evidences
have accumulated of a progressive disruption of the Spite plateau in stars of
extremely low metallicity. What appears to be a flat, thin plateau above
[Fe/H]\sim-2.8 turns, at lower metallicities, into a broader distribution for
which the plateau level constitutes the upper limit, but more and more stars
show lower Li abundances. The sample we have collected currently counts
abundances or upper limits for 44 EMP TO stars between [Fe/H]=-2.5 and -3.5,
plus the ultra-metal poor star SDSS J102915+172927 at [Fe/H]=-4.9. The
"meltdown" of the Spite plateau is quite evident and, at the current status of
the sample, does not appear to be restricted to the cool end of the effective
temperature distribution. SDSS J102915+172927 displays an extreme Li depletion
that contrasts with its otherwise quite ordinary set of [X/Fe] ratios.Comment: 6 pages, 4 figures, proceedings of the "Lithium in the Cosmos"
conference, Paris, 27-29 February 201
The Semiclassical and Quantum Regimes of Superradiant Light Scattering from a Bose-Einstein Condensate
We show that many features of the recent experiments of Schneble et al. [D.
Schneble, Y. Torii, M. Boyd, E.W. Streed, D.E. Pritchard and W. Ketterle,
Science vol. 300, p. 475 (2003)], which demonstrate two different regimes of
light scattering by a Bose-Einstein condensate, can be described using a
one-dimensional mean-field quantum CARL model, where optical amplification
occurs simultaneously with the production of a periodic density modulation in
the atomic medium. The two regimes of light scattering observed in these
experiments, originally described as ``Kapiza-Dirac scattering'' and
``Superradiant Rayleigh scattering'', can be interpreted as the semiclassical
and quantum limits respectively of CARL lasing.Comment: 10 pages, 5 figures - to appear in Journal of Optics
MyGIsFOS: an automated code for parameter determination and detailed abundance analysis in cool stars
The current and planned high-resolution, high-multiplexity stellar
spectroscopic surveys, as well as the swelling amount of under-utilized data
present in public archives have led to an increasing number of efforts to
automate the crucial but slow process to retrieve stellar parameters and
chemical abundances from spectra. We present MyGIsFOS, a code designed to
derive atmospheric parameters and detailed stellar abundances from medium -
high resolution spectra of cool (FGK) stars. We describe the general structure
and workings of the code, present analyses of a number of well studied stars
representative of the parameter space MyGIsFOS is designed to cover, and
examples of the exploitation of MyGIsFOS very fast analysis to assess
uncertainties through Montecarlo tests. MyGIsFOS aims to reproduce a
``traditional'' manual analysis by fitting spectral features for different
elements against a precomputed grid of synthetic spectra. Fe I and Fe II lines
can be employed to determine temperature, gravity, microturbulence, and
metallicity by iteratively minimizing the dependence of Fe I abundance from
line lower energy and equivalent width, and imposing Fe I - Fe II ionization
equilibrium. Once parameters are retrieved, detailed chemical abundances are
measured from lines of other elements. MyGIsFOS replicates closely the results
obtained in similar analyses on a set of well known stars. It is also quite
fast, performing a full parameter determination and detailed abundance analysis
in about two minutes per star on a mainstream desktop computer. Currently, its
preferred field of application are high-resolution and/or large spectral
coverage data (e.g UVES, X-Shooter, HARPS, Sophie).Comment: 15 pages, 14 figures, accepted for publication by A&
Neutrinos And Big Bang Nucleosynthesis
The early universe provides a unique laboratory for probing the frontiers of
particle physics in general and neutrino physics in particular. The primordial
abundances of the relic nuclei produced during the first few minutes of the
evolution of the Universe depend on the electron neutrinos through the
charged-current weak interactions among neutrons and protons (and electrons and
positrons and neutrinos), and on all flavors of neutrinos through their
contributions to the total energy density which regulates the universal
expansion rate. The latter contribution also plays a role in determining the
spectrum of the temperature fluctuations imprinted on the Cosmic Background
Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer
and helium-4 as a chronometer, the predictions of BBN and the CBR are compared
to observations. The successes of, as well as challenges to the standard models
of particle physics and cosmology are identified. While systematic
uncertainties may be the source of some of the current tensions, it could be
that the data are pointing the way to new physics. In particular, BBN and the
CBR are used to address the questions of whether or not the relic neutrinos
were fully populated in the early universe and, to limit the magnitude of any
lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129,
"Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O.
Botner, P. Carlson, P. O. Hulth, and T. Ohlsso
Automatic abundance analysis of high resolution spectra
We describe an automatic procedure for determining abundances from high
resolution spectra. Such procedures are becoming increasingly important as
large amounts of data are delivered from 8m telescopes and their
high-multiplexing fiber facilities, such as FLAMES on ESO-VLT. The present
procedure is specifically targeted for the analysis of spectra of giants in the
Sgr dSph; however, the procedure may be, in principle, tailored to analyse
stars of any type. Emphasis is placed on the algorithms and on the stability of
the method; the external accuracy rests, ultimately, on the reliability of the
theoretical models (model-atmospheres, synthetic spectra) used to interpret the
data. Comparison of the results of the procedure with the results of a
traditional analysis for 12 Sgr giants shows that abundances accurate at the
level of 0.2 dex, comparable with that of traditional analysis of the same
spectra, may be derived in a fast and efficient way. Such automatic procedures
are not meant to replace the traditional abundance analysis, but as an aid to
extract rapidly a good deal of the information contained in the spectra.Comment: A&A accepte
The low Sr/Ba ratio on some extremely metal-poor stars
It has been noted that, in classical extremely metal-poor (EMP) stars, the
abundance ratio of Sr and Ba, is always higher than [Sr/Ba] = -0.5, the value
of the solar r-only process; however, a handful of EMP stars have recently been
found with a very low Sr/Ba ratio. We try to understand the origin of this
anomaly by comparing the abundance pattern of the elements in these stars and
in the classical EMP stars. Four stars with very low Sr/Ba ratios were observed
and analyzed within LTE approximation through 1D (hydrostatic) model
atmosphere, providing homogeneous abundances of nine neutron-capture elements.
In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in
fact, found to be higher than the r-only solar ratio, so the star is discarded.
The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved
giants. They do not present a clear carbon enrichment. The abundance patterns
of the neutron-capture elements in the three stars are strikingly similar to a
theoretical s-process pattern. This pattern could at first be attributed to
pollution by a nearby AGB, but none of the stars presents a clear variation in
the radial velocity indicating the presence of a companion. The stellar
parameters seem to exclude any internal pollution in a TP-AGB phase for at
least two of these stars. The possibility that the stars are early-AGB stars
polluted during the core He flash does not seem compatible with the theory.Comment: Accepted for publication in Astronomy and Astrophysic
Communicative and linguistic development in preterm children: a longitudinal study from 12 to 24 months.
BACKGROUND: Research conducted on preterm children's linguistic skills has provided varying pictures, and the question of whether and to what extent preterm children are delayed in early language acquisition remains largely unresolved. AIMS: To examine communicative and linguistic development during the second year in a group of Italian children born prematurely using the 'Primo Vocabolario del Bambino' (PVB), the Italian version of the MacArthur-Bates Communicative Development Inventory. The primary goal was to compare action/gesture production, word comprehension, and word production, and the relationship between these three domains in preterm children and to normative data obtained from a large sample of Italian children born at term. A second aim was to address the longstanding debate regarding the use of chronological versus corrected gestational age in the assessment of preterm children's abilities. METHODS & PROCEDURES: Parents of twelve preterm children completed the PVB questionnaire at five age points during the children's second year, and scores were compared with those from a normative sample of full-term children and those of 59 full-term children selected as a control group from the normative sample for the PVB. OUTCOMES & RESULTS: Preterm children exhibited a delay in all three aspects of communication and language. In particular, communicative-linguistic age tended to lag approximately 3 months behind chronological age when children were between the ages of 12 and 24 months. When chronological age was used, preterm children's percentile scores for all three components of communication and language fell within the lower limits of the normal range, while scores calculated using corrected age either fell at or above the 50th percentile. CONCLUSIONS & IMPLICATIONS: Findings suggest that despite the significant biological risk engendered by premature birth, early communicative and linguistic development appears to proceed in a relatively robust fashion among preterm children, with tight relations across communicative domains as in full-term children. Employing both chronological and corrected gestational age criteria in the evaluation of preterm children's abilities may provide important information about their progress in language acquisition. This may be especially important during the initial stages of communicative and linguistic development, inasmuch as comparisons of the two sets of scores may provide clinicians with a way to distinguish children who may be at risk for language problems from those who may be expected to progress normally
6Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem?
The presence of 6Li in the atmospheres of metal-poor halo stars is usually
inferred from the detection of a subtle extra depression in the red wing of the
7Li doublet line at 670.8 nm. However, the intrinsic line asymmetry caused by
convective flows in the photospheres of cool stars is almost indistinguishable
from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li
profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D
model atmospheres, ignoring the convection-induced line asymmetry, must
therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D
non-LTE line profiles of the Li 670.8 nm feature, we quantify the differential
effect of the convective line asymmetry on the derived 6Li abundance as a
function of effective temperature, gravity, and metallicity. As expected, we
find that the asymmetry effect systematically reduces the resulting 6Li/7Li
ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges
between -0.005 and -0.020. When this purely theoretical correction is taken
into account for the Asplund 2006 sample of stars, the number of significant
6Li detections decreases from 9 to 5 (2 sigma criterion), or from 5 to 2 (3
sigma criterion).
We also present preliminary results of a re-analysis of high-resolution, high
S/N spectra of individual metal-poor turn-off stars, to see whether the "second
Lithium problem" actually disappears when accounting properly for convection
and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD84937
seems to be the only significant (2 sigma) detection of 6Li. In view of our
results, the existence of a 6Li plateau appears questionable.Comment: To appear in the proceedings of 'Lithium in the Cosmos', Paris, Feb.
27-29, 2012, Memorie della Societa' Astronomica Italiana Supplement
Optical nonlinear dynamics with cold atoms in a cavity
This paper presents the nonlinear dynamics of laser cooled and trapped cesium
atoms placed inside an optical cavity and interacting with a probe light beam
slightly detuned from the 6S1/2(F=4) to 6P3/2(F=5) transition. The system
exhibits very strong bistability and instabilities. The origin of the latter is
found to be a competition between optical pumping and non-linearities due to
saturation of the optical transition.Comment: 6 pages, 7 figures, LaTe
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