568 research outputs found
Ekman Pumping in Compact Astrophysical Bodies
We examine the dynamics of a rotating viscous fluid following an abrupt
change in the angular velocity of the solid bounding surface. We include the
effects of a density stratification and compressibility which are important in
astrophysical objects such as neutron stars. We confirm and extend the
conclusions of previous studies that stratification restricts the Ekman pumping
process to a relatively thin layer near the boundary, leaving much of the
interior fluid unaffected. We find that finite compressibility further inhibits
Ekman pumping by decreasing the extent of the pumped layer and by increasing
the time for spin-up. The results of this paper are important for interpreting
the spin period discontinuities (``glitches'') observed in rotating neutron
stars.Comment: Latex, 21 pages, 5 ps figures. Revised version includes extended
discussion in the introduction and references to previous works. Various
minor corrections and clarifications include
Precise Determination of Minimum Achievable Temperature for Solid-State Optical Refrigeration
We measure the minimum achievable temperature (MAT) as a function of
excitation wavelength in anti-Stokes fluorescence cooling of high purity
Yb3+-doped LiYF4 (Yb:YLF) crystal. Such measurements were obtained by
developing a sensitive noncontact thermometry that is based on a two-band
differential luminescence spectroscopy using balanced photo-detectors. These
measurements are in excellent agreement with the prediction of the laser
cooling model and identify MAT of 110 K at 1020 nm, corresponding to E4-E5
Stark manifold transition in Yb:YLF crystal.Comment: 10 pages, 6 figure
Are We Seeing Magnetic Axis Reorientation in the Crab and Vela Pulsars?
Variation in the angle between a pulsar's rotational and magnetic
axes would change the torque and spin-down rate. We show that sudden increases
in , coincident with glitches, could be responsible for the persistent
increases in spin-down rate that follow glitches in the Crab pulsar. Moreover,
changes in at a rate similar to that inferred for the Crab pulsar
account naturally for the very low braking index of the Vela pulsar. If
increases with time, all pulsar ages obtained from the conventional
braking model are underestimates. Decoupling of the neutron star liquid
interior from the external torque cannot account for Vela's low braking index.
Variations in the Crab's pulse profile due to changes in might be
measurable.Comment: 14 pages and one figure, Latex, uses aasms4.sty. Accepted to ApJ
Letter
Electrocaloric devices based on thini-film heat switches
We describe a new approach to refrigeration and electrical generation that exploits the attractive properties of thin films of electrocaloric materials. Layers of electrocaloric material coupled with thin-film heat switches can work as either refrigerators or electrical generators, depending on the phasing of the applied voltages and heat switching. With heat switches based on thin layers of liquid crystals, the efficiency of these thin-film heat engines can be at least as high as that of current thermoelectric devices. Advanced heat switches would enable thin-film heat engines to outperform conventional vaporcompression devices
Anti-Stokes luminescence cooling of Tm3+ doped BaY2F8
We report laser-induced cooling with thulium-doped BaY2F8 single crystals grown using the Czochralski technique. The spectroscopic characterization of the crystals has been used to evaluate the laser cooling performance of the samples. Cooling by 3 degrees below ambient temperature is obtained in a single-pass geometry with 4.4 Watts of pump laser power at lambda = 1855 nm
Observational constraints on the Internal Structure and Dynamics of the Vela Pulsar
We show that the short spin-up time observed for the Vela pulsar during the
1988 ``Christmas'' glitch implies that the coupling time of the pulsar core to
its crust is less than 10 seconds. Ekman pumping cannot explain the fast
core-crust coupling and a more effective coupling is necessary. The internal
magnetic field of the Vela pulsar can provide the necessary coupling if the
field threads the core with a magnitude that exceeds Gauss for a
normal interior and Gauss for a superconducting interior. These
lower bounds favor the hypothesis that the interior of neutron stars contains
superfluid neutrons and protons and challenge the notion that pulsar magnetic
fields decay over million year time scales or that magnetic flux is expelled
from the core as the star slows.Comment: Latex with aasms4 style file, 15 pages, 1 ps figur
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