656 research outputs found
The Peculiar-Velocity-Field in Structure Formation Theories with Cosmic Strings
We investigate the peculiar velocity field due to long cosmic strings in
several cosmological models and analyse the influence of a nonscaling behaviour
of the string network, which is expected in open cosmological models or models
with a cosmological constant. It is shown that the deviation of the propability
distribution of the peculiar velocity field from the normal distribution is
only weak in all models. It is further argued that one can not necessarily
obtain the parameter from density and velocity
fields, where is the density parameter and the linear biasing
parameter, if cosmic strings are responsible for structure formation in the
universe. An explanation for this finding is given.Comment: 11 pages, 5 figure
On the Origin of Very Wide Ly-Absorption-Lines in Quasar Spectra
We present a new explanation for the very wide absorption features in quasar
spectra. In our model, a very wide absorption feature will originate, when the
line of sight crosses a bubble wall tangentially. We demonstrate this on the
quasar pair (2138-4427), (2139-4434). Both show two very wide absorption lines
in their spectra at the same redshift. The bubble wall model can explain these
observations in low density Friedmann-Lemaitre models with spherical metric. It
contradicts models with euclidian or hyperbolic metric.Comment: Latex, 6 pages, Figures available at [email protected]
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
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