4,093 research outputs found
Disorder effect in low dimensional superconductors
The quasiparticle density of states (DOS), the energy gap, the superfluid
density , and the localization effect in the s- and d-wave
superconductors with non-magnetic impurity in two dimensions (2D) are studied
numerically. For strong (unitary) scatters, we find that it is the range of the
scattering potential rather than the symmetry of the superconducting pairing
which is more important in explaining the impurity dependences of the specific
heat and the superconducting transition temperature in Zn doped YBCO. The
localization length is longer in the d-wave superconducting state than in the
normal state, even in the vicinity of the Fermi energy.Comment: 2 pages, uuencoded compressed postscript file, IRC-940610
GALEX ultraviolet observations of stellar variability in the Hyades and Pleiades clusters
We present GALEX near ultraviolet (NUV:1750 - 2750A) and far ultraviolet
(FUV: 1350 - 1750A) imaging observations of two 1.2 degree diameter fields in
the Hyades and Pleiades open clusters in order to detect possible UV
variability of the member stars. We have performed a detailed software search
for short-term UV flux variability during these observations of the approx 400
sources detected in each of the Hyades and Pleiades fields to identify
flare-like (dMe) stellar objects. This search resulted in the detection of 16
UV variable sources, of which 13 can be directly associated with probable
M-type stars. The other UV sources are G-type stars and one newly discovered RR
Lyrae star, USNOB1.0 1069-0046050, of period 0.624 day and distance 4.5-7.0
kpc. Light curves of photon flux versus time are shown for 7 flare events
recorded on six probable dMe stars. UV energies for these flares span the range
2E27 to 5E29 erg, with a corresponding NUV variability change of 1.82 mag. Only
one of these flare events (on the star Cl* Melotte 25 LH129) can definitely be
associated with an origin on a member the Hyades cluster itself. Finally, many
of our M-type candidates show long periods of enhanced UV activity but without
the associated rapid increase in flux that is normally associated with a flare
event. However, the total UV energy output during such periods of increased
activity is greater than that of many short-term UV flares. These intervals of
enhanced low-level UV activity concur with the idea that, even in quiescence,
the UV emission from dMe stars may be related to a superposition of many small
flare events possessing a wide range of energies.Comment: PASP Submitte
Calculating the inherent visual structure of a landscape (inherent viewshed) using high-throughput computing
This paper describes a method of calculating the inherent visibility at all locations in a landscape (‘total viewshed’) by making use of redundant computer cycles. This approach uses a simplified viewshed program that is suitable for use within a distributed environment, in this case managed by the Condor system. Distributing the calculation in this way reduced the calculation time of our example from an estimated 34 days to slightly over 25 hours using a cluster of 43 workstations. Finally, we discuss the example ‘total viewshed’ raster for the Avebury region, and briefly highlight some of its implications
Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman- line
The TRAPPIST-1 system offers the opportunity to characterize terrestrial,
potentially habitable planets orbiting a nearby ultracool dwarf star. We
performed a four-orbit reconnaissance with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope to study the stellar emission
at Lyman-, to assess the presence of hydrogen exospheres around the two
inner planets, and to determine their UV irradiation. We detect the
Lyman- line of TRAPPIST-1, making it the coldest exoplanet host star
for which this line has been measured. We reconstruct the intrinsic line
profile, showing that it lacks broad wings and is much fainter than expected
from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as
Proxima Cen but a much lower Ly- emission. This suggests that
TRAPPIST-1 chromosphere is only moderately active compared to its transition
region and corona. We estimated the atmospheric mass loss rates for all
planets, and found that despite a moderate extreme UV emission the total XUV
irradiation could be strong enough to strip the atmospheres of the inner
planets in a few billions years. We detect marginal flux decreases at the times
of TRAPPIST-1b and c transits, which might originate from stellar activity, but
could also hint at the presence of extended hydrogen exospheres. Understanding
the origin of these Lyman- variations will be crucial in assessing the
atmospheric stability and potential habitability of the TRAPPIST-1 planets.Comment: Published in A&A as a Letter to the Edito
Investigating the suitability of carbon nanotube reinforced polymer in transcatheter valve applications
The current delivery size of transcatheter aortic valves, limited by the thickness of their pericardial leaflets, correlates with a high prevalence of major vascular complications. Polyurethane valves can be developed to a fraction of the thickness of pericardial valves through the addition of carbon nanotubes to reinforce their leaflets. This study investigates the suitability of a novel carbon nanotube reinforced leaflet to reduce the delivery profile of transcatheter aortic valves. Carbon nanotube polyurethane composites were developed with thicknesses of 50 μm and their mechanical properties were determined in relation to various environmental effects. The composites demonstrated improvements to the material stiffness, particularly at increasing strain rates compared to the neat polymer. However, increasing nanotube concentrations significantly decreased the fatigue life of the composites. Key findings highlighted a potential for carbon nanotube reinforcement in valve replacement which experience very high strain rates during the cardiac cycle. Further testing is needed to achieve a strong nanotube-matrix interface which will prolong the cyclic fatigue life and further strengthen tensile properties. Testing on the durability and haemocompatibility of these composite heart valves are ongoing
Physical properties and radius variations in the HAT-P-5 planetary system from simultaneous four-colour photometry
The radii of giant planets, as measured from transit observations, may vary
with wavelength due to Rayleigh scattering or variations in opacity. Such an
effect is predicted to be large enough to detect using ground-based
observations at multiple wavelengths. We present defocussed photometry of a
transit in the HAT-P-5 system, obtained simultaneously through Stromgren u,
Gunn g and r, and Johnson I filters. Two more transit events were observed
through a Gunn r filter. We detect a substantially larger planetary radius in
u, but the effect is greater than predicted using theoretical model atmospheres
of gaseous planets. This phenomenon is most likely to be due to systematic
errors present in the u-band photometry, stemming from variations in the
transparency of Earth's atmosphere at these short wavelengths. We use our data
to calculate an improved orbital ephemeris and to refine the measured physical
properties of the system. The planet HAT-P-5b has a mass of 1.06 +/- 0.11 +/-
0.01 Mjup and a radius of 1.252 +/- 0.042 +/- 0.008 Rjup (statistical and
systematic errors respectively), making it slightly larger than expected
according to standard models of coreless gas-giant planets. Its equilibrium
temperature of 1517 +/- 29 K is within 60K of that of the extensively-studied
planet HD 209458b.Comment: Version 2 corrects the accidental omission of one author in the arXiv
metadata. Accepted for publication in MNRAS. 9 pages, 4 figures, 7 tables.
The properties of HAT-P-5 have been added to the Transiting Extrasolar Planet
Catalogue at http://www.astro.keele.ac.uk/~jkt/tepcat
- …