2,943 research outputs found
Correlation of gravimetric and satellite geodetic data Interim progress report, 11 Sep. 1967 - 29 Feb. 1968
Gravimetric and geodetic data correlation for satellite position prediction accuracy with error analysi
Two-dimensional electron gas in a modulation-doped SrTiO3/Sr(Ti,Zr)O3 heterostructure
A two-dimensional electron gas (2DEG) in SrTiO3 is created via modulation
doping by interfacing undoped SrTiO3 with a wider-band-gap material,
SrTi1-xZrxO3, that is doped n-type with La. All layers are grown using hybrid
molecular beam epitaxy. Using magnetoresistance measurements, we show that
electrons are transferred into the SrTiO3, and a 2DEG is formed. In particular,
Shubnikov-de Haas oscillations are shown to depend only on the perpendicular
magnetic field. Experimental Shubnikov-de Haas oscillations are compared with
calculations that assume multiple occupied subbands.Comment: Submitted to Applied Physics Letter
Correlation of gravimetric and satellite geodetic data, part 2 Interim scientific report, 11 Sep. 1967 - 29 Feb. 1968
Graphical output from computer correlated gravimetric and satellite geodetic dat
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
First Evidence of Circumstellar Disks around Blue Straggler Stars
We present an analysis of optical HST/STIS and HST/FOS spectroscopy of 6 blue
stragglers found in the globular clusters M3, NGC6752 and NGC6397. These stars
are a subsample of a set of ~50 blue stragglers and stars above the main
sequence turn-off in four globular clusters which will be presented in an
forthcoming paper. All but the 6 stars presented here can be well fitted with
non-LTE model atmospheres. The 6 misfits, on the other hand, possess Balmer
jumps which are too large for the effective temperatures implied by their
Paschen continua. We find that our data for these stars are consistent with
models only if we account for extra absorption of stellar Balmer photons by an
ionized circumstellar disk. Column densities of HI and CaII are derived as are
the the disks' thicknesses. This is the first time that a circumstellar disk is
detected around blue stragglers. The presence of magnetically-locked disks
attached to the stars has been suggested as a mechanism to lose the large
angular momentum imparted by the collision event at the birth of these stars.
The disks implied by our study might not be massive enough to constitute such
an angular momentum sink, but they could be the leftovers of once larger disks.Comment: Accepted by ApJ Letters 10 pages, 2 figure
VETA-1 x ray detection system
The alignment and X-ray imaging performance of the Advanced X-ray Astrophysics Facility (AXAF) Verification Engineering Test Article-I (VETA-I) was measured by the VETA-I X-Ray Detection System (VXDS). The VXDS was based on the X-ray detection system utilized in the AXAF Technology Mirror Assembly (TMA) program, upgraded to meet the more stringent requirements of the VETA-I test program. The VXDS includes two types of X-ray detectors: (1) a High Resolution Imager (HRI) which provides X-ray imaging capabilities, and (2) sealed and flow proportional counters which, in conjunction with apertures of various types and precision translation stages, provide the most accurate measurement of VETA-I performance. Herein we give an overview of the VXDS hardware including X-ray detectors, translation stages, apertures, proportional counters and flow counter gas supply system and associated electronics. We also describe the installation of the VXDS into the Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF). We discuss in detail the design and performance of those elements of the VXDS which have not been discussed elsewhere; translation systems, flow counter gas supply system, apertures and thermal monitoring system
Path lengths in turbulence
By tracking tracer particles at high speeds and for long times, we study the
geometric statistics of Lagrangian trajectories in an intensely turbulent
laboratory flow. In particular, we consider the distinction between the
displacement of particles from their initial positions and the total distance
they travel. The difference of these two quantities shows power-law scaling in
the inertial range. By comparing them with simulations of a chaotic but
non-turbulent flow and a Lagrangian Stochastic model, we suggest that our
results are a signature of turbulence.Comment: accepted for publication in Journal of Statistical Physic
Simultaneous measurements of three-dimensional trajectories and wingbeat frequencies of birds in the field
This is the author accepted manuscript. The final version is available from the Royal Society via the DOI in this recordData accessibility:
We provide data including images recorded by four cameras, camera parameters, videos showing the time variation of the bird 3D positions, and plain text files that include bird id number, positions, times, velocities, accelerations, and wingbeat frequencies at every time step. We also provide the Matlab codes that were used to: (i) detect birds on images; (ii) reconstruct birds' 3D locations using the new stereo-matching algorithm; (iii) track individual's 3D motions; and (iv) calculate wing motion and wingbeat frequency from tracking results. The code and data are available at: https://github.com/linghj/3DTracking.git and https://figshare.com/s/3c572f91b07b06ed30aa.Tracking the movements of birds in three dimensions is integral to a wide range of problems in animal ecology, behaviour and cognition. Multi-camera stereo-imaging has been used to track the three-dimensional (3D) motion of birds in dense flocks, but precise localization of birds remains a challenge due to imaging resolution in the depth direction and optical occlusion. This paper introduces a portable stereo-imaging system with improved accuracy and a simple stereo-matching algorithm that can resolve optical occlusion. This system allows us to decouple body and wing motion, and thus measure not only velocities and accelerations but also wingbeat frequencies along the 3D trajectories of birds. We demonstrate these new methods by analysing six flocking events consisting of 50 to 360 jackdaws (Corvus monedula) and rooks (Corvus frugilegus) as well as 32 jackdaws and 6 rooks flying in isolated pairs or alone. Our method allows us to (i) measure flight speed and wingbeat frequency in different flying modes; (ii) characterize the U-shaped flight performance curve of birds in the wild, showing that wingbeat frequency reaches its minimum at moderate flight speeds; (iii) examine group effects on individual flight performance, showing that birds have a higher wingbeat frequency when flying in a group than when flying alone and when flying in dense regions than when flying in sparse regions; and (iv) provide a potential avenue for automated discrimination of bird species. We argue that the experimental method developed in this paper opens new opportunities for understanding flight kinematics and collective behaviour in natural environments.Human Frontier Science Progra
Galactic populations of radio and gamma-ray pulsars in the polar cap model
We simulate the characteristics of the Galactic population of radio and
-ray pulsars using Monte Carlo techniques. At birth, neutron stars are
spatially distributed in the Galactic disk, with supernova-kick velocities, and
randomly dispersed in age back to years. They are evolved in the
Galactic gravitational potential to the present time. From a radio luminosity
model, the radio flux is filtered through a selected set of radio-survey
parameters. -ray luminosities are assigned using the features of recent
polar cap acceleration models invoking space-charge-limited flow, and a pulsar
death valley further attenuates the population of radio-loud pulsars. Assuming
a simple emission geometry with aligned radio and -ray beams of 1
steradian solid angle, our model predicts that EGRET should have seen 7
radio-loud and 1 radio-quiet, -ray pulsars. With much improved
sensitivity, GLAST, on the other hand, is expected to observe 76 radio-loud and
74 radio-quiet, -ray pulsars of which 7 would be identified as pulsed
sources. We also explore the effect of magnetic field decay on the
characteristics of the radio and -ray pulsar populations. Including
magnetic field decay on a timescale of 5 Myr improves agreement with the radio
pulsar population and increases the predicted number of GLAST detected pulsars
to 90 radio-loud and 101 radio-quiet (9 pulsed) -ray pulsars. The lower
flux threshold allows GLAST to detect -ray pulsars at larger distances
than those observed by the radio surveys used in this study.Comment: 38 pages, 11 figures, accepted for publication v565 n1 Ap
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