507 research outputs found
Optical links in the angle-data assembly of the 70-meter antennas
In the precision-pointing mode the 70 meter antennas utilize an optical link provided by an autocollimator. In an effort to improve reliability and performance, commercial instruments were evaluated as replacement candidates, and upgraded versions of the existing instruments were designed and tested. The latter were selected for the Neptune encounter, but commercial instruments with digital output show promise of significant performance improvement for the post-encounter period
Effects of temperature and diet on the growth rate of year 0 oyster toadfish, Opsanus tau
Author Posting. © Marine Biological Laboratory, 2006. This article is posted here by permission of Marine Biological Laboratory for personal use, not for redistribution. The definitive version was published in Biological Bulletin 210 (2006): 64-71.The effects of temperature and diet on the growth of captive year 0 specimens of Opansus tau were examined for two consecutive year classes. The 2001 year class was raised at about 23, 26, or 29°C and provided with live brine shrimp, frozen butterfish and squid, or commercial food pellets (45% protein, 19% fat, and 3% fiber) three times per week. Maximal growth was achieved with the pellet diet, and fish raised at 29°C attained the highest mean wet weight (84.0 g ± 14.6 g SE) and fastest instantaneous relative growth rate (IRGR, 1.79% body weight/d). The 2002 year class was raised on the same pellet diet at 31.5°C and fed 3, 5, or 7 times per week. Although more frequent feedings led to significantly greater mean wet weight in the first half of the year, by month 12 there was no significant difference between the three feeding frequencies. These fish weighed approximately 68g and had an average IRGR of 1.74% body weight/d. The pellet diet during both years was correlated with high survival (> 75%).Support was provided by the Marine Models in Biological
Research Program, the University of Minnesota Grant in
Aid, the NASA Life Science Fellowship, the MBL Associates
Fellowship, and NIH grant DC01837
Ranging system which compares an object reflected component of a light beam to a reference component of the light beam
A system is described for measuring the distance to an object by comparing a first component of a light pulse that is reflected off the object with a second component of the light pulse that passes along a reference path of known length, which provides great accuracy with a relatively simple and rugged design. The reference path can be changed in precise steps so that it has an equivalent length approximately equal to the path length of the light pulse component that is reflected from the object. The resulting small difference in path lengths can be precisely determined by directing the light pulse components into opposite ends of a detector formed of a material that emits a second harmonic light output at the locations where the opposite going pulses past simultaneously across one another
Estimating the phase in ground-based interferometry: performance comparison between single-mode and multimode schemes
In this paper we compare the performance of multi and single-mode
interferometry for the estimation of the phase of the complex visibility. We
provide a theoretical description of the interferometric signal which enables
to derive the phase error in presence of detector, photon and atmospheric
noises, for both multi and single-mode cases. We show that, despite the loss of
flux occurring when injecting the light in the single-mode component (i.e.
single-mode fibers, integrated optics), the spatial filtering properties of
such single-mode devices often enable higher performance than multimode
concepts. In the high flux regime speckle noise dominated, single-mode
interferometry is always more efficient, and its performance is significantly
better when the correction provided by adaptive optics becomes poor, by a
factor of 2 and more when the Strehl ratio is lower than 10%. In low light
level cases (detector noise regime), multimode interferometry reaches better
performance, yet the gain never exceeds 20%, which corresponds to the
percentage of photon loss due to the injection in the guides. Besides, we
demonstrate that single-mode interferometry is also more robust to the
turbulence in both cases of fringe tracking and phase referencing, at the
exception of narrow field of views (<1 arcsec).Comment: 9 pages (+ 11 online material appendices) -- 8 Figures. Accepted in
A&
Nucleon Spin Fluctuations and the Supernova Emission of Neutrinos and Axions
In the hot and dense medium of a supernova (SN) core, the nucleon spins
fluctuate so fast that the axial-vector neutrino opacity and the axion
emissivity are expected to be significantly modified. Axions with
m_a\alt10^{-2}\,{\rm eV} are not excluded by SN~1987A. A substantial transfer
of energy in neutrino-nucleon () collisions is enabled which may alter
the spectra of SN neutrinos relative to calculations where energy-conserving
collisions had been assumed near the neutrinosphere.Comment: 8 pages. REVTeX. 2 postscript figures, can be included with epsf.
Small modifications of the text, a new "Note Added", and three new
references. To be published in Phys. Rev. Let
Near-infrared and optical observations of galactic warps: A common, unexplained feature of most discs
Context: Warps occurring in galactic discs have been studied extensively in
HI and in the optical, but rarely in the near-infrared (NIR) bands that trace
the older stellar populations.
Aims: We provide NIR data of nearby edge-on galaxies, combined with optical
observations, for direct comparison of the properties of galactic warps as a
function of wavelength, and calculate warp curves for each galaxy and obtain
the characteristic warp parameters. We discuss these properties as possible
constraints to the different mechanisms that have been proposed for the
development and persistence of galactic warps.
Methods: We observed 20 galaxies that were selected from a statistically
complete diameter-limited subsample of edge-on disc galaxies. We used the Cerro
Tololo Infrared Imager (CIRIM) at the CTIO 1.5m Ritchey-Chretien telescope to
acquire the NIR data. We used the 1.54m Danish and 0.92m Dutch telescopes at
the European Southern Observatory's La Silla site for our optical observations.
Results: Our results show that 13 of our 20 sample galaxies are warped, with
the warp more pronounced in the optical than at NIR wavelengths. In the
remaining seven galaxies, no warp is apparent within the limitations of our
automated detection method. The transition between the unperturbed inner disc
and the outer, warped region is rather abrupt. S0 galaxies exhibit very small
or no warps. The magnetic model remains one of a number of interesting
formation scenarios.Comment: 16 page
Refractive Index of Humid Air in the Infrared: Model Fits
The theory of summation of electromagnetic line transitions is used to
tabulate the Taylor expansion of the refractive index of humid air over the
basic independent parameters (temperature, pressure, humidity, wavelength) in
five separate infrared regions from the H to the Q band at a fixed percentage
of Carbon Dioxide. These are least-squares fits to raw, highly resolved spectra
for a set of temperatures from 10 to 25 C, a set of pressures from 500 to 1023
hPa, and a set of relative humidities from 5 to 60%. These choices reflect the
prospective application to characterize ambient air at mountain altitudes of
astronomical telescopes.Comment: Corrected exponents of c0ref, c1ref and c1p in Table
Neutrino Interactions in Hot and Dense Matter
We study the charged and neutral current weak interaction rates relevant for
the determination of neutrino opacities in dense matter found in supernovae and
neutron stars. We establish an efficient formalism for calculating differential
cross sections and mean free paths for interacting, asymmetric nuclear matter
at arbitrary degeneracy. The formalism is valid for both charged and neutral
current reactions. Strong interaction corrections are incorporated through the
in-medium single particle energies at the relevant density and temperature. The
effects of strong interactions on the weak interaction rates are investigated
using both potential and effective field-theoretical models of matter. We
investigate the relative importance of charged and neutral currents for
different astrophysical situations, and also examine the influence of
strangeness-bearing hyperons. Our findings show that the mean free paths are
significantly altered by the effects of strong interactions and the
multi-component nature of dense matter. The opacities are then discussed in the
context of the evolution of the core of a protoneutron star.Comment: 41 pages, 25 figure
Neutrino propagation and spin zero sound in hot neutron matter with Skyrme interactions
We present microscopic calculations of neutrino propagation in hot neutron
matter above nuclear density within the framework of the Random Phase
Approximation . Calculations are performed for non- degenerate neutrinos using
various Skyrme effective interactions. We find that for densities just above
nuclear density, spin zero sound is present at zero temperature for all Skyrme
forces considered. However it disappears rapidly with increasing temperature
due to a strong Landau damping. As a result the mean-free path is given, to a
good approximation, by the mean field value. Because of the renormalization of
the bare mass in the mean field, the medium is more transparent as compared to
the free case. We find, in contrast, that at several times nuclear density, a
new type of behavior sets in due to the vicinity of a magnetic instability. It
produces a strong reduction of the mean free path. The corresponding transition
density however occurs in a region where inputs from more realistic
calculations are necessary for the construction of a reliable Skyrme type
parametrization.Comment: 17 pages, 4 figure
Particle-hole state densities with non-equidistant single-particle levels
The correct use of energy-dependent single-particle level (s.p.l.) densities
within particle-hole state densities based on the equidistant spacing model
(ESM) is analysed. First, an analytical expression is obtained following the
convolution of energy-dependent excited-particle and hole densities. Next, a
comparison is made with results of the ESM formula using average s.p.l.
densities for the excited particles and holes, respectively. The Fermi-gas
model (FGM) s.p.l. densities calculated at the corresponding average excitation
energies are used in both cases. The analysis concerns also the density of
particle-hole bound states. The pairing correlations are taken into account
while the comparison of various effects includes the exact correction for the
Pauli exclusion principle. Quantum-mechanical s.p.l. densities and the
continuum effect can also match a corresponding FGM formula, suitable for use
within the average energy-dependent partial state density in multistep reaction
models.Comment: 29 pages, ReVTeX, 11 postscript figures, submitted to Phys.Rev.
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