4,296 research outputs found
Coherent population trapping in two-electron three-level systems with aligned spins
The possibility of coherent population trapping in two electron states with
aligned spins (ortho-system) is evidenced. From the analysis of a three-level
atomic system containing two electrons, and driven by the two laser fields
needed for coherent population trapping, a conceptually new kind of
two-electron dark state appears. The properties of this trapping are studied
and are physically interpreted in terms of a dark hole, instead of a dark
two-electron state. This technique, among many other applications, offers the
possibility of measuring, with subnatural resolution, some superposition-state
matrix-elements of the electron-electron correlation that due to their time
dependent nature are inaccesible by standard measuring procedures.Comment: 10 pages and 4 figure
On the gluon content of the eta and eta' mesons
A phenomenological analysis of radiative and
decays is performed with the purpose of determining the gluonic content of the
and wave functions. Our results show that within our model
there is no evidence for a gluonium contribution in the ,
, or the , . In terms of a mixing angle description this corresponds to
and . In
addition, the - mixing angle is found to be if we don't allow for a gluonium component.Comment: 16 pages, 3 figures, JHEP style. First part of data fitting changed,
conclusions not modifie
The 21cm Signature of the First Stars
We predict the 21-cm signature of the first metal-free stars. The soft X-rays
emitted by these stars penetrate the atomic medium around their host halos,
generating Lyman alpha photons that couple the spin and kinetic temperatures.
These creates a region we call the Lyman alpha sphere, visible in 21-cm against
the CMB, which is much larger than the HII region produced by the same star.
The spin and kinetic temperatures are strongly coupled before the X-rays can
substantially heat the medium, implying that a strong 21-cm absorption signal
from the adiabatically cooled gas in Hubble expansion around the star is
expected when the medium has not been heated previously. A central region of
emission from the gas heated by the soft X-rays is also present although with a
weaker signal than the absorption. The Lyman alpha sphere is a universal
signature that should be observed around any first star illuminating its
vicinity for the first time. The 21-cm radial profile of the Lyman alpha sphere
can be calculated as a function of the luminosity, spectrum and age of the
star. For a star of a few hundred solar masses and zero metallicity (as
expected for the first stars), the physical radius of the Lyman alpha sphere
can reach tens of kiloparsecs. The first metal-free stars should be strongly
clustered because of high cosmic biasing; this implies that the regions
producing a 21-cm absorption signal may contain more than one star and will
generally be irregular and not spherical, because of the complex distribution
of the gas. We discuss the feasiblity of detecting these Lyman alpha spheres,
which would be present at redshifts in the Cold Dark Matter model.
Their observation would represent a direct proof of the detection of a first
star.Comment: replaced with ApJ accepted version. Many minor revisions and
additional references, major results unchange
Nonequilibrium corrections in the pressure tensor due to an energy flux
The physical interpretation of the nonequilibrium corrections in the pressure
tensor for radiation submitted to an energy flux obtained in some previous
works is revisited. Such pressure tensor is shown to describe a moving
equilibrium system but not a real nonequilibrium situation.Comment: 4 pages, REVTeX, Brief Report to appear in PRE Dec 9
The Fundamental Plane in RX J0142.0+2131: a galaxy cluster merger at z=0.28
We present the Fundamental Plane (FP) in the z = 0.28 cluster of galaxies RX
J0142.0+2131. There is no evidence for a difference in the slope of the FP when
compared with the Coma cluster, although the internal scatter is larger. On
average, stellar populations in RX J0142.0+2131 have rest-frame V-band
mass-to-light ratios (M/L_V) 0.29+-0.03 dex lower than in Coma. This is
significantly lower than expected for a passively-evolving cluster formed at
z_f=2. Lenticular galaxies have lower average M/L_V and a distribution of M/L_V
with larger scatter than ellipticals. Lower mass-to-light ratios are not due to
recent star formation: our previous spectroscopic observations of RX
J0142.0+2131 E/S0 galaxies showed no evidence for significant star-formation
within the past ~4 Gyr. However, cluster members have enhanced alpha-element
abundance ratios, which may act to decrease M/L_V. The increased scatter in the
RX J0142.0+2131 FP reflects a large scatter in M/L_V implying that galaxies
have undergone bursts of star formation over a range of epochs. The seven
easternmost cluster galaxies, including the second brightest member, have M/L_V
consistent with passive evolution and z_f = 2. We speculate that RX
J0142.0+2131 is a cluster-cluster merger where the galaxies to the east are yet
to fall into the main cluster body or have not experienced star formation as a
result of the merger.Comment: 4 pages, 2 figures, accepted for publication in ApJ Letter
Cosmogenic gamma-rays and the composition of cosmic rays
We discuss the prospects of detecting the sources of ultra-high energy (UHE)
cosmic ray (CR) nuclei via their emission of cosmogenic gamma-rays in the GeV
to TeV energy range. These gamma-rays result from electromagnetic cascades
initiated by high energy photons, electrons and positrons that are emitted by
CRs during their propagation in the cosmic radiation background and are
independent of the simultaneous emission of gamma-rays in the vicinity of the
source. The corresponding production power by UHE CR nuclei (with mass number A
and charge Z) is dominated by pion photo-production (~ A) and Bethe-Heitler
pair production (~ Z^2). We show that the cosmogenic gamma-ray signal from a
single steady UHE CR source is typically more robust with respect to variations
of the source composition and injection spectrum than the accompanying signal
of cosmogenic neutrinos. We study the diffuse emission from the sum of
extragalactic CR sources as well as the point source emission of the closest
sources.Comment: 14 pages, 5 figure
Impact of nutritional vitamin D supplementation on parathyroid hormone and 25-hydroxyvitamin D levels in non-dialysis chronic kidney disease : a meta-analysis
Background. Secondary hyperparathyroidism (SHPT) is a common and major complication in chronic kidney disease (CKD), reflecting the increase of parathyroid hormone (PTH) in response to reduced vitamin D signalling and hypocalcaemia. This meta-analysis evaluated the impact of nutritional vitamin D (NVD) (cholecalciferol or ergocalciferol) on SHPT-related biomarkers. Methods. A systematic literature search was performed in PubMed to identify relevant randomized control trials to be included in the meta-analysis. Fixed- and random-effects models were used to pool study-level results. Effects were studied within NVD study arms and relative to control groups (placebo/no treatment); the former in order to identify the effect of actively altering biomarkers levels. Results. Reductions in PTH from supplementation with NVD were small when observed within the NVD study arms (pooled reduction: 10.5 pg/mL) and larger when compared with placebo/no treatment (pooled reduction: 49.7 pg/mL). NVD supplementation increased levels of 25-hydroxyvitamin D [25(OH)D] in both analyses (increase within NVD study arm: 20.6 ng/mL, increase versus placebo/no treatment: 26.9 ng/mL). While small and statistically non-significant changes in phosphate and fibroblast growth factor 23 were observed, NVD supplementation caused calcium levels to increase when compared with placebo/no treatment (increase: 0.23 mg/dL). Conclusions. Our results suggest that supplementation with NVD can be used to increase 25(OH)D to a certain extent, while the potential of NVD to actively reduce PTH in non-dialysis-CKD patients with SHPT is limited
Tunable-filter imaging of quasar fields at z~1. I. A cluster around MRC B0450-221
Using a combination of multicolour broad- and narrow-band imaging techniques
and follow-up spectroscopy, we have detected an overdensity of galaxies in the
field of quasar MRC B0450-221, whose properties are consistent with a cluster
at the quasar redshift z=0.9. An excess of red galaxies (V-I>2.2, I-K'>3.8) is
evident within 1' of the quasar, with the colours expected for galaxies at
z=0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting
galaxies (nine candidates with equivalent widths EW>70A) are also detected in
the field using the TAURUS Tunable Filter (TTF). Three have been confirmed
spectroscopically to indeed lie at z=0.9. The TTF candidates with the strongest
[O II] line emission cluster in a group which lies 200-700 kpc away from the
quasar and the red galaxy excess, and therefore most likely on the outskirts of
the cluster. These observations are the first in a series probing quasar
environments at z~1 with TTF.Comment: Accepted for publication in AJ. 25 pages, 24 figs (large files in jpg
or gif format), uses emulateapj.st
Cooling Flows and Metallicity Gradients in Clusters of Galaxies
The X-ray emission by hot gas at the centers of clusters of galaxies is
commonly modeled assuming the existence of steady-state, inhomogeneous cooling
flows. We derive the metallicity profiles of the intracluster medium expected
from such models. The inflowing gas is chemically enriched by type Ia
supernovae and stellar mass loss in the outer parts of the central galaxy,
which may give rise to a substantial metallicity gradient. The amplitude of the
expected metallicity enhancement towards the cluster center is proportional to
the ratio of the central galaxy luminosity to the mass inflow rate. The
metallicity of the hotter phases is expected to be higher than that of the
colder, denser phases. The metallicity profile expected for the Centaurus
cluster is in good agreement with the metallicity gradient recently inferred
from ASCA measurements (Fukazawa et al. 1994). However, current data do not
rule out alternative models where cooling is balanced by some heat source. The
metallicity gradient does not need to be present in all clusters, depending on
the recent merging history of the gas around the central cluster galaxy, and on
the ratio of the stellar mass in the central galaxy to the gas mass in the
cooling flow.Comment: uuencoded postscript, 8 pages of text + 2 figures, accepted by The
Astrophysical Journal (Letters
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