117,638 research outputs found
H2 formation via the UV photo-processing of a-C:H nano-particles
Context. The photolysis of hydrogenated amorphous carbon, a-C(:H), dust by UV
photon-irradiation in the laboratory leads to the release of H2 as well as
other molecules and radicals. This same process is also likely to be important
in the interstellar medium. Aims. To investigate molecule formation arising
from the photo-dissociatively-driven, regenerative processing of a-C(:H) dust.
Methods. We explore the mechanism of a-C(:H) grain photolysis leading to the
formation of H2 and other molecules/radicals. Results. The rate constant for
the photon-driven formation of H2 from a-C(:H) grains is estimated to be
2x10^-17 cm^3 s^-1. In intense radiation fields photon-driven grain
decomposition will lead to fragmentation into daughter species rather than H2
formation. Conclusions. The cyclic re-structuring of arophatic a-C(:H)
nano-particles appears to be a viable route to formation of H2 for low to
moderate radiation field intensities (1 < G_0 < 10^2), even when the dust is
warm (T ~ 50 - 100 K).Comment: 7 pages, 2 figures, accepted for publication in A&
Random Curves by Conformal Welding
We construct a conformally invariant random family of closed curves in the
plane by welding of random homeomorphisms of the unit circle given in terms of
the exponential of Gaussian Free Field. We conjecture that our curves are
locally related to SLE for .Comment: 5 page
Bulk Viscosity in Neutron Stars from Hyperons
The contribution from hyperons to the bulk viscosity of neutron star matter
is calculated. Compared to previous works we use for the weak interaction the
one-pion exchange model rather than a current-current interaction, and include
the neutral current process. Also the sensitivity
to details of the equation of state is examined. Compared to previous works we
find that the contribution from hyperons to the bulk viscosity is about two
orders of magnitude smaller.Comment: 18 pages, to appear in Physical Review
Modulation of endoglin expression in islets of langerhans by VEGF reveals a novel regulator of islet endothelial cell function
BACKGROUND: Endoglin/CD105 is an auxiliary receptor for transforming growth factor-β with established roles in vascular remodelling. It has recently been shown that heterozygous endoglin deficiency in mice decreases insulin secretion in an animal model of obesity, highlighting a potential role for endoglin in the regulation of islet function. We have previously identified two different populations of endoglin expressing cells in human and mouse islets which are: (i) endothelial cells (ECs) and (ii) islet mesenchymal stromal cells. The contribution of islet EC endoglin expression to islet development and sensitivity to VEGF is unknown and is the focus of this study. RESULTS: In vitro culture of mouse islets with VEGF164 for 48 h increased endoglin mRNA levels above untreated controls but VEGF did not modulate VEGFR2, CD31 or CD34 mRNA expression or islet viability. Removal of EC-endoglin expression in vivo reduced islet EC area but had no apparent effect on islet size or architecture. CONCLUSION: EC-specific endoglin expression in islets is sensitive to VEGF and plays partial roles in driving islet vascular development, however such regulation appears to be distinct to mechanisms required to modulate islet viability and size
Theoretical confirmation of Feynman's hypothesis on the creation of circular vortices in Bose-Einstein condensates: III
In two preceding papers (Infeld and Senatorski 2003 J. Phys.: Condens. Matter
15 5865, and Senatorski and Infeld 2004 J. Phys.: Condens. Matter 16 6589) the
authors confirmed Feynman's hypothesis on how circular vortices can be created
from oppositely polarized pairs of linear vortices (first paper), and then gave
examples of the creation of several different circular vortices from one linear
pair (second paper). Here in part III, we give two classes of examples of how
the vortices can interact. The first confirms the intuition that the
reconnection processes which join two interacting vortex lines into one,
practically do not occur. The second shows that new circular vortices can also
be created from pairs of oppositely polarized coaxial circular vortices. This
seems to contradict the results for such pairs given in Koplik and Levine 1996
Phys. Rev. Lett. 76 4745.Comment: 10 pages, 7 figure
Opening angles, Lorentz factors and confinement of X-ray binary jets
We present a collation of the available data on the opening angles of jets in
X-ray binaries, which in most cases are small (less than 10 degrees). Under the
assumption of no confinement, we calculate the Lorentz factors required to
produce such small opening angles via the transverse relativistic Doppler
effect. The derived Lorentz factors, which are in most cases lower limits, are
found to be large, with a mean greater than 10, comparable to those estimated
for AGN and much higher than the commonly-assumed values for X-ray binaries of
2 to 5. Jet power constraints do not in most cases rule out such high Lorentz
factors. The upper limits on the opening angles show no evidence for smaller
Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those
sources in which deceleration has been observed (notably XTE J1550-564 and
Cygnus X-3), some confinement of the jets must be occurring, and we briefly
discuss possible confinement mechanisms. It is however possible that all the
jets could be confined, in which case the requirement for high bulk Lorentz
factors can be relaxed.Comment: 11 pages, 4 figures (2 colour), accepted for publication in MNRA
Vortex Rings and Lieb Modes in a Cylindrical Bose-Einstein Condensate
We present a calculation of a solitary wave propagating along a cylindrical
Bose-Einstein trap, which is found to be a hybrid of a one-dimensional (1D)
soliton and a three-dimensional (3D) vortex ring. The calculated
energy-momentum dispersion exhibits characteristics similar to those of a mode
proposed sometime ago by Lieb within a 1D model, as well as some rotonlike
features.Comment: 4 pages, 4 figure
Nematic cells with defect-patterned alignment layers
Using Monte Carlo simulations of the Lebwohl--Lasher model we study the
director ordering in a nematic cell where the top and bottom surfaces are
patterned with a lattice of point topological defects of lattice
spacing . We find that the nematic order depends crucially on the ratio of
the height of the cell to . When the system is very
well--ordered and the frustration induced by the lattice of defects is relieved
by a network of half--integer defect lines which emerge from the point defects
and hug the top and bottom surfaces of the cell. When the
system is disordered and the half--integer defect lines thread through the cell
joining point defects on the top and bottom surfaces. We present a simple
physical argument in terms of the length of the defect lines to explain these
results. To facilitate eventual comparison with experimental systems we also
simulate optical textures and study the switching behavior in the presence of
an electric field
Single stage experimental evaluation of variable geometry guide vanes and stator blading. Part 5 - Overall performance for variable camber guide vane and stator B with radial and circumferential inlet flow distortion
Inlet flow distortion effects on single stage variable geometry guide vanes and stator
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