9,699 research outputs found
Exploring the Structure of Distant Galaxies with Adaptive Optics on the Keck-II Telescope
We report on the first observation of cosmologically distant field galaxies
with an high order Adaptive Optics (AO) system on an 8-10 meter class
telescope. Two galaxies were observed at 1.6 microns at an angular resolution
as high as 50 milliarcsec using the AO system on the Keck-II telescope. Radial
profiles of both objects are consistent with those of local spiral galaxies and
are decomposed into a classic exponential disk and a central bulge. A
star-forming cluster or companion galaxy as well as a compact core are detected
in one of the galaxies at a redshift of 0.37+/-0.05. We discuss possible
explanations for the core including a small bulge, a nuclear starburst, or an
active nucleus. The same galaxy shows a peak disk surface brightness that is
brighter than local disks of comparable size. These observations demonstrate
the power of AO to reveal details of the morphology of distant faint galaxies
and to explore galaxy evolution.Comment: 5 pages, Latex, 3 figures. Accepted for publication in P.A.S.
Melatonin treatment in children with therapy-resistant monosymptomatic nocturnal enuresis
Objective: To evaluate the effects of exogenous melatonin on the frequency of wet nights, on the sleep-wake cycle, and on the melatonin profile in children with therapy-resistant MNE. Patients and methods: 24 patients were included. Patients had to maintain a diary including time of sleep and arousal, and whether they had a dry or a wet bed in the morning. We measured baseline melatonin profiles in saliva. Hereafter, patients were randomized to synthetic melatonin or placebo. After 3 and 6 months we evaluated the frequency of enuresis and the melatonin profiles. Results: 11 patients were randomized to melatonin, 13 to placebo. We evaluated melatonin profiles of 7 patients in the melatonin group and of 8 in the placebo group. We observed a change in profile in the melatonin group, but we did not observe a difference in the sleep-wake cycle or the frequency of wet nights in either group. Conclusion: This is the first time exogenous melatonin has been evaluated in the treatment of MNE. Although we observed a change in melatonin profile after the use of exogenous melatonin, we did not observe a change in enuresis frequency or in the sleep-wake cycle of this select group of patients. (C) 2011 Journal of Pediatric Urology Company. Published by Elsevier Ltd. All rights reserved
Superconducting proximity effect in a mesoscopic ferromagnetic wire
We present an experimental study of the transport properties of a
ferromagnetic metallic wire (Co) in metallic contact with a superconductor
(Al). As the temperature is decreased below the Al superconducting transition,
the Co resistance exhibits a significant dependence on both temperature and
voltage. The differential resistance data show that the decay length for the
proximity effect is much larger than we would simply expect from the exchange
field of the ferromagnet.Comment: 4 pages, 6 included epsf figures, published version with small
change
Relation Between the Thickness of Stellar Disks and the Relative Mass of Dark Halo in Galaxies
We consider a thickness of stellar disks of late-type galaxies by analyzing
the R and K_s band photometric profiles for two independent samples of edge-on
galaxies. The main goal is to verify a hypotesis that a thickness of old
stellar disks is related to the relative masses of the spherical and disk
components of galaxies. We confirm that the radial-to-vertical scale length
ratio for galactic disks increases (the disks become thinner) with the
increasing of total mass-to-light ratio of the galaxies, which characterize the
contribution of dark halo to the total mass, and with the decreasing of central
deprojected disk brightness (surface density). Our results are in good
agreement with numerical models of collisionless disks evolved from subcritical
velocity dispersion state to a marginally stable equilibrium state. This
suggests that in most galaxies the vertical stellar velocity dispersion, which
determine the equilibrium disk thickness, is close to the minimum value, that
ensures disk stability. The thinnest edge-on disks appear to be low brightness
galaxies (after deprojection) in which a dark halo mass far exceeds a mass of
the stellar disk.Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002
The discovery of two extremely low luminosity Milky Way globular clusters
We report the discovery of two extremely low luminosity globular clusters in
the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey
Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory.
The clusters, Koposov 1 and Koposov 2, are located at kpc and
appear to have old stellar populations and luminosities of only
mag. Their observed sizes of pc are well within the expected tidal
limit of 10 pc at that distance. Together with Palomar 1, AM 4 and
Whiting 1, these new clusters are the lowest luminosity globulars orbiting the
Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to
distant branch of the Sagittarius stream. The half-mass relaxation times of
Koposov 1 and 2 are only and Myr respectively (2 orders of
magnitude shorter than the age of the stellar populations), so it would seem
that they have undergone drastic mass segregation. Since they do not appear to
be very concentrated, their evaporation timescales may be as low as . These discoveries show that the structural parameter space of
globular clusters in the Milky Way halo is not yet fully explored. They also
add, through their short remaining survival times, significant direct evidence
for a once much larger population of globular clusters.Comment: accepted for publication in ApJ, minor revision
Observation of Andreev Reflection Enhanced Shot Noise
We have experimentally investigated the quasiparticle shot noise in
NbN/MgO/NbN superconductor - insulator - superconductor tunnel junctions. The
observed shot noise is significantly larger than theoretically expected. We
attribute this to the occurrence of multiple Andreev reflection processes in
pinholes present in the MgO barrier. This mechanism causes the current to flow
in large charge quanta (Andreev clusters), with a voltage dependent average
value of m = 1+ 2 Delta/eV times the electron charge. Because of this charge
enhancement effect, the shot noise is increased by the factor m.Comment: 4 pages, 5 figures include
How exactly did the Universe become neutral?
We present a refined treatment of H, He I, and He II recombination in the
early Universe. The difference from previous calculations is that we use
multi-level atoms and evolve the population of each level with redshift by
including all bound-bound and bound-free transitions. In this framework we
follow several hundred atomic energy levels for H, He I, and He II combined.
The main improvements of this method over previous recombination calculations
are: (1) allowing excited atomic level populations to depart from an
equilibrium distribution; (2) replacing the total recombination coefficient
with recombination to and photoionization from each level directly at each
redshift step; and (3) correct treatment of the He I atom, including the
triplet and singlet states. We find that the ionization fraction x_e = n_e/n_H
is approximately 10% smaller at redshifts <~800 than in previous calculations,
due to the non-equilibrium of the excited states of H, which is caused by the
strong but cool radiation field at those redshifts. In addition we find that He
I recombination is delayed compared with previous calculations, and occurs only
just before H recombination. These changes in turn can affect the predicted
power spectrum of microwave anisotropies at the few percent level. Other
improvements such as including molecular and ionic species of H, including
complete heating and cooling terms for the evolution of the matter temperature,
including collisional rates, and including feedback of the secondary spectral
distortions on the radiation field, produce negligible change to x_e. The lower
x_e at low z found in this work affects the abundances of H molecular and ionic
species by 10-25%. However this difference is probably not larger than other
uncertainties in the reaction rates.Comment: 24 pages, including 18 figures, using emulateapj.sty, to appear in
ApJ, the code recfast can be obtained at
http://www.astro.ubc.ca/people/scott/recfast.html (in FORTRAN) and
http://cfa-www.harvard.edu/~sasselov/rec/ (in C
Star formation rate indicators in the Sloan Digital Sky Survey
The Sloan Digital Sky Survey (SDSS) first data release provides a database of
106000 unique galaxies in the main galaxy sample with measured spectra. A
sample of star-forming (SF) galaxies are identified from among the 3079 of
these having 1.4 GHz luminosities from FIRST, by using optical spectral
diagnostics. Using 1.4 GHz luminosities as a reference star formation rate
(SFR) estimator insensitive to obscuration effects, the SFRs derived from the
measured SDSS Halpha, [OII] and u-band luminosities, as well as far-infrared
luminosities from IRAS, are compared. It is established that straightforward
corrections for obscuration and aperture effects reliably bring the SDSS
emission line and photometric SFR estimates into agreement with those at 1.4
GHz, although considerable scatter (~60%) remains in the relations. It thus
appears feasible to perform detailed investigations of star formation for large
and varied samples of SF galaxies through the available spectroscopic and
photometric measurements from the SDSS. We provide herein exact prescriptions
for determining the SFR for SDSS galaxies. The expected strong correlation
between [OII] and Halpha line fluxes for SF galaxies is seen, but with a median
line flux ratio F_[OII]/F_Halpha=0.23, about a factor of two smaller than that
found in the sample of Kennicutt (1992). This correlation, used in deriving the
[OII] SFRs, is consistent with the luminosity-dependent relation found by
Jansen et al. (2001). The median obscuration for the SDSS SF systems is found
to be A_Halpha=1.2 mag, while for the radio detected sample the median
obscuration is notably higher, 1.6 mag, and with a broader distribution.Comment: Accepted for publication in ApJ, 40 pages, 26 figure
Drop impact upon micro- and nanostructured superhydrophobic surfaces
We experimentally investigate drop impact dynamics onto different
superhydrophobic surfaces, consisting of regular polymeric micropatterns and
rough carbon nanofibers, with similar static contact angles. The main control
parameters are the Weber number \We and the roughness of the surface. At small
\We, i.e. small impact velocity, the impact evolutions are similar for both
types of substrates, exhibiting Fakir state, complete bouncing, partial
rebouncing, trapping of an air bubble, jetting, and sticky vibrating water
balls. At large \We, splashing impacts emerge forming several satellite
droplets, which are more pronounced for the multiscale rough carbon nanofiber
jungles. The results imply that the multiscale surface roughness at nanoscale
plays a minor role in the impact events for small \We \apprle 120 but an
important one for large \We \apprge 120. Finally, we find the effect of
ambient air pressure to be negligible in the explored parameter regime \We
\apprle 150Comment: 8 pages, 7 figure
Photometric deprojection of edge-on galaxies
Photometric deprojection is used to determine the stellardisk and bulge
parameters for several edgeon galaxies from the FGC catalog. The assumption
that the galaxies of our sample belonging to the fourth (i.e., lowest)
surfacebrightness class in the FGC are edgeon, lowsurfacebrightness (LSB)
galaxies is considered.Comment: 6 pages including 3 figures. Have been published in Astronomy
Letters, v.26, p.219, 2000 (Translated from Pis'ma v Astronomicheskiy
Zhurnal, V.26, N.4, 2000, p.266.
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