1,063 research outputs found
Sediment transport and erosion in the Fourchon area of Lafourche parish
NASA aerial photography in the form of color infrared and color positive transparencies is used as an aid in evaluating the rate and effect of erosion and sediment transport in Bay Champagne Louisiana
Building Collaborative Reference and Instructional Services for Distance Education Students
To meet the needs of rigorous educational programs, academic libraries must make a commitment to serve growing distance education (DE) populations. Students who participate in distance education are typically older, nontraditional students with unique characteristics who need special services provided by their university library in order to obtain an education equal to their on-campus counterparts. Creating a successful reference and instructional service for distance education students not only takes planning, collaboration, and assessment, but also requires knowing your audience, constantly experimenting with new technologies, expecting the unexpected, and being ready to problem-solve at a momentâs notice. In this article, the authors will discuss the process of creating reference and instructional services for distance students and faculty: challenges to expect, ways to address these challenges, and the constant need to plan for the future
Smoothed Particle Hydrodynamics Simulations of Apsidal and Nodal Superhumps
In recent years a handful of systems have been observed to show "negative"
(nodal) superhumps, with periods slightly shorter than the orbital period. It
has been suggested that these modes are a consequence of the slow retrograde
precession of the line of nodes in a disk tilted with respect to the orbital
plane. Our simulations confirm and refine this model: they suggest a roughly
axisymmetric, retrogradely-precessing, tilted disk that is driven at a period
slightly less than half the orbital period as the tidal field of the orbiting
secondary encounters in turn the two halves of the disk above and below the
midplane. Each of these passings leads to viscous dissipation on one face of an
optically-thick disk -- observers on opposite sides of the disk would each
observe one brightening per orbit, but 180 degrees out of phase with each
other.Comment: 11 pages. Accepted for publication in The ApJ Letter
Hydrodynamics of the stream-disk impact in interacting binaries
We use hydrodynamic simulations to provide quantitative estimates of the
effects of the impact of the accretion stream on disks in interacting binaries.
For low accretion rates, efficient radiative cooling of the hotspot region can
occur, and the primary consequence of the stream impact is stream overflow
toward smaller disk radii. The stream is well described by a ballistic
trajectory, but larger masses of gas are swept up and overflow at smaller, but
still highly supersonic, velocities. If cooling is inefficient, overflow still
occurs, but there is no coherent stream inward of the disk rim. Qualitatively,
the resulting structure appears as a bulge extending downstream along the disk
rim. We calculate the mass fraction and velocity of the overflowing component
as a function of the important system parameters, and discuss the implications
of the results for X-ray observations and doppler tomography of cataclysmic
variables, low-mass X-ray binaries and supersoft X-ray sources.Comment: 16 pages, including 8 figures. 1 color figure as a jpeg. ApJ, in
pres
Periodic Variability During the X-ray Decline of 4U 1636-53
We report the onset of a large amplitude, statistically significant
periodicity (~46 d) in the RXTE/ASM data of the prototype X-ray burster 4U
1636-53, the X-ray flux of which has been gradually declining over the last
four years. This behaviour is remarkably similar to that observed in the
neutron star LMXB KS 1731-260, which is a long-term transient. We also report
on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and
find that the hard X-ray flux (20-100 keV) indicates an apparent
anti-correlation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is
transiting from activity to quiescence, as occurred in KS 1731-260. We also
suggest that the variability during the X-ray decline is the result of an
accretion rate variability related to the X-ray irradiation of the disc.Comment: 6 pages. Accepted by MNRA
MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence
We report on the analysis of new and previously published MMT optical spectra
of the black hole binary XTE J1118+480 during the decline from the 2000
outburst to true quiescence. From cross-correlation with template stars, we
measure the radial velocity of the secondary to derive a new spectroscopic
ephemeris. The observations acquired during approach to quiescence confirm the
earlier reported modulation in the centroid of the double-peaked Halpha
emission line. Additionally, our data combined with the results presented by
Zurita et al. (2002) provide support for a modulation with a periodicity in
agreement with the expected precession period of the accretion disk of ~52 day.
Doppler images during the decline phase of the Halpha emission line show
evidence for a hotspot and emission from the gas stream: the hotspot is
observed to vary its position, which may be due to the precession of the disk.
The data available during quiescence show that the centroid of the Halpha
emission line is offset by about -100 km/s from the systemic velocity which
suggests that the disk continues to precess. A Halpha tomogram reveals emission
from near the donor star after subtraction of the ring-like contribution from
the accretion disk which we attribute to chromospheric emission. No hotspot is
present suggesting that accretion from the secondary has stopped (or decreased
significantly) during quiescence. Finally, a comparison is made with the black
hole XRN GRO J0422+32: we show that the Halpha profile of this system also
exhibits a behaviour consistent with a precessing disk.Comment: 11 pages, 5 figures, accepted by Ap
The anomalous accretion disk of the Cataclysmic Variable RW Sextantis
Synthetic spectra covering the wavelength range 900\AA~to 3000\AA~provide an
accurate fit, established by a analysis, to a combined
observed spectrum of RW Sextantis. Two separately calibrated distances to the
system establish the synthetic spectrum comparison on an absolute flux basis
but with two alternative scaling factors, requiring alternative values of
for final models. Based on comparisons for a range of
values, the observed spectrum does not follow the standard model. Rather than
the exponent 0.25 in the expression for the radial temperature profile, a value
close to 0.125 produces a synthetic spectrum with an accurate fit to the
combined spectrum. A study of time-series spectra shows that a proposed
warped or tilted disk is not supported by the data; an alternative proposal is
that an observed non-axisymmetric wind results from an interaction with the
mass transfer stream debris.Comment: 56 pages, 15 figures, 11 tables. Accepted for The Astrophysical
Journa
Levels of evidence in plastic surgery â Trends and comparison with five other surgical specialties
Background: Categorising research by level of evidence (LOE) is an important evidence-based medicine initiative. Our objective was to assess the change in LOEs in plastic surgery from 2003 to 2013 and compared this with five other surgical specialties. Methods: A search for all research articles published in the top three general plastic surgery journals (ranked by impact factor) was conducted for 2003 and 2013. Articles were then labelled as LOE 1â5 and compared to other specialties. Results: Mean LOE for plastic surgery improved by 4.1ĂÂ % from 3.86 (95ĂÂ % confidence interval 3.81â3.91) to 3.70 (95ĂÂ % confidence interval 3.64â3.74) from 2003 to 2013 respectively. All six surgical specialties improved their mean LOE (range 3.7 to 10.9ĂÂ %). By mean LOE, plastic surgery continues to rank five out of six. Conclusions: Plastic surgery is tending towards higher levels of evidence at a slow pace. The specialty must continue to drive towards higher levels of evidence. Level of Evidence: Not ratable
Detection of orbital and superhump periods in Nova V2574 Ophiuchi (2004)
We present the results of 37 nights of CCD unfiltered photometry of nova
V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 d (~3.40 h)
and 0.14773 d (~3.55 h) in the 2005 data. The 2004 data show variability on a
similar timescale, but no coherent periodicity was found. We suggest that the
longer periodicity is the orbital period of the underlying binary system and
that the shorter period represents a negative superhump. The 3.40 h period is
about 4% shorter than the orbital period and obeys the relation between
superhump period deficit and binary period. The detection of superhumps in the
light curve is evidence of the presence of a precessing accretion disk in this
binary system shortly after the nova outburst. From the maximum magnitude -
rate of decline relation, we estimate the decay rate t_2 = 17+/-4 d and a
maximum absolute visual magnitude of M_Vmax = -7.7+/-1.7 mag.Comment: 6 pages, 6 figures, 2 .sty files, AJ accepted, minor change to one of
reference
Using binary stars to bound the mass of the graviton
Interacting white dwarf binary star systems, including helium cataclysmic
variable (HeCV) systems, are expected to be strong sources of gravitational
radiation, and should be detectable by proposed space-based laser
interferometer gravitational wave observatories such as LISA. Several HeCV star
systems are presently known and can be studied optically, which will allow
electromagnetic and gravitational wave observations to be correlated.
Comparisons of the phases of a gravitational wave signal and the orbital light
curve from an interacting binary white dwarf star system can be used to bound
the mass of the graviton. Observations of typical HeCV systems by LISA could
potentially yield an upper bound on the inverse mass of the graviton as strong
as km (
eV), more than two orders of magnitude better than present solar system derived
bounds.Comment: 21 pages plus 4 figures; ReVTe
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