26,423 research outputs found
Feasibility study of an Integrated Program for Aerospace vehicle Design (IPAD). Volume 2: The design process
The extent to which IPAD is to support the design process is identified. Case studies of representative aerospace products were developed as models to characterize the design process and to provide design requirements for the IPAD computing system
G28.17+0.05: An unusual giant HI cloud in the inner Galaxy
New 21 cm HI observations have revealed a giant HI cloud in the Galactic
plane that has unusual properties. It is quite well defined, about 150 pc in
diameter at a distance of 5 kpc, and contains as much as 100,000 Solar Masses
of atomic hydrogen. The outer parts of the cloud appear in HI emission above
the HI background, while the central regions show HI self-absorption. Models
which reproduce the observations have a core with a temperature <40 K and an
outer envelope as much as an order of magnitude hotter. The cold core is
elongated along the Galactic plane, whereas the overall outline of the cloud is
approximately spherical. The warm and cold parts of the HI cloud have a
similar, and relatively large, line width of approximately 7 km/s. The cloud
core is a source of weak, anomalously-excited 1720 MHz OH emission, also with a
relatively large line width, which delineates the region of HI self-absorption
but is slightly blue-shifted in velocity. The intensity of the 1720 MHz OH
emission is correlated with N(H) derived from models of the cold core. There is
12CO emission associated with the cloud core. Most of the cloud mass is in
molecules, and the total mass is > 200,000 Solar Masses. In the cold core the
HI mass fraction may be 10 percent. The cloud has only a few sites of current
star formation. There may be about 100 more objects like this in the inner
Galaxy; every line of sight through the Galactic plane within 50 degrees of the
Galactic center probably intersects at least one. We suggest that G28.17+0.05
is a cloud being observed as it enters a spiral arm and that it is in the
transition from the atomic to the molecular state.Comment: 35 pages, inludes 12 figure
Curvature Dependence of Peaks in the Cosmic Microwave Background Distribution
The widely cited formula for the multipole
number of the first Doppler peak is not even a crude approximation in the case
of greatest current interest, in which the cosmic mass density is less than the
vacuum energy density. For instance, with fixed at 0.3, the position
of any Doppler peak varies as near .Comment: 7 pages, Late
Superfluid Density in a Highly Underdoped YBCO Superconductor
The superfluid density rho_s(T) = 1/lambda^2(T) has been measured at 2.64 GHz
in highly underdoped YBCO, at 37 dopings with T_c between 3 K and 17 K. Within
limits set by the transition width Delta T_c ~ 0.4 K, rho_s(T) shows no
evidence of critical fluctuations as T goes to T_c, with a mean-field-like
transition and no indication of vortex unbinding. Instead, we propose that
rho_s displays the behaviour expected for a quantum phase transition in the (3
+ 1)-dimensional XY universality class, with rho_s0 ~ (p - p_c), T_c ~ (p -
p_c)^1/2 and rho_s(T) ~ (T_c - T)^1 as T goes to T_c.Comment: 4 pages, 5 figures; final version of pape
Hawking Radiation via Tunneling from Hot NUT-Kerr-Newman-Kasuya Spacetime
We study the Hawking thermal spectrum in dragging coordinate system and the
tunneling radiation characteristics of hot NUT-Kerr-Newman-Kasuya spacetime.
The tunneling rates at the event and cosmological horizon are found to be
related to the change of Bekenstein-Hawking entropy. The radiation spectrum is
not pure thermal and thus there is a correction to the Hawking thermal
spectrum.Comment: To appear in Class. Quant. Gra
Dynamics of a Quantum Reference Frame
We analyze a quantum mechanical gyroscope which is modeled as a large spin
and used as a reference against which to measure the angular momenta of
spin-1/2 particles. These measurements induce a back-action on the reference
which is the central focus of our study. We begin by deriving explicit
expressions for the quantum channel representing the back-action. Then, we
analyze the dynamics incurred by the reference when it is used to sequentially
measure particles drawn from a fixed ensemble. We prove that the reference
thermalizes with the measured particles and find that generically, the thermal
state is reached in time which scales linearly with the size of the reference.
This contrasts a recent conclusion of Bartlett et al. that this takes a
quadratic amount of time when the particles are completely unpolarized. We now
understand their result in terms of a simple physical principle based on
symmetries and conservation laws. Finally, we initiate the study of the
non-equilibrium dynamics of the reference. Here we find that a reference in a
coherent state will essentially remain in one when measuring polarized
particles, while rotating itself to ultimately align with the polarization of
the particles
Autocatalytic plume pinch-off
A localized source of buoyancy flux in a non-reactive fluid medium creates a
plume. The flux can be provided by either heat, a compositional difference
between the fluid comprising the plume and its surroundings, or a combination
of both. For autocatalytic plumes produced by the iodate-arsenous acid
reaction, however, buoyancy is produced along the entire reacting interface
between the plume and its surroundings. Buoyancy production at the moving
interface drives fluid motion, which in turn generates flow that advects the
reaction front. As a consequence of this interplay between fluid flow and
chemical reaction, autocatalytic plumes exhibit a rich dynamics during their
ascent through the reactant medium. One of the more interesting dynamical
features is the production of an accelerating vortical plume head that in
certain cases pinches-off and detaches from the upwelling conduit. After
pinch-off, a new plume head forms in the conduit below, and this can lead to
multiple generations of plume heads for a single plume initiation. We
investigated the pinch-off process using both experimentation and simulation.
Experiments were performed using various concentrations of glycerol, in which
it was found that repeated pinch-off occurs exclusively in a specific
concentration range. Autocatalytic plume simulations revealed that pinch-off is
triggered by the appearance of accelerating flow in the plume conduit.Comment: 10 figures. Accepted for publication in Phys Rev E. See also
http://www.physics.utoronto.ca/nonlinear/papers_chemwave.htm
Supernovae as a probe of particle physics and cosmology
It has very recently been demonstrated by Csaki, Kaloper and Terning (CKT)
that the faintness of supernovae at high redshift can be accommodated by mixing
of a light axion with the photon in the presence of an intergalactic magnetic
field, as opposed to the usual explanation of an accelerating universe by a
dark energy component. In this paper we analyze further aspects of the CKT
mechanism and its generalizations. The CKT mechanism also passes various
cosmological constraints from the fluctuations of the CMB and the formation of
structure at large scales, without requiring an accelerating phase in the
expansion of the Universe. We investigate the statistical significance of
current supernova data for pinning down the different components of the
cosmological energy-momentum tensor and for probing physics beyond the standard
models.Comment: 17 pages, LaTeX, 4 figures; v2: typos corrected, minor changes,
references added; v3: updated figures, details regarding fits include
A Fresh Look at Axions and SN 1987A
We re-examine the very stringent limits on the axion mass based on the
strength and duration of the neutrino signal from SN 1987A, in the light of new
measurements of the axial-vector coupling strength of nucleons, possible
suppression of axion emission due to many-body effects, and additional emission
processes involving pions. The suppression of axion emission due to nucleon
spin fluctuations induced by many-body effects degrades previous limits by a
factor of about 2. Emission processes involving thermal pions can strengthen
the limits by a factor of 3-4 within a perturbative treatment that neglects
saturation of nucleon spin fluctuations. Inclusion of saturation effects,
however, tends to make the limits less dependent on pion abundances. The
resulting axion mass limit also depends on the precise couplings of the axion
and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.Comment: 32 latex pages, 13 postscript figures included, uses revtex.sty,
submitted to Physical Review
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