25,732 research outputs found
Models for application of radiation boundary condition for MHD waves in collapse calculations
The problem of reflection of magnetohydrodynamic (MHD) waves at the boundary of a numerical grid has to be resolved in order to obtain reliable results for the end state of the (isothermal) collapse of a rotating, magnetic protostellar cloud. Since the goal of investigating magnetic braking in collapse simulations is to see if the transport of angular momentum via alfven waves is large enough to solve the angular momentum problem an approximation that artificially suppresses large amplitudes in the MHD waves can be self-defeating. For this reason, four alternate methods of handling reflected waves where no assumptions are made regarding the amplitudes of the waves were investigated. In order to study this problem (of reflection) without interference from other effects these methods were tried on two simpler cases. The four methods are discussed
Microwave Remote Sensing of Ocean Surface Wind Speed and Rain Rates over Tropical Storms
The value of using narrowly spaced frequencies within a microwave band to measure wind speeds and rain rates over tropical storms with radiometers is reviewed. The technique focuses on results obtained in the overflights of Hurricane Allen during 5 and 8 of August, 1980
Parity Effect in a Small Superconducting Particle
Matveev and Larkin calculated the parity effect on the ground state energy of
a small superconducting particle in the regimes where the mean level spacing is
either large or small compared to the bulk gap. We perform a numerical
calculation which extends their results into the intermediate regime, where the
level spacing is of the same order as the bulk gap.Comment: 6 LaTeX pages, including 2 EPS figures; corrected reference and
spellin
Light forces in ultracold photoassociation
We study the time-resolved photoassociation of ultracold sodium in an optical
dipole trap. The photoassociation laser excites pairs of atoms to molecular
states of large total angular momentum at high intensities (above 20
kW/cm). Such transitions are generally suppressed at ultracold
temperatures by the centrifugal barriers for high partial waves. Time-resolved
ionization measurements reveal that the atoms are accelerated by the dipole
potential of the photoassociation beam. We change the collision energy by
varying the potential depth, and observe a strong variation of the
photoassociation rate. These results demonstrate the important role of light
forces in cw photoassociation at high intensities.Comment: 7 pages, 3 figure
Gate-Voltage Studies of Discrete Electronic States in Al Nanoparticles
We have investigated the spectrum of discrete electronic states in single,
nm-scale Al particles incorporated into new tunneling transistors, complete
with a gate electrode. The addition of the gate has allowed (a) measurements of
the electronic spectra for different numbers of electrons in the same particle,
(b) greatly improved resolution and qualitatively new results for spectra
within superconducting particles, and (c) detailed studies of the gate-voltage
dependence of the resonance level widths, which have directly demonstrated the
effects of non-equilibrium excitations.Comment: 4 pages, 7 figure
Neutron interferometric measurement of the scattering length difference between the triplet and singlet states of n-He
We report a determination of the n-He scattering length difference
(
(statistical) (systematic)) fm between the triplet and singlet
states using a neutron interferometer. This revises our previous result (-5.610 (statistical) (systematic)
fm obtained using the same technique in 2008. This revision is due to a
re-analysis of the 2008 experiment that includes a more robust treatment of the
phase shift caused by magnetic field gradients near the He cell.
Furthermore, we more than doubled our original data set from 2008 by acquiring
six months of additional data in 2013. Both the new data set and a re-analysis
of the older data are in good agreement. Scattering lengths of low Z isotopes
are valued for use in few-body nuclear effective field theories, provide
important tests of modern nuclear potential models and in the case of He
aid in the interpretation of neutron scattering from quantum liquids. The
difference was determined by measuring the relative phase
shift between two incident neutron polarizations caused by the spin-dependent
interaction with a polarized He target. The target He gas was sealed
inside a small, flat windowed glass cell that was placed in one beam path of
the interferometer. The relaxation of He polarization was monitored
continuously with neutron transmission measurements. The neutron polarization
and spin flipper efficiency were determined separately using He analyzers
and two different polarimetry analysis methods. A summary of the measured
scattering lengths for n-He with a comparison to nucleon interaction models
is given
Detection of a dense clump in a filament interacting with W51e2
In the framework of the Herschel/PRISMAS Guaranteed Time Key Program, the
line of sight to the distant ultracompact HII region W51e2 has been observed
using several selected molecular species. Most of the detected absorption
features are not associated with the background high-mass star-forming region
and probe the diffuse matter along the line of sight. We present here the
detection of an additional narrow absorption feature at ~70 km/s in the
observed spectra of HDO, NH3 and C3. The 70 km/s feature is not uniquely
identifiable with the dynamic components (the main cloud and the large-scale
foreground filament) so-far identified toward this region. The narrow
absorption feature is similar to the one found toward low-mass protostars,
which is characteristic of the presence of a cold external envelope. The
far-infrared spectroscopic data were combined with existing ground-based
observations of 12CO, 13CO, CCH, CN, and C3H2 to characterize the 70 km/s
component. Using a non-LTE analysis of multiple transitions of NH3 and CN, we
estimated the density (n(H2) (1-5)x10^5 cm^-3) and temperature (10-30 K) for
this narrow feature. We used a gas-grain warm-up based chemical model with
physical parameters derived from the NH3 data to explain the observed
abundances of the different chemical species. We propose that the 70 km/s
narrow feature arises in a dense and cold clump that probably is undergoing
collapse to form a low-mass protostar, formed on the trailing side of the
high-velocity filament, which is thought to be interacting with the W51 main
cloud. While the fortuitous coincidence of the dense clump along the line of
sight with the continuum-bright W51e2 compact HII region has contributed to its
non-detection in the continuum images, this same attribute makes it an
appropriate source for absorption studies and in particular for ice studies of
star-forming regions.Comment: Accepted for publication in A&
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