8,429 research outputs found
Development and evaluation of television tubes for space astronomy
An improved vidicon image sensor for space astronomy applications is developed that features reliably sealed ultraviolet transmitting windows and achieves acceptable photocathode quantum efficiency on these MgF2 windows. New preamplifier designs improve the dynamic range by reduction of the readout noise
New coherent states and a new proof of the Scott correction
We introduce new coherent states and use them to prove semi-classical
estimates for Schr\"odinger operators with regular potentials. This can be
further applied to the Thomas-Fermi potential yielding a new proof of the Scott
correction for molecules. This is the short version of a paper by the authors
archived at math-ph/0208044.Comment: A misprint in definition of new coherent states corrected. To appear
in Proceedings of the UAB 2002 Int'l Conference on Differential Equations and
Mathematics Physic
Dynamics of Protoplanetary Disks
Protoplanetary disks are quasi-steady structures whose evolution and
dispersal determine the environment for planet formation. I review the theory
of protoplanetary disk evolution and its connection to observations.
Substantial progress has been made in elucidating the physics of potential
angular momentum transport processes - including self-gravity,
magnetorotational instability, baroclinic instabilities, and magnetic braking -
and in developing testable models for disk dispersal via photoevaporation. The
relative importance of these processes depends upon the initial mass, size and
magnetization of the disk, and subsequently on its opacity, ionization state,
and external irradiation. Disk dynamics is therefore coupled to star formation,
pre-main-sequence stellar evolution, and dust coagulation during the early
stages of planet formation, and may vary dramatically from star to star. The
importance of validating theoretical models is emphasized, with the key
observations being those that probe disk structure on the scales, between 1 AU
and 10 AU, where theory is most uncertain.Comment: Annual Review of Astronomy and Astrophysics (2011). Final edited
version at
http://www.annualreviews.org/doi/abs/10.1146/annurev-astro-081710-102521
.High resolution versions of illustrations at
http://jila.colorado.edu/~pja/araa.htm
On the dynamics of interfaces in the ferromagnetic XXZ chain under weak perturbations
We study the time evolution of interfaces of the ferromagnetic XXZ chain in a
magnetic field. A scaling limit is introduced where the strength of the
magnetic field tends to zero and the microscopic time to infinity while keeping
their product constant. The leading term and its first correction are
determined and further analyzed in more detail for the case of a uniform
magnetic field.Comment: 20 pages, 2 figures, uses conm-p-l.cls. 1 reference adde
Environmental Impact on the Southeast Limb of the Cygnus Loop
We analyze observations from the Chandra X-ray Observatory of the southeast
knot of the Cygnus Loop supernova remnant. In this region, the blast wave
propagates through an inhomogeneous environment. Extrinsic differences and
subsequent multiple projections along the line of sight rather than intrinsic
shock variations, such as fluid instabilities, account for the apparent
complexity of the images. Interactions between the supernova blast wave and
density enhancements of a large interstellar cloud can produce the
morphological and spectral characteristics. Most of the X-ray flux arises in
such interactions, not in the diffuse interior of the supernova remnant.
Additional observations at optical and radio wavelengths support this account
of the existing interstellar medium and its role in shaping the Cygnus Loop,
and they demonstrate that the southeast knot is not a small cloud that the
blast wave has engulfed. These data are consistent with rapid equilibration of
electron and ion temperatures behind the shock front, and the current blast
wave velocity v_{bw} approx 330 km/s. Most of this area does not show strong
evidence for non-equilibrium ionization conditions, which may be a consequence
of the high densities of the bright emission regions.Comment: To appear in ApJ, April 1, 200
Toward High-Precision Astrometry with WFPC2. I. Deriving an Accurate PSF
The first step toward doing high-precision astrometry is the measurement of
individual stars in individual images, a step that is fraught with dangers when
the images are undersampled. The key to avoiding systematic positional error in
undersampled images is to determine an extremely accurate point-spread function
(PSF). We apply the concept of the {\it effective} PSF, and show that in images
that consist of pixels it is the ePSF, rather than the often-used instrumental
PSF, that embodies the information from which accurate star positions and
magnitudes can be derived. We show how, in a rich star field, one can use the
information from dithered exposures to derive an extremely accurate effective
PSF by iterating between the PSF itself and the star positions that we measure
with it. We also give a simple but effective procedure for representing spatial
variations of the HST PSF. With such attention to the PSF, we find that we are
able to measure the position of a single reasonably bright star in a single
image with a precision of 0.02 pixel (2 mas in WF frames, 1 mas in PC), but
with a systematic accuracy better than 0.002 pixel (0.2 mas in WF, 0.1 mas in
PC), so that multiple observations can reliably be combined to improve the
accuracy by .Comment: 33 pp. text + 15 figs.; accepted by PAS
Density-Dependent Response of an Ultracold Plasma to Few-Cycle Radio-Frequency Pulses
Ultracold neutral plasmas exhibit a density-dependent resonant response to
applied radio-frequency (RF) fields in the frequency range of several MHz to
hundreds of MHz for achievable densities. We have conducted measurements where
short bursts of RF were applied to these plasmas, with pulse durations as short
as two cycles. We still observed a density-dependent resonant response to these
short pulses. However, the too rapid timescale of the response, the dependence
of the response on the sign of the driving field, the response as the number of
pulses was increased, and the difference in plasma response to radial and
axially applied RF fields are inconsistent with the plasma response being due
to local resonant heating of electrons in the plasma. Instead, our results are
consistent with rapid energy transfer from collective motion of the entire
electron cloud to electrons in high-energy orbits. In addition to providing a
potentially more robust way to measure ultracold neutral plasma densities,
these measurements demonstrate the importance of collective motion in the
energy transport in these systems.Comment: 5 pages, 4 figure
A flowing plasma model to describe drift waves in a cylindrical helicon discharge
A two-fluid model developed originally to describe wave oscillations in the
vacuum arc centrifuge, a cylindrical, rapidly rotating, low temperature and
confined plasma column, is applied to interpret plasma oscillations in a RF
generated linear magnetised plasma (WOMBAT), with similar density and field
strength. Compared to typical centrifuge plasmas, WOMBAT plasmas have slower
normalised rotation frequency, lower temperature and lower axial velocity.
Despite these differences, the two-fluid model provides a consistent
description of the WOMBAT plasma configuration and yields qualitative agreement
between measured and predicted wave oscillation frequencies with axial field
strength. In addition, the radial profile of the density perturbation predicted
by this model is consistent with the data. Parameter scans show that the
dispersion curve is sensitive to the axial field strength and the electron
temperature, and the dependence of oscillation frequency with electron
temperature matches the experiment. These results consolidate earlier claims
that the density and floating potential oscillations are a resistive drift
mode, driven by the density gradient. To our knowledge, this is the first
detailed physics model of flowing plasmas in the diffusion region away from the
RF source. Possible extensions to the model, including temperature
non-uniformity and magnetic field oscillations, are also discussed
Impact of Dark Matter Microhalos on Signatures for Direct and Indirect Detection
Detecting dark matter as it streams through detectors on Earth relies on
knowledge of its phase space density on a scale comparable to the size of our
solar system. Numerical simulations predict that our Galactic halo contains an
enormous hierarchy of substructures, streams and caustics, the remnants of the
merging hierarchy that began with tiny Earth mass microhalos. If these bound or
coherent structures persist until the present time, they could dramatically
alter signatures for the detection of weakly interacting elementary particle
dark matter (WIMP). Using numerical simulations that follow the coarse grained
tidal disruption within the Galactic potential and fine grained heating from
stellar encounters, we find that microhalos, streams and caustics have a
negligible likelihood of impacting direct detection signatures implying that
dark matter constraints derived using simple smooth halo models are relatively
robust. We also find that many dense central cusps survive, yielding a small
enhancement in the signal for indirect detection experiments.Comment: 6 pages, revision in response to referees report. Now accepted by
Phys. Rev D., in pres
- …