262 research outputs found
Observational signatures of lithium depletion in the metal-poor globular cluster NGC6397
The "stellar" solution to the cosmological lithium problem proposes that
surface depletion of lithium in low-mass, metal-poor stars can reconcile the
lower abundances found for Galactic halo stars with the primordial prediction.
Globular clusters are ideal environments for studies of the surface evolution
of lithium, with large number statistics possible to obtain for main sequence
stars as well as giants. We discuss the Li abundances measured for >450 stars
in the globular cluster NGC6397, focusing on the evidence for lithium depletion
and especially highlighting how the inferred abundances and interpretations are
affected by early cluster self-enrichment and systematic uncertainties in the
effective temperature determination.Comment: 6 pages, 2 figures, conference proceedings for IAU symposium 26
Classical Cepheids, what else?
We present new and independent estimates of the distances to the Magellanic
Clouds (MCs) using near-infrared (NIR) and optical--NIR period--Wesenheit (PW)
relations. The slopes of the PW relations are, within the dispersion, linear
over the entire period range and independent of metal content. The absolute
zero points were fixed using Galactic Cepheids with distances based on the
infrared surface-brightness method. The true distance modulus we found for the
Large Magellanic Cloud--- mag---and the
Small Magellanic Cloud--- mag---agree quite
well with similar distance determinations based on robust distance indicators.
We also briefly discuss the evolutionary and pulsation properties of MC
Cepheids
Beryllium in the Ultra-Lithium-Deficient,Metal-Poor Halo Dwarf, G186-26
The vast majority of low-metal halo dwarfs show a similar amount of Li; this
has been attributed to the Li that was produced in the Big Bang. However, there
are nine known halo stars with T 5900 K and [Fe/H] 1.0 that are
ultra-Li-deficient. We have looked for Be in the very low metallicity star, G
186-26 at [Fe/H] = 2.71, which is one of the ultra-Li-deficient stars. This
star is also ultra-Be deficient. Relative to Be in the Li-normal stars at
[Fe/H] = 2.7, G 182-26 is down in Be by more than 0.8 dex. Of two potential
causes for the Li-deficiency -- mass-transfer in a pre-blue straggler or extra
rotationally-induced mixing in a star that was initially a very rapid rotator
-- the absence of Be favors the blue-straggler hypothesis, but the rotation
model cannot be ruled-out completely.Comment: Accepted for Ap.J. Letters 10 pages, 4 figure
How Events Come Into Being: EEQT, Particle Tracks, Quantum Chaos, and Tunneling Time
In sections 1 and 2 we review Event Enhanced Quantum Theory (EEQT). In
section 3 we discuss applications of EEQT to tunneling time, and compare its
quantitative predictions with other approaches, in particular with
B\"uttiker-Larmor and Bohm trajectory approach. In section 4 we discuss quantum
chaos and quantum fractals resulting from simultaneous continuous monitoring of
several non-commuting observables. In particular we show self-similar,
non-linear, iterated function system-type, patterns arising from quantum jumps
and from the associated Markov operator. Concluding remarks pointing to
possible future development of EEQT are given in section 5.Comment: latex, 27 pages, 7 postscript figures. Paper submitted to Proc.
Conference "Mysteries, Puzzles And Paradoxes In Quantum Mechanics, Workshop
on Entanglement And Decoherence, Palazzo Feltrinelli, Gargnano, Garda Lake,
Italy, 20-25 September, 199
On the distance of the Magellanic Clouds using Cepheid NIR and optical-NIR Period Wesenheit Relations
We present the largest near-infrared (NIR) data sets, , ever collected
for classical Cepheids in the Magellanic Clouds (MCs). We selected fundamental
(FU) and first overtone (FO) pulsators, and found 4150 (2571 FU, 1579 FO)
Cepheids for Small Magellanic Cloud (SMC) and 3042 (1840 FU, 1202 FO) for Large
Magellanic Cloud (LMC). Current sample is 2--3 times larger than any sample
used in previous investigations with NIR photometry. We also discuss optical
photometry from OGLE-III. NIR and optical--NIR Period-Wesenheit (PW)
relations are linear over the entire period range () and their slopes are, within the intrinsic dispersions, common between the
MCs. These are consistent with recent results from pulsation models and
observations suggesting that the PW relations are minimally affected by the
metal content. The new FU and FO PW relations were calibrated using a sample of
Galactic Cepheids with distances based on trigonometric parallaxes and Cepheid
pulsation models. By using FU Cepheids we found a true distance moduli of
mag (LMC) and
mag (SMC). These estimates
are the weighted mean over ten PW relations and the systematic errors account
for uncertainties in the zero-point and in the reddening law. We found similar
distances using FO Cepheids
( mag [LMC] and
mag [SMC]). These new MC
distances lead to the relative distance, mag (FU, ) and mag (FO, ),which agrees quite
well with previous estimates based on robust distance indicators.Comment: 17 pages, 7 figure
On the metallicity distribution of classical Cepheids in the Galactic inner disk
We present homogeneous and accurate iron abundances for almost four dozen
(47) of Galactic Cepheids using high-spectral resolution (R40,000) high
signal-to-noise ratio (S/N 100) optical spectra collected with UVES at
VLT. A significant fraction of the sample (32) is located in the inner disk (RG
6.9 kpc) and for half of them we provide new iron abundances. Current
findings indicate a steady increase in iron abundance when approaching the
innermost regions of the thin disk. The metallicity is super-solar and ranges
from 0.2 dex for RG 6.5 kpc to 0.4 dex for RG 5.5 kpc. Moreover,
we do not find evidence of correlation between iron abundance and distance from
the Galactic plane. We collected similar data available in the literature and
ended up with a sample of 420 Cepheids. Current data suggest that the mean
metallicity and the metallicity dispersion in the four quadrants of the
Galactic disk attain similar values. The first-second quadrants show a more
extended metal-poor tail, while the third-fourth quadrants show a more extended
metal-rich tail, but the bulk of the sample is at solar iron abundance.
Finally, we found a significant difference between the iron abundance of
Cepheids located close to the edge of the inner disk ([Fe/H]0.4) and
young stars located either along the Galactic bar or in the nuclear bulge
([Fe/H]0). Thus suggesting that the above regions have had different
chemical enrichment histories. The same outcome applies to the metallicity
gradient of the Galactic bulge, since mounting empirical evidence indicates
that the mean metallicity increases when moving from the outer to the inner
bulge regions.Comment: 10 pages, 5 figures; Corrected typos, corrected Table
Implications of a new temperature scale for halo dwarfs on LiBeB and chemical evolution
Big bang nucleosynthesis (BBN) and the cosmic baryon density from cosmic
microwave background anisotropies together predict a primordial Li7 abundance a
factor of 2--3 higher than that observed in galactic halo dwarf stars. A recent
analysis of Li7 observations in halo stars, using significantly higher surface
temperature for these stars, found a higher Li plateau abundance. These results
go a long way towards resolving the discrepancy with BBN. Here, we examine the
implications of the higher surface temperatures on the abundances of Be and B
which are thought to have been produced in galactic cosmic-ray nucleosynthesis
by spallation of CNO together with Li (produced in alpha + alpha collisions).
While the Be abundance is not overly sensitive to the surface temperature, the
derived B abundances and more importantly the derived oxygen abundances are
very temperature dependent. If the new temperature scale is correct, the
implied increased abundances of these elements poses a serious challenge to
models of galactic cosmic ray nucleosynthesis and galactic chemical evolution.Comment: 23 pages, 10 eps figure
The early days of the Sculptor dwarf spheroidal galaxy
We present the high resolution spectroscopic study of five -3.9<=[Fe/H]<=-2.5
stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the
number of stars with comparable observations in this metallicity range. We
carry out a detailed analysis of the chemical abundances of alpha, iron peak,
light and heavy elements, and draw comparisons with the Milky Way halo and the
ultra faint dwarf stellar populations. We show that the bulk of the Sculptor
metal-poor stars follows the same trends in abundance ratios versus metallicity
as the Milky Way stars. This suggests similar early conditions of star
formation and a high degree of homogeneity of the interstellar medium. We find
an outlier to this main regime, which seems to miss the products of the most
massive of the TypeII supernovae. In addition to its value to help refining
galaxy formation models, this star provides clues to the production of cobalt
and zinc. Two of our sample stars have low odd-to-even barium isotope abundance
ratios, suggestive of a fair proportion of s-process; we discuss the
implication for the nucleosynthetic origin of the neutron capture elements.Comment: Replacement after language editio
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