355 research outputs found
Gravitational instability and star formation in disk galaxies
We present a general star formation law where star formation rate depends
upon efficiency , timescale of star formation, gas component
of surface mass density and a real exponent . A given exponent
determines which however yields the corresponding star formation
rate. Current nominal Schmidt exponent for our model is .
Based on a gravitational instability parameter and another
dimensionless parameter , where =
pressure, = column density of molecular clouds, we suggest a
general equation for star formation rate which depends upon relative competence
of the two parameters for various physical circumstances. We find that
emerges to be a better parameter for star formation scenario than Toomre
Q-parameter. Star formation rate in the solar neighbourhood is found to be in
good agreement with values inferred from previous studies. Under closed box
approximation model, we obtain a relation between metallicity of gas and the
efficiency of star formation. Our model calculations of metallicity in the
solar neighbourhood agree with earlier estimates. We conclude that metallicity
dispersion for stars of same age may result due to a change in efficiency
through which different sample stars were processed. For no significant change
of metallicity with age, we suggest that all sample stars were born with almost
similar efficiency.Comment: 10 pages, 3 figures, submitted to MNRA
Brane-World Cosmology, Bulk Scalars and Perturbations
We investigate aspects of cosmology in brane world theories with a bulk
scalar field. We concentrate on a recent model motivated from supergravity in
singular spaces. After discussing the background evolution of such a
brane-world, we present the evolution of the density contrast. We compare our
results to those obtained in the (second) Randall-Sundrum scenario and usual 4D
scalar-tensor theories.Comment: 29 pages, one figure, JHEP3-styl
Time-Varying Fine-Structure Constant Requires Cosmological Constant
Webb et al. presented preliminary evidence for a time-varying fine-structure
constant. We show Teller's formula for this variation to be ruled out within
the Einstein-de Sitter universe, however, it is compatible with cosmologies
which require a large cosmological constant.Comment: 3 pages, no figures, revtex, to be published in Mod. Phys. Lett.
Cosmological tensor perturbations in the Randall-Sundrum model: evolution in the near-brane limit
We discuss the evolution of cosmological tensor perturbations in the RSII
model. In Gaussian normal coordinates the wave equation is non-separable, so we
use the near-brane limit to perform the separation and study the evolution of
perturbations. Massive excitations, which may also mix, decay outside the
horizon which could lead to some novel cosmological signatures.Comment: 18 pages, 1 figur
CMB Anisotropies in the Presence of Extra Dimensions
We discuss the effect of the time evolution of extra dimensions on CMB
anisotropies and large-scale structure formation. We study the impact of scalar
fields in a low-energy effective description of a general class of brane world
models on the temperature anisotropy power spectrum. We show that when the
coupling between these scalar fields and matter evolves over cosmological
timescales, current observations of the CMB anisotropies can constrain
primordial values of the fields in a manner complementary to local, late-time
tests of gravity. We also present the effect of these fields on the
polarization anisotropy spectra and the growth of large-scale structure,
showing that future CMB observations will constrain theories of the Universe
involving extra dimensions even further.Comment: 17 pages, 15 figure
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