430 research outputs found
Long-term cortisol levels in hair of children and adolescents with Prader-Willi Syndrome
Context: Prader-Willi syndrome (PWS) is characterized by hypothalamic dysfunction. In children with PWS, stress-induced central adrenal insufficiency (CAI) has been described, however, daily life cortisol production may be normal. Hair cortisol concentration (HCC) is a marker of long-term systemic cortisol production. Cortisol awakening response (CAR) is the increase in cortisol level after awakening. A negative CAR might suggest hypothalamic-pituitary-adrenal (HPA)-axis reactivity problems. Little is known about HCC and CAR in children with PWS. Objective: To investigate long-term cortisol levels in hair and CAR in children with PWS. Design: Cross-sectional study. Patients: 41 children with PWS. Setting: Dutch PWS Reference Center. Main outcome measures: HCC and salivary cortisol measured by LCMS. Results: Median (IQR) HCC was 1.90 (1.02â3.30) pg/mg at a median (IQR) age of 14.5 (8.20â19.0) years, with median HCC in age-matched references being 2.63 pg/mg. Five patients (13.2%) had HCC < 2.5th percentile for age and these patients had a repeatedly negative CAR. Median HCC was significantly lower in patients with negative CAR than in patients with normal CAR (1.00 (0.22â1.59) vs. 2.25 (1.47â3.26) pg/mg, p = 0.007). One patient had both HCC < 2.5th percentile and repeatedly low morning salivary cortisol levels and negative CAR, and was diagnosed with adrenal insufficiency by overnight metyrapone test. Conclusions: HCC were normal in the majority of children with PWS. Our data suggest that children with HCC < 2.5th percentile and (repeatedly) negative CAR might possibly have adrenal insufficiency or delayed HPA-axis responsiveness.</p
Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process
Manufacturing superconducting circuits out of ultrathin films is a
challenging task when it comes to patterning complex compounds, which are
likely to be deteriorated by the patterning process. With the purpose of
developing high-T superconducting photon detectors, we designed a novel
route to pattern ultrathin YBCO films down to the nanometric scale. We believe
that our method, based on a specific use of a focused-ion beam, consists in
locally implanting Ga^{3+} ions and/or defects instead of etching the film.
This protocol could be of interest to engineer high-T superconducting
devices (SQUIDS, SIS/SIN junctions and Josephson junctions), as well as to
treat other sensitive compounds.Comment: 13 pages, 7 figure
Spin splitting in a polarized quasi-two-dimensional exciton gas
We have observed a large spin splitting between "spin" and
heavy-hole excitons, having unbalanced populations, in undoped GaAs/AlAs
quantum wells in the absence of any external magnetic field. Time-resolved
photoluminescence spectroscopy, under excitation with circularly polarized
light, reveals that, for high excitonic density and short times after the
pulsed excitation, the emission from majority excitons lies above that of
minority ones. The amount of the splitting, which can be as large as 50% of the
binding energy, increases with excitonic density and presents a time evolution
closely connected with the degree of polarization of the luminescence. Our
results are interpreted on the light of a recently developed model, which shows
that, while intra-excitonic exchange interaction is responsible for the spin
relaxation processes, exciton-exciton interaction produces a breaking of the
spin degeneracy in two-dimensional semiconductors.Comment: Revtex, four pages; four figures, postscript file Accepted for
publication in Physical Review B (Rapid Commun.
The Impact of cold gas accretion above a mass floor on galaxy scaling relations
Using the cosmological baryonic accretion rate and normal star formation
efficiencies, we present a very simple model for star-forming galaxies (SFGs)
that accounts for the mass and redshift dependencies of the SFR-Mass and
Tully-Fisher relations from z=2 to the present. The time evolution follows from
the fact that each modelled galaxy approaches a steady state where the SFR
follows the (net) cold gas accretion rate. The key feature of the model is a
halo mass floor M_{min}~10^{11} below which accretion is quenched in order to
simultaneously account for the observed slopes of the SFR-Mass and
Tully-Fischer relations. The same successes cannot be achieved via a
star-formation threshold (or delay) nor by varying the SF efficiency or the
feedback efficiency. Combined with the mass ceiling for cold accretion due to
virial shock heating, the mass floor M_{min} explains galaxy "downsizing",
where more massive galaxies formed earlier and over a shorter period of time.
It turns out that the model also accounts for the observed galactic baryon and
gas fractions as a function of mass and time, and the cosmic SFR density from
z~6 to z=0, which are all resulting from the mass floor M_{min}. The model
helps to understand that it is the cosmological decline of accretion rate that
drives the decrease of cosmic SFR density between z~2 and z=0 and the rise of
the cosmic SFR density allows us to put a constraint on our main parameter
M_{min}~10^{11} solar masses. Among the physical mechanisms that could be
responsible for the mass floor, we view that photo-ionization feedback (from
first in-situ hot stars) lowering the cooling efficiency is likely to play a
large role.Comment: 19pages, 14 figures, accepted to ApJ, updated reference
Bose-Einstein statistics in thermalization and photoluminescence of quantum well excitons
Quasi-equilibrium relaxational thermodynamics is developed to understand
LA-phonon-assisted thermalization of Bose-Einstein distributed excitons in
quantum wells. We study the quantum-statistical effects in the relaxational
dynamics of the effective temperature of excitons . When is less
than the degeneracy temperature , well-developed Bose-Einstein statistics
of quantum well excitons leads to nonexponential and density-dependent
thermalization. At low bath temperatures the thermalization of
quantum-statistically degenerate excitons effectively slows down and . We also analyze the optical decay of Bose-Einstein
distributed excitons in perfect quantum wells and show how nonclassical
statistics influences the effective lifetime . In particular,
of a strongly degenerate gas of excitons is given by ,
where is the intrinsic radiative lifetime of quasi-two-dimensional
excitons. Kinetics of resonant photoluminescence of quantum well excitons
during their thermalization is studied within the thermodynamic approach and
taking into account Bose-Einstein statistics. We find density-dependent
photoluminescence dynamics of statistically degenerate excitons. Numerical
modeling of the thermalization and photoluminescence kinetics of
quasi-two-dimensional excitons are given for GaAs/AlGaAs quantum wells.Comment: 19 pages, 9 figures. Phys. Rev. B (accepted for publication
Three years of growth hormone treatment in young adults with Prader-Willi Syndrome previously treated with growth hormone in childhood: Effects on glucose homeostasis and metabolic syndrome
Context: Growth hormone (GH) has been approved for children with Prader-Willi syndrome (PWS) and significantly improves body composition in adults with PWS. Adults with PWS are predisposed to develop impaired glucose tolerance (IGT) and diabetes mellitus type 2 (DMT2). Continuation of GH maintains body composition, but GH is known to induce insulin resistance, which might affect glucose homeostasis. Studies on long-term effects of GH treatment in adults are very limited. Objective: To investigate effects of 3 years of GH treatment on glucose homeostasis and prevalence of metabolic syndrome (MS) in adults with PWS. Design: Open-label, prospective study. Patients: 43 young adults with PWS. Setting: Dutch PWS Reference Center. Main outcome measures: Glucose and insulin during oral glucose tolerance test. Results: Estimated mean (95% CI) fasting glucose and insulin levels remained stable during 3 years of GH treatment. Glucose being 4.6 (4.4-4.8) mmol/l at start and 4.7 (4.6-4.9) mmol/l after 3 years (P =.07); insulin being 59.5 (45.2-75.8) pmol/l and 56.7 (45.2-69.6) pmol/l resp. (P =.72). Sex, ethnicity and fat mass percentage were significantly associated with fasting glucose levels, while IGF-I or GH-dose were not. Blood pressure, lipids and prevalence of MS remained stable during 3 years of GH. IGT prevalence was variable over time, six patients had IGT at start and eleven after 3 years of GH. One patient developed DMT2. However, prevalence of IGT or DMT2 was not significantly higher after 3 years than at study start. Conclusions: Three years of GH treatment in adults with PWS does not impair glucose homeostasis and does not lead to an increased prevalence of DMT2
Spin sensitive bleaching and monopolar spin orientation in quantum wells
Spin sensitive bleaching of the absorption of far-infrared radiation has been
observed in -type GaAs/AlGaAs quantum well structures. The absorption of
circularly polarized radiation saturates at lower intensities than that of
linearly polarized light due to monopolar spin orientation in the first heavy
hole subband. Spin relaxation times of holes in -type material in the range
of tens of ps were derived from the intensity dependence of the absorption.Comment: Figures have been updated due to technical printing problems
(Postscript mismatch
Herschel PEP: The star-formation rates of 1.5<z<2.5 massive galaxies
The star formation rate (SFR) is a key parameter in the study of galaxy
evolution. The accuracy of SFR measurements at z~2 has been questioned
following a disagreement between observations and theoretical models. The
latter predict SFRs at this redshift that are typically a factor 4 or more
lower than the measurements. We present star-formation rates based on
calorimetric measurements of the far-infrared (FIR) luminosities for massive
1.5<z<2.5, normal star-forming galaxies (SFGs), which do not depend on
extinction corrections and/or extrapolations of spectral energy distributions.
The measurements are based on observations in GOODS-N with the Photodetector
Array Camera & Spectrometer (PACS) onboard Herschel, as part of the PACS
Evolutionary Probe (PEP) project, that resolve for the first time individual
SFGs at these redshifts at FIR wavelengths. We compare FIR-based SFRs to the
more commonly used 24 micron and UV SFRs. We find that SFRs from 24 micron
alone are higher by a factor of ~4-7.5 than the true SFRs. This overestimation
depends on luminosity: gradually increasing for log L(24um)>12.2 L_sun. The
SFGs and AGNs tend to exhibit the same 24 micron excess. The UV SFRs are in
closer agreement with the FIR-based SFRs. Using a Calzetti UV extinction
correction results in a mean excess of up to 0.3 dex and a scatter of 0.35 dex
from the FIR SFRs. The previous UV SFRs are thus confirmed and the mean excess,
while narrowing the gap, is insufficient to explain the discrepancy between the
observed SFRs and simulation predictions.Comment: Accepted for publication in the A&A Herschel first results special
issue. v2 Correction to the meta data onl
The Radio - X-ray relation as a star formation indicator: Results from the VLA--E-CDFS Survey
In order to trace the instantaneous star formation rate at high redshift, and
hence help understanding the relation between the different emission mechanisms
related to star formation, we combine the recent 4 Ms Chandra X-ray data and
the deep VLA radio data in the Extended Chandra Deep Field South region. We
find 268 sources detected both in the X-ray and radio band. The availability of
redshifts for of the sources in our sample allows us to derive
reliable luminosity estimates and the intrinsic properties from X-ray analysis
for the majority of the objects. With the aim of selecting sources powered by
star formation in both bands, we adopt classification criteria based on X-ray
and radio data, exploiting the X-ray spectral features and time variability,
taking advantage of observations scattered across more than ten years. We
identify 43 objects consistent with being powered by star formation. We also
add another 111 and 70 star forming candidates detected only in the radio or
X-ray band, respectively. We find a clear linear correlation between radio and
X-ray luminosity in star forming galaxies over three orders of magnitude and up
to . We also measure a significant scatter of the order of 0.4 dex,
higher than that observed at low redshift, implying an intrinsic scatter
component. The correlation is consistent with that measured locally, and no
evolution with redshift is observed. Using a locally calibrated relation
between the SFR and the radio luminosity, we investigate the L_X(2-10keV)-SFR
relation at high redshift. The comparison of the star formation rate measured
in our sample with some theoretical models for the Milky Way and M31, two
typical spiral galaxies, indicates that, with current data, we can trace
typical spirals only at z<0.2, and strong starburst galaxies with
star-formation rates as high as , up to .Comment: 21 pages, 10 figures, 5 table
A Public, K-Selected, Optical-to-Near-Infrared Catalog of the Extended Chandra Deep Field South (ECDFS) from the MUltiwavelength Survey by Yale-Chile (MUSYC)
We present a new K-selected, optical-to-near-infrared photometric catalog of
the Extended Chandra Deep Field South (ECDFS), making it publicly available to
the astronomical community. The dataset is founded on publicly available
imaging, supplemented by original zJK imaging data obtained as part of the
MUltiwavelength Survey by Yale-Chile (MUSYC). The final photometric catalog
consists of photometry derived from nine band U-K imaging covering the full
0.5x0.5 sq. deg. of the ECDFS, plus H band data for approximately 80% of the
field. The 5sigma flux limit for point-sources is K = 22.0 (AB). This is also
the nominal completeness and reliability limit of the catalog: the empirical
completeness for 21.75 < K < 22.00 is 85+%. We have verified the quality of the
catalog through both internal consistency checks, and comparisons to other
existing and publicly available catalogs. As well as the photometric catalog,
we also present catalogs of photometric redshifts and restframe photometry
derived from the ten band photometry. We have collected robust spectroscopic
redshift determinations from published sources for 1966 galaxies in the
catalog. Based on these sources, we have achieved a (1sigma) photometric
redshift accuracy of Dz/(1+z) = 0.036, with an outlier fraction of 7.8%. Most
of these outliers are X-ray sources. Finally, we describe and release a utility
for interpolating restframe photometry from observed SEDs, dubbed InterRest.
Particularly in concert with the wealth of already publicly available data in
the ECDFS, this new MUSYC catalog provides an excellent resource for studying
the changing properties of the massive galaxy population at z < 2. (Abridged)Comment: Re-submitted to ApJSS after a first referee report. 27 pages, 17
figures. MUSYC data is freely available from http://astro.yale.edu/MUSYC .
Links to phot-z and restframe photometry catalogs, as well as to InterRest
access and documentation, including a full walkthrough, can be found at
http://www.strw.leidenuniv.nl/~ent
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