6,897 research outputs found
The Effect of Shear Flow on the Helfrich Interaction in Lyotropic Lamellar Systems
We study the effect of shear flow on the entropic Helfrich interaction in
lyotropic surfactant smectic fluids. Arguing that flow induces an effective
anisotropic surface tension in bilayers due to a combination of intermonolayer
friction, bilayer collisions and convection, we calculate the reduction in
fluctuations and hence the renormalised change in effective compression modulus
and steady-state layer spacing. We demonstrate that non-permeable or slowly
permeating membranes can be susceptible to a undulatory instability of the
Helfrich-Hurault type, and speculate that such an instability could be one
source of a transition to multilamellar vesicles.Comment: 14 pages, to appear in Eur Phys J
NICMOS and VLBA observations of the gravitational lens system B1933+503
NICMOS observations of the complex gravitational lens system B1933+503 reveal
infrared counterparts to two of the inverted spectrum radio images. The
infrared images have arc-like structures. The corresponding radio images are
also detected in a VLBA map made at 1.7 GHz with a resolution of 6 mas. We fail
to detect two of the four inverted radio spectrum components with the VLBA even
though they are clearly visible in a MERLIN map at the same frequency at a
different epoch. The absence of these two components could be due to rapid
variability on a time-scale less than the time delay, or to broadening of the
images during propagation of the radio waves through the ISM of the lensing
galaxy to an extent that they fall below the surface brightness detectability
threshold of the VLBA observations. The failure to detect the same two images
with NICMOS is probably due to extinction in the ISM of the lensing galaxy.Comment: 5 pages, 4 figures, submitted to MNRA
Gravitational lensing statistics with extragalactic surveys. II. Analysis of the Jodrell Bank-VLA Astrometric Survey
We present constraints on the cosmological constant from
gravitational lensing statistics of the Jodrell Bank-VLA Astrometric Survey
(JVAS). Although this is the largest gravitational lens survey which has been
analysed, cosmological constraints are only comparable to those from optical
surveys. This is due to the fact that the median source redshifts of JVAS are
lower, which leads to both relatively fewer lenses in the survey and a weaker
dependence on the cosmological parameters. Although more approximations have to
be made than is the case for optical surveys, the consistency of the results
with those from optical gravitational lens surveys and other cosmological tests
indicate that this is not a major source of uncertainty in the results.
However, joint constraints from a combination of radio and optical data are
much tighter. Thus, a similar analysis of the much larger Cosmic Lens All-Sky
Survey should provide even tighter constraints on the cosmological constant,
especially when combined with data from optical lens surveys.
At 95% confidence, our lower and upper limits on ,
using the JVAS lensing statistics information alone, are respectively -2.69 and
0.68. For a flat universe, these correspond to lower and upper limits on
\lambda_{0} of respectively -0.85 and 0.84. Using the combination of JVAS
lensing statistics and lensing statistics from the literature as discussed in
Quast & Helbig (Paper I) the corresponding values are
-1.78 and 0.27. For a flat universe, these correspond to lower and upper limits
on of respectively -0.39 and 0.64.Comment: LaTeX, 9 pages, 18 PostScript files in 6 figures. Paper version
available on request. Data available from
http://gladia.astro.rug.nl:8000/ceres/data_from_papers/papers.htm
Lensing galaxies: light or dark?
In a recent paper, Hawkins (1997) argues on the basis of statistical studies
of double-image gravitational lenses and lens candidates that a large
population of dark lenses exists and that these outnumber galaxies with more
normal mass-to-light ratios by a factor of 3:1. If correct, this is a very
important result for many areas of astronomy including galaxy formation and
cosmology. In this paper we discuss our new radio-selected gravitational lens
sample, JVAS/CLASS, in order to test and constrain this proposition. We have
obtained ground-based and HST images of all multiple-image lens systems in our
sample and in 12 cases out of 12 we find the lensing galaxies in the optical
and/or near infrared. Our success in finding lensing galaxies creates problems
for the dark lens hypothesis. If it is to survive, ad hoc modifications seem to
be necessary: only very massive galaxies (more than about one trillion solar
masses) can be dark, and the cutoff in mass must be sharp. Our finding of lens
galaxies in all the JVAS/CLASS systems is complementary evidence which supports
the conclusion of Kochanek et al. (1997) that many of the wide-separation
optically-selected pairs are physically distinct quasars rather than
gravitational lens systems.Comment: 4 pages, 2 included figures, accepted for publication in Astronomy
and Astrophysics. Paper version available on request. This replacement amends
the text to allow more discussion of the overlap with astro-ph/971016
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