19,081 research outputs found
Observation of thermo-mechanical equilibration in the presence of a solid 4He conduit
We observe a thermo-mechanical effect when a chemical potential difference is
created by a temperature difference imposed between two liquid reservoirs
connected to each other through Vycor rods in series with solid hcp 4He. By
creating a temperature difference, , between the two reservoirs, we
induce a rate-limited growth of a pressure difference between the two
reservoirs, . In equilibrium is in
quantitative agreement with the thermo-mechanical effect in superfluid helium.
These observations confirm that below 600 mK a flux-limited flow exists
through the solid helium.Comment: 4 pages, 4 figure
Radial deformation of the earth by oceanic tidal loading
A high-degree spherical harmonic series is used to compute the radial deformation of the Earth by oceanic tidal loading. By exploiting fast numerical transforms, this approach is found to be much more efficient, but no less accurate, than the traditional Green's function approach. The method is used to derive an atlas of load tide maps for 10 constitutents of the NSWC ocean tide model
Growth of solid hcp \^4He off the melting curve
We report studies of the growth of solid hcp \4he at pressures higher than
the bulk freezing pressure using a cell design that allows us to inject atoms
into the solid. Near the melting curve during injection we observe random
events during which the pressure recorded in the cell drops abruptly. These
events are accompanied by transient increases in the temperature of the cell.
We discuss these transients and conclude that they represent the solidification
of meta-stable liquid regions and the associated relief of strain in the local
solid. We also observe that further from the melting curve the transients are
no longer recorded, but that we can continue to add atoms to the solid,
increasing its density at fixed volume. We document these changes in density
with respect to changes in the chemical potential as a function of temperature
and discuss these in the context of recent theoretical work.Comment: 7 pages, 8 figure
Lunar and Hawaiian lava tubes: Analogs and uses based on terrestrial field data
Presented here is an analysis of the data collected for a large number of Hawaiian lava tubes on the islands of Oahu, Molokai, and Hawaii. The results are extrapolated to lunar conditions. It is argued that lava tubes that formed on the Earth and the Moon are relatively stable over time, as illustrated by the ridigity of the Hawaiian prehistoric lava tubes as well as the historic tubes located in the bombing range near Mauna Loa. These natural structures should be considered for use in planning for the expansion and advanced stages of the future manned lunar base
Mass flow through solid 4He induced by the fountain effect
Using an apparatus that allows superfluid liquid 4He to be in contact with
hcp solid \4he at pressures greater than the bulk melting pressure of the
solid, we have performed experiments that show evidence for 4He mass flux
through the solid and the likely presence of superfluid inside the solid. We
present results that show that a thermomechanical equilibrium in quantitative
agreement with the fountain effect exists between two liquid reservoirs
connected to each other through two superfluid-filled Vycor rods in series with
a chamber filled with solid 4He. We use the thermomechanical effect to induce
flow through the solid and measure the flow rate. On cooling, mass flux appears
near T = 600 mK and rises smoothly as the temperature is lowered. Near T = 75
mK a sharp drop in the flux is present. The flux increases as the temperature
is reduced below 75 mK. We comment on possible causes of this flux minimum.Comment: 20 pages, 22 figures, 7 table
Low-Mass Dileptons at the CERN-SpS: Evidence for Chiral Restoration?
Using a rather complete description of the in-medium spectral function
- being constrained by various independent experimental information - we
calculate pertinent dilepton production rates from hot and dense hadronic
matter. The strong broadening of the resonance entails a reminiscence to
perturbative annihilation rates in the vicinity of the phase
boundary. The application to dilepton observables in Pb(158AGeV)+Au collisions
- incorporating recent information on the hadro-chemical composition at
CERN-SpS energies - essentially supports the broadening scenario. Possible
implications for the nature of chiral symmetry restoration are outlined.Comment: 6 pages ReVTeX including 5 eps-figure
Advanced passive communication satellite systems comparison studies. Volume 1 - Summary Final report
Passive communication satellites feasibility for Comsat system - Vol.
Simulation of flight test conditions in the Langley pilot transonic cryogenic tunnel
The theory and advantages of the cryogenic tunnel concept are briefly reviewed. The unique ability to vary temperature independently of pressure and Mach number allows, in addition to large reductions in model loads and tunnel power, the independent determination of Reynolds number, Mach number, and aeroelastic effects on the aerodynamic characteristics of the model. Various combinations of Reynolds number and dynamic pressure are established to represent accurately flight variations of aeroelastic deformation with altitude changes. The consequences of the thermal and caloric imperfections of the test gas under cryogenic conditions were examined and found to be insignificant for operating pressures up to 5 atm. The characteristics of the Langley pilot transonic cryogenic tunnel are described and the results of initial tunnel operation are presented. Tests of a two-dimensional airfoil at a Mach number of 0.85 show identical pressure distributions for a chord Reynolds number of 8,600,000 obtained first at a stagnation pressure of 4.91 atm at a stagnation temperature of 322.0 K and then at a stagnation pressure of 1.19 atm at a stagnation temperature of 116.5 K
Advanced passive communication satellite systems comparison studies. Volume 2 - Technical discussion Final report
Passive communication satellites feasibility for Comsat system - Vol.
Emission Line Variability of the Accreting Young Brown Dwarf 2MASSW J1207334-393254: From Hours to Years
We have obtained a series of high-resolution optical spectra for the brown
dwarf 2MASSW J1207334-393254 (2M1207) using the ESO Very Large Telescope with
the UVES spectrograph during two consecutive observing nights (time resolution
of ~12 min) and the Magellan Clay telescope with the MIKE spectrograph.
Combined with previously published results, these data allow us to investigate
changes in the emission line spectrum of 2M1207 on timescales of hours to
years. Most of the emission line profiles of 2M1207 are broad, in particular
that of Halpha, indicating that the dominant fraction of the emission must be
attributed to disk accretion rather than to magnetic activity. From the Halpha
10% width we deduce a relatively stable accretion rate between
10^(-10.1...-9.8) Msun/yr for two nights of consecutive observations.
Therefore, either the accretion stream is nearly homogeneous over (sub-)stellar
longitude or the system is seen face-on. Small but significant variations are
evident throughout our near-continuous observation, and they reach a maximum
after ~8 h, roughly the timescale on which maximum variability is expected
across the rotation cycle. Together with past measurements, we confirm that the
accretion rate of 2M1207 varies by more than one order of magnitude on
timescales of months to years. Such variable mass accretion yields a plausible
explanation for the observed spread in the accretion rate vs. mass diagram. The
magnetic field required to drive the funnel flow is on the order of a few
hundred G. Despite the obvious presence of a magnetic field, no radio nor X-ray
emission has been reported for 2M1207. Possibly strong accretion suppresses
magnetic activity in brown dwarfs, similar to the findings for higher mass T
Tauri stars.Comment: accepted for publication in Ap
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