571 research outputs found
Method and apparatus for slicing crystals
The crystal slicing method is described as follows. A crystal is sliced in a plane parallel to flat, opposed parallel end faces of the crystal. The end faces of the crystal are gripped by a pair of opposed, perforated platens of a pair of vacuum chambers, one of which is translatable relative to the other. A blade cuts the crystal through the desired plane. A spring biases one of the vacuum chambers away from the other vacuum chamber while both of the faces are gripped by the vacuum chambers and the blade is cleaving the crystal. A sliced portion of the crystal gripped by one of the vacuum chambers is pulled away from the remainder of the crystal gripped by the second vacuum chamber when the crystal was cleaved by the blade through the plane
Radial distribution of Fe XIV emission in the Cygnus Loop
The one dimensional distribution of Fe 14 emission has been determined along a radius of the Cygnus Loop through the use of a tilting filter photometer. The observed emission extends at least 5 arc minutes outside the optical filaments. A simple Sedov solution model of the temperature and density distribution behind the shock agrees with the observations if the shock front is near the extent of the Fe 14 emission, the shock velocity is from 300 to 250/kms and the density external to the remnant is about 0.7-1.4 cm to three minus 3 power. These parameters are in reasonable agreement with X-ray maps and optical radial velocities
Mystery of the Lyα Blobs
We present Spitzer Space Telescope observations of the extended Lyman α blobs associated with the z=2.38 over-density J2143-4423, the largest known structure (110 Mpc) above z=2. We detect all 4 of the Lyα blobs in all four IRAC channels and we also detect 3 out of 4 of the blobs with MIPS 24μm. Conversion from rest-wavelength 7μm to total far-infrared luminosity using locally derived correlations suggests all the detected sources are in the class of ULIRGs or even Hyper-LIRGs. We find a weak correlation between Lyα and mid-infrared emission for the Lyα blobs (L_(Lyα)/L_(bol) = 0.05-0.2%). Nearly all Lyα blobs show some evidence for interaction, either in HST imaging, or the proximity of multiple MIPS sources within the Lyα cloud. This suggests that interaction or even mergers may be related to the production of Lyα blobs. Optical through infrared SEDs of the Lyα blobs do not show a clear 1.6μm bump, but rather are indicative of a composite of star formation and AGN energy sources
Unveiling Su Aurigae in the near Infrared: New high spatial resolution results using Adaptive Optics
We present here new results on circumstellar nebulosity around SU Aurigae, a
T-Tauri star of about 2 solar mass and 5 Myrs old at 152 pc in the J, H and K
bands using high resolution adaptive optics imaging (0\farcs30) with the Penn
state IR Imaging Spectrograph (PIRIS) at the 100 inch Mt. Wilson telescope.
A comparison with HST STIS optical (0.2 to 1.1 micron) images shows that the
orientation of the circumstellar nebulosity in the near-IR extends from PAs 210
to 270 degrees in H and K bands and up to 300 degrees in the J band. We call
the circumstellar nebulosity seen between 210 to 270 degrees as 'IR
nebulosity'. We find that the IR nebulosity (which extends up to 3.5 arcsecs in
J band and 2.5 arcsecs in the K band) is due to scattered light from the
central star. The IR nebulosity is either a cavity formed by the stellar
outflows or part of the circumstellar disk. We present a schematic
3-dimensional geometrical model of the disk and jet of SU Aur based on STIS and
our near-IR observations. According to this model the IR nebulosity is a part
of the circumstellar disk seen at high inclination angles. The extension of the
IR nebulosity is consistent with estimates of the disk diameter of 50 to 400 AU
in radius, from earlier mm, K band interferometric observations and SED
fittings.Comment: Accepted for publications in the Astronomical Journal, to appear in
the May issue of the Journa
Quantum dynamics of the avian compass
The ability of migratory birds to orient relative to the Earth's magnetic
field is believed to involve a coherent superposition of two spin states of a
radical electron pair. However, the mechanism by which this coherence can be
maintained in the face of strong interactions with the cellular environment has
remained unclear. This Letter addresses the problem of decoherence between two
electron spins due to hyperfine interaction with a bath of spin 1/2 nuclei.
Dynamics of the radical pair density matrix are derived and shown to yield a
simple mechanism for sensing magnetic field orientation. Rates of dephasing and
decoherence are calculated ab initio and found to yield millisecond coherence
times, consistent with behavioral experiments
Luminosities of H alpha emitting regions in a pair of interacting galaxies in the Bootes void
Luminosities of H alpha emission from a pair of interacting galaxies in the low density environment of the Bootes void are presented. CG 692 (IRAS 1519+5050) has an H alpha luminosity of 2 x 10(exp 42) ergs s(exp -1), indicating a star formation rate of 18.4 solar mass yr(exp -1). Individual extranuclear H alpha regions have luminosities of approximately 10(exp 40) ergs s(exp -1). These luminosities are similar to those found for H II regions in bright, late-type galaxies in more densely populated parts of the Universe
Approximate Jacobians for the Solution of the Euler and Navier-Stokes Equations. G.U. Aero Report 9705
This paper describes a method for efficiently solving the steady-state Euler and Navier-Stokes equations. Robustness is achieved through the use of an upwind TVD scheme for discretising the convective terms. The approximate solution is advanced in time implicitly and the linear system arising at each implicit step is solved using a Conjugate Gradient type method. The main emphasis of this paper is on the use of Jacobian matrices associated with a simpler spatial discretisation. This leads to better conditioned linear systems. The resulting method has lower memory and CPU-time requirements when compared with the one using exact Jacobians
Solution of the Euler Equations in Three Dimensional Complex Geometries Using a Fully Unfactored Method. Aerospace Engineering Report 9907
An unfactored implicit time-marching method for the
solution of the three dimensional Euler equations on
multiblock curvilinear grids is presented. For robustness
the convective terms are discretised using an upwind
TVD scheme. The linear system arising from each
implicit time step is solved using a Krylov subspace
method with preconditioning based on an block incomplete
lower-upper (BELU(O)) factorisation. Results are
shown for the ONERA M6 wing, a wing/body configuration
and the NLR-F5 wing with launcher and missile.
It was found that the simulation cost is relatively independent
of the number of blocks used and their orientation.
Comparison is made with experiment where available
and good agreement is obtained
Interplay between magnetism and short-range order in medium- and high-entropy alloys: CrCoNi, CrFeCoNi, and CrMnFeCoNi
The impact of magnetism on predicted atomic short-range order in three
medium- and high-entropy alloys is studied using a first-principles,
all-electron, Landau-type linear response theory, coupled with lattice-based
atomistic modelling. We perform two sets of linear-response calculations: one
in which the paramagnetic state is modelled within the disordered local moment
picture, and one in which systems are modelled in a magnetically ordered state,
which is ferrimagnetic for the alloys considered in this work. We show that the
treatment of magnetism can have significant impact both on the predicted
temperature of atomic ordering and also the nature of atomic order itself. In
CrCoNi, we find that the nature of atomic order changes from being
-like when modelled in the paramagnetic state to MoPt-like
when modelled assuming the system has magnetically ordered. In CrFeCoNi, atomic
correlations between Fe and the other elements present are dramatically
strengthened when we switch from treating the system as magnetically disordered
to magnetically ordered. Our results show it is necessary to consider the
magnetic state when modelling multicomponent alloys containing mid- to
late- elements. Further, we suggest that there may be high-entropy alloy
compositions containing transition metals that will exhibit specific
atomic short-range order when thermally treated in an applied magnetic field.
This has the potential to provide a route for tuning physical and mechanical
properties in this class of materials.Comment: 26 pages, 4 figures, 2 table
Revisiting N\'eel 60 years on: the magnetic anisotropy of FeNi (tetrataenite)
The magnetocrystalline anisotropy energy of atomically ordered
FeNi (the meteoritic mineral tetrataenite) is studied within a
first-principles electronic structure framework. Two compositions are examined:
equiatomic FeNi and an Fe-rich composition,
FeNi. It is confirmed that, for the single crystals modelled
in this work, the leading-order anisotropy coefficient dominates the
higher-order coefficients and . To enable comparison with
experiment, the effects of both imperfect atomic long-range order and finite
temperature are included. While our computational results initially appear to
undershoot the measured experimental values for this system, careful scrutiny
of the original analysis due to N\'{e}el et al. [J. Appl. Phys. 35, 873 (1964)]
suggests that our computed value of is, in fact, consistent with
experimental values, and that the noted discrepancy has its origins in the
nanoscale polycrystalline, multivariant nature of experimental samples, that
yields much larger values of and than expected a priori. These
results provide fresh insight into the existing discrepancies in the literature
regarding the value of tetrataenite's uniaxial magnetocrystalline anisotropy in
both natural and synthetic samples.Comment: 9 pages, 3 figures, 2 table
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