3,786 research outputs found
Viability of Noether symmetry of F(R) theory of gravity
Canonization of F(R) theory of gravity to explore Noether symmetry is
performed treating R - 6(\frac{\ddot a}{a} + \frac{\dot a^2}{a^2} +
\frac{k}{a^2}) = 0 as a constraint of the theory in Robertson-Walker
space-time, which implies that R is taken as an auxiliary variable. Although it
yields correct field equations, Noether symmetry does not allow linear term in
the action, and as such does not produce a viable cosmological model. Here, we
show that this technique of exploring Noether symmetry does not allow even a
non-linear form of F(R), if the configuration space is enlarged by including a
scalar field in addition, or taking anisotropic models into account.
Surprisingly enough, it does not reproduce the symmetry that already exists in
the literature (A. K. Sanyal, B. Modak, C. Rubano and E. Piedipalumbo,
Gen.Relativ.Grav.37, 407 (2005), arXiv:astro-ph/0310610) for scalar tensor
theory of gravity in the presence of R^2 term. Thus, R can not be treated as an
auxiliary variable and hence Noether symmetry of arbitrary form of F(R) theory
of gravity remains obscure. However, there exists in general, a conserved
current for F(R) theory of gravity in the presence of a non-minimally coupled
scalar-tensor theory (A. K. Sanyal, Phys.Lett.B624, 81 (2005),
arXiv:hep-th/0504021 and Mod.Phys.Lett.A25, 2667 (2010), arXiv:0910.2385
[astro-ph.CO]). Here, we briefly expatiate the non-Noether conserved current
and cite an example to reveal its importance in finding cosmological solution
for such an action, taking F(R) \propto R^{3/2}.Comment: 16 pages, 1 figure. appears in Int J Theoretical Phys (2012
Energy dependent wavelength of the ion induced nanoscale ripple
Wavelength variation of ion beam induced nanoscale ripple structure has
received much attention recently due to its possible application in
nanotechnology. We present here results of Ar bombarded Si in the energy
range 50 to 140 keV to demonstrate that with beam scanning the ripple
wavelength increases with ion energy and decreases with energy for irradiation
without ion beam scanning. An expression for the energy dependence of ripple
wavelength is proposed taking into simultaneous effect of thermally activated
surface diffusion and ion induced effective surface diffusion.Comment: REVTeX (4 pages), 3 EPS figure
Formation of rectifier with gold nanoclusters
Gold nanoclusters encapsulated with organic molecules are of great interest
for its possible applications in the fields of molecular electronics, catalysis
and medical science. Here we demonstrate that monolayer and bilayer films of
thiol-capped gold nanoclusters can exhibit diode-like properties provided
controlled spatial asymmetry exist between two tunnel junctions used to connect
a thiol capped gold nanoclusters. Current-voltage characteristics of this
rectifier were obtained from conducting probe atomic force microscopy
measurements and also from conventional two probe resistance measurements.
Systematic x-ray reflectivity and atomic force microscopy measurements were
carried out to characterize the spatial asymmetry introduced by a monolayer of
fatty acid salt gadolinium stearate used to deposit thiol-capped gold
nanocluster molecules on hydrophilic SiO2-Si(001) substrate by Langmuir
Blodgett technique. This information was used to explain prominent
rectification observed in these nano-structured films.Comment: 13 pages, 3 figure
The metallicity dependence of envelope inflation in massive stars
Recently it has been found that models of massive stars reach the Eddington
limit in their interior, which leads to dilute extended envelopes. We perform a
comparative study of the envelope properties of massive stars at different
metallicities, with the aim to establish the impact of the stellar metallicity
on the effect of envelope inflation. We analyse published grids of
core-hydrogen burning massive star models computed with metallicities
appropriate for massive stars in the Milky Way, the LMC and the SMC, the very
metal poor dwarf galaxy I Zwicky 18, and for metal-free chemical composition.
Stellar models of all the investigated metallicities reach and exceed the
Eddington limit in their interior, aided by the opacity peaks of iron, helium
and hydrogen, and consequently develop inflated envelopes. Envelope inflation
leads to a redward bending of the zero-age main sequence and a broadening of
the main sequence band in the upper part of the Hertzsprung-Russell diagram. We
derive the limiting L/M-values as function of the stellar surface temperature
above which inflation occurs, and find them to be larger for lower metallicity.
While Galactic models show inflation above ~29 Msun, the corresponding mass
limit for Population III stars is ~150 Msun. While the masses of the inflated
envelopes are generally small, we find that they can reach 1-100 Msun in models
with effective temperatures below ~8000 K, with higher masses reached by models
of lower metallicity. Envelope inflation is expected to occur in sufficiently
massive stars at all metallicities, and is expected to lead to rapidly growing
pulsations, high macroturbulent velocities, and might well be related to the
unexplained variability observed in Luminous Blue Variables like S Doradus and
Eta Carina.Comment: 16 pages (with Appendix), accepted in A&
Characterisation and neutralisation of Aeromonas hydrophila enterotoxin in the rabbit ileal-loop model
Cell-free culture filtrates and crude enterotoxin preparations from six strains of Aeromonas hydrophiza caused the accumulation of fluid in rabbit ileal loops. This activity was due to a non-dialysable, heat and acid-labile antigenic protein and was lost when culture filtrates and crude enterotoxin preparations were heated at 60°C for 20 min. or 56°C for 30 min. respectively. Maximum activity was observed at pH 8.0-10.0; there was a gradual loss at lower pH and activity was abolished in culture filtrates held at pH 3.0 and crude enterotoxin preparations held at pH 4.0. Titration of the crude enterotoxin preparations in rabbit ileal loops showed that the ED50 (equivalent to 1 unit of toxin) was contained in 25 μg of protein; a logarithmic plot of the neutralisation coefficients against antiserum concentrations showed that one unit of antitoxin was contained in 42×10−4 ml of the antiserum
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