8,830 research outputs found
Hysteresis and competition between disorder and crystallization in sheared and vibrated granular flow
Experiments on spherical particles in a 3D Couette cell vibrated from below
and sheared from above show a hysteretic freezing/melting transition. Under
sufficient vibration a crystallized state is observed, which can be melted by
sufficient shear. The critical line for this transition coincides with equal
kinetic energies for vibration and shear. The force distribution is
double-peaked in the crystalline state and single-peaked with an approximately
exponential tail in the disordered state. A linear relation between pressure
and volume () exists for a continuum of partially and/or
intermittently melted states over a range of parameters
Two-body effects in the decay rate of atomic levels
Recoil corrections to the atomic decay rate are considered in the order of
Zm/M . The expressions are treated exactly without any expansion over Z alpha.
The expressions obtained are valid both for muonic atoms (for which they
contribute on the level of a few percent in high Z ions) and for electronic
atoms. Explicit results for Lyman-alpha transitions for low-Z of the order
(Zm/M)(Z alpha)^2 are also presented.Comment: 5 pages, 1 table, email: [email protected]
Design Considerations for Unmagnetized Collisionless-shock Measurements in Homologous Flows
The subject of this paper is the design of practical laser experiments that can produce collisionless shocks mediated by the Weibel instability. Such shocks may be important in a wide range of astrophysical systems. Three issues are considered. The first issue is the implications of the fact that such experiments will produce expanding flows that are approximately homologous. As a result, both the velocity and the density of the interpenetrating plasma streams will be time dependent. The second issue is the implications of the linear theory of the Weibel instability. For the experiments, the instability is in a regime where standard simplifications do not apply. It appears feasible but non-trivial to obtain adequate growth. The third issue is collisionality. The need to keep resistive magnetic-field dissipation small enough implies that the plasmas should not be allowed to cool substantially.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98554/1/0004-637X_749_2_171.pd
Redshifted X-rays from the material accreting onto TW Hya: evidence of a low-latitude accretion spot
High resolution spectroscopy, providing constraints on plasma motions and
temperatures, is a powerful means to investigate the structure of accretion
streams in CTTS. In particular, the accretion shock region, where the accreting
material is heated to temperatures of a few MK as it continues its inward bulk
motion, can be probed by X-ray spectroscopy. To attempt to detect for the first
time the motion of this X-ray-emitting post-shock material, we searched for a
Doppler shift in the deep Chandra/HETGS observation of the CTTS TW Hya. This
test should unveil the nature of this X-ray emitting plasma component in CTTS,
and constrain the accretion stream geometry. We searched for a Doppler shift in
the X-ray emission from TW Hya with two different methods, by measuring the
position of a selected sample of emission lines, and by fitting the whole TW
Hya X-ray spectrum, allowing the line-of-sight velocity to vary. We found that
the plasma at T~2-4 MK has a line-of-sight velocity of 38.3+/-5.1 km/s with
respect to the stellar photosphere. This result definitively confirms that this
X-ray-emitting material originates in the post-shock region, at the base of the
accretion stream, and not in coronal structures. The comparison of the observed
velocity along the line of sight, 38.3+/-5.1 km/s, with the inferred intrinsic
velocity of the post shock of TW Hya, v_post~110-120 km/s, indicates that the
footpoints of the accretion streams on TW Hya are located at low latitudes on
the stellar surface. Our results indicate that complex magnetic field
geometries, such as that of TW Hya, permit low-latitude accretion spots.
Moreover, since on TW Hya the redshift of the soft X-ray emission is very
similar to that of the narrow component of the CIV resonance doublet at 1550
Ang, as found by Ardila et al. (2013), then the plasma at 2-4 MK and that at
0.1 MK likely originate in the same post-shock regions.Comment: Accepted for publication in Astronomy & Astrophysics; 2nd version
after language editor corrections; 16 pages, 8 figures, 6 table
Sonobuoy Mils
The purpose of this paper is to describe a new technique for locating missile impact positions in the open ocean using Navy sonobuoys. Figure 1 shows a typical Navy ASW sonobuoy, an air-dropped, expendable VHF radio that relays the underwater acoustic signals received on a hydrophone beneath it to an aircraft overhead* This missile impact location system has the advantages of being low cost, portable and capable of high accuracy. Basically the sonobuoys monitor the hydroacoustic signal of a missile impact on the ocean\u27s surface and use fixed ocean bottom transponders as a geodetic reference for the sonobuoys* Heretofore we have used aircraft dropped sofar charges as an acoustic tie between the surface sonobuoys and the ocean bottom transponders. Impact accuracy in this sonobuoy system of 0.1 NM is possible. In the future, with a little more hardware development, the use of active sonobuoys will eliminate the need for the sofar charge reference tie and the upgraded SMILS will have an accuracy of 250\u27
Infrared Observations During the Secondary Eclipse of HD 209458 b II. Strong Limits on the Infrared Spectrum Near 2.2 Microns
We report observations of the transiting extrasolar planet, HD 209458 b,
designed to detect the secondary eclipse. We employ the method of `occultation
spectroscopy', which searches in combined light (star and planet) for the
disappearance and reappearance of weak infrared spectral features due to the
planet as it passes behind the star and reappears. Our observations cover two
predicted secondary eclipse events, and we obtained 1036 individual spectra of
the HD 209458 system using the SpeX instrument at the NASA IRTF in September
2001. Our spectra extend from 1.9 to 4.2 microns with a spectral resolution of
1500. We have searched for a continuum peak near 2.2 microns (caused by CO and
water absorption bands), as predicted by some models of the planetary
atmosphere to be approximately 6E-4 of the stellar flux, but no such peak is
detected at a level of about 3E-4 of the stellar flux. Our results represent
the strongest limits on the infrared spectrum of the planet to date and carry
significant implications for understanding the planetary atmosphere. In
particular, some models that assume the stellar irradiation is re-radiated
entirely on the sub-stellar hemisphere predict a flux peak inconsistent with
our observations. Several physical mechanisms can improve agreement with our
observations, including the re-distribution of heat by global circulation, a
nearly isothermal atmosphere, and/or the presence of a high cloud.Comment: Accepted to the Astrophysical Journal 17 pages, 6 figure
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