6,378 research outputs found
Tracer sensitive tapes
A leak detection system has been developed, consisting of a tape that can be wrapped around possible leak sites on a system pressurized with air or gaseous nitrogen. Carbon monoxide, at a level of 100 to 1000 parts per million is used as a trace gas in the pressurized system. The sensitive element of the tape is palladium chloride supported on specially prepared silica gel and specially dried. At a CO level of 100 ppm and a leak rate of 10-20 ml/hr, discoloration of the sensitive element is observed in 1.5 to 3 min. The tape and trace gas are compatible with aerospace hardware, safe to handle, and economically reasonable to produce and handle
QCD: Challenges for the Future
Despite many experimental verifications of the correctness of our basic
understanding of QCD, there remain numerous open questions in strong
interaction physics and we focus on the role of future colliders in addressing
these questions. We discuss possible advances in the measurement of ,
in the study of parton distribution functions, and in the understanding of low
physics at present colliders and potential new facilities. We also touch
briefly on the role of spin physics in advancing our understanding of QCD.Comment: 12 pages, LATEX2e with snow2e, epsfig and 2 figures. Also available
at http://penguin.phy.bnl.gov/~dawson/qcdsnow.ps . QCD working group summary
at DPF/DPB Summer Study on New Directions for High Energy Physics, Snowmass,
CO, June 25- July 12, 199
Epitaxial growth and surface reconstruction of CrSb(0001)
Smooth CrSb(0001) films have been grown by molecular beam epitaxy on MnSb(0001) – GaAs(111) substrates. CrSb(0001) shows (2 × 2), triple domain (1 × 4) and (√3×√3)R30° reconstructed surfaces as well as a (1 × 1) phase. The dependence of reconstruction on substrate temperature and incident fluxes is very similar to MnSb(0001)
Possible Solutions to the Radius Anomalies of Transiting Giant Planets
We calculate the theoretical evolution of the radii of all fourteen of the
known transiting extrasolar giant planets (EGPs) for a variety of assumptions
concerning atmospheric opacity, dense inner core masses, and possible internal
power sources. We incorporate the effects of stellar irradiation and customize
such effects for each EGP and star. Looking collectively at the family as a
whole, we find that there are in fact two radius anomalies to be explained. Not
only are the radii of a subset of the known transiting EGPs larger than
expected from previous theory, but many of the other objects are smaller than
the default theory would allow. We suggest that the larger EGPs can be
explained by invoking enhanced atmospheric opacities that naturally retain
internal heat. This explanation might obviate the necessity for an extra
internal power source. We explain the smaller radii by the presence in perhaps
all the known transiting EGPs of dense cores, such as have been inferred for
Saturn and Jupiter. Importantly, we derive a rough correlation between the
masses of our "best-fit" cores and the stellar metallicity that seems to
buttress the core-accretion model of their formation. Though many caveats and
uncertainties remain, the resulting comprehensive theory that incorporates
enhanced-opacity atmospheres and dense cores is in reasonable accord with all
the current structural data for the known transiting giant planets.Comment: 22 pages in emulateapj format, including 10 figures (mostly in
color), accepted to the Astrophysical Journal (February 9, 2007); to appear
in volume 661, June 200
A Theory for the Radius of the Transiting Giant Planet HD 209458b
Using a full frequency-dependent atmosphere code that can incorporate
irradiation by a central primary star, we calculate self-consistent boundary
conditions for the evolution of the radius of the transiting planet HD 209458b.
Using a well-tested extrasolar giant planet evolutionary code, we then
calculate the behavior of this planet's radius with age. The measured radius is
in fact a transit radius that resides high in HD 209458b's inflated atmosphere.
Using our derived atmospheric and interior structures, we find that irradiation
plus the proper interpretation of the transit radius can yield a theoretical
radius that is within the measured error bars. We conclude that if HD 209458b's
true transit radius is at the lower end of the measured range, an extra source
of core heating power is not necessary to explain the transit observations.Comment: 6 pages in emulateapj format, plus 2 figures (one color), accepted to
the Astrophysical Journa
Dimensional Dependence of the Hydrodynamics of Core-Collapse Supernovae
The multidimensional character of the hydrodynamics in core-collapse
supernova (CCSN) cores is a key facilitator of explosions. Unfortunately, much
of this work has necessarily been performed assuming axisymmetry and it remains
unclear whether or not this compromises those results. In this work, we present
analyses of simplified two- and three-dimensional CCSN models with the goal of
comparing the multidimensional hydrodynamics in setups that differ only in
dimension. Not surprisingly, we find many differences between 2D and 3D models.
While some differences are subtle and perhaps not crucial to understanding the
explosion mechanism, others are quite dramatic and make interpreting 2D CCSN
models problematic. In particular, we find that imposing axisymmetry
artificially produces excess power at the largest spatial scales, power that
has been deemed critical in the success of previous explosion models and has
been attributed solely to the standing accretion shock instability.
Nevertheless, our 3D models, which have an order of magnitude less power on
large scales compared to 2D models, explode earlier. Since we see explosions
earlier in 3D than in 2D, the vigorous sloshing associated with the large scale
power in 2D models is either not critical in any dimension or the explosion
mechanism operates differently in 2D and 3D. Possibly related to the earlier
explosions in 3D, we find that about 25% of the accreted material spends more
time in the gain region in 3D than in 2D, being exposed to more integrated
heating and reaching higher peak entropies, an effect we associate with the
differing characters of turbulence in 2D and 3D. Finally, we discuss a simple
model for the runaway growth of buoyant bubbles that is able to quantitatively
account for the growth of the shock radius and predicts a critical luminosity
relation.Comment: Submitted to the Astrophysical Journa
Testing the models: NIR imaging and spectroscopy of the benchmark T-dwarf binary Eps Indi B
The relative roles of metallicity and surface gravity on the near-infrared
spectra of late-T brown dwarfs are not yet fully understood, and evolutionary
models still need to be calibrated in order to provide accurate estimates of
brown dwarf physical parameters from measured spectra. The T-type brown dwarfs
Eps Indi Ba and Bb forming the tightly bound binary Eps Indi B, which orbits
the K4V star Eps Indi A, are nowadays the only such benchmark T dwarfs for
which all important physical parameters such as metallicity, age and mass are
(or soon will be) known. We present spatially resolved VLT/NACO images and low
resolution spectra of Eps Indi B in the J, H and K near-infrared bands. The
spectral types of Eps Indi Ba and Bb are determined by direct comparison of the
flux-calibrated JHK spectra with T dwarf standard template spectra and also by
NIR spectral indices. Eps Indi Bb is confirmed as a T6 while the spectral type
of Eps Indi Ba is T1.5 so somewhat later than the previously reported T1.
Constrained values for surface gravity and effective temperature are derived by
comparison with model spectra. The evolutionary models predict masses around
about 53 M_J for Eps Indi Ba and about 34 M_J for Eps Indi Bb, slightly higher
than previously reported values. The suppressed J-band and enhanced K-band flux
of Eps Indi Ba indicates that a noticeable cloud layer is still present in a
T1.5 dwarf while no clouds are needed to model the spectrum of Eps Indi Bb.Comment: 7 pages, 5 figures, accepted by Ap
Spatial distribution and broad-band spectral characteristics of the diffuse X-ray background, 0.1 - 1.0 keV
Preliminary maps covering more than 85 percent of the sky are presented for three energy bands: the B band, the C band, and the M band. The study was undertaken to find evidence that most of the diffuse X-ray background at energies less than 1 keV is local to the galaxy and that it is most probably due to thermal radiation from a low density plasma which fills a substantial fraction of interstellar space. A preliminary analysis of the data is provided including a report that most of the B and C band flux has a common origin, probably in a 10 to the 6th power K region surrounding the Sun, and that most of the M band flux does not originate from the same material
Limits on soft X-ray flux from distant emission regions
The all-sky soft X-ray data of McCammon et al. and the new N sub H survey (Stark et al. was used to place limits on the amount of the soft X-ray diffuse background that can originate beyond the neutral gas of the galactic disk. The X-ray data for two regions of the sky near the galactic poles are shown to be uncorrelated with 21 cm column densities. Most of the observed x-ray flux must therefore originate on the near side of the most distant neutral gas. The results from these regions are consistent with X-ray emission from a locally isotropic, unabsorbed source, but require large variations in the emission of the local region over large angular scales
The soft X-ray diffuse background
Maps of the diffuse X-ray background intensity covering essentially the entire sky with approx. 7 deg spatial resolution are presented for seven energy bands. The data were obtained on a series of ten sounding rocket flights conducted over a seven-year period. The different nature of the spatial distributions in different bands implies at least three distinct origins for the diffuse X-rays, none of which is well-understood. At energies or approx. 2000 eV, an isotropic and presumably extraglalactic 500 and 1000 eV, an origin which is at least partially galactic seems called for. At energies 284 eV, the observed intensity is anticorrelated with neutral hydrogen column density, but we find it unlikely that this anticorrelation is simply due to absorption of an extragalactic or halo source
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