332 research outputs found
Continuous matter creation and the acceleration of the universe: the growth of density fluctuations
Cosmologies including continuous matter creation are able to reproduce the
main properties of the standard CDM model, in particular in cases
where the particle and entropy production rates are equal. These specific
models, characterized by a mass density equal to the critical value, behave
like the standard CDM model at early times whereas their late
evolution is similar to the steady-state cosmology. The maximum amplitude of
density fluctuations in these models depends on the adopted creation rate,
related here to the parameter and this limitation could be a
difficulty for the formation of galaxies and large-scale structure in this
class of universe. Additional problems are related with predictions either of
the random peculiar velocities of galaxies or the present density of massive
clusters of galaxies, both being largely overestimated with respect to
observational data.Comment: 11 pages, 2 figures, accepted for publication in General Relativity
and Gravitatio
Gravitation Wave Emission from Radio Pulsars Revisited
We report a new pulsar population synthesis based on Monte Carlo techniques,
aiming to estimate the contribution of galactic radio pulsars to the continuous
gravitational wave emission. Assuming that the rotation periods of pulsars at
birth have a Gaussian distribution, we find that the average initial period is
290 ms. The number of objects with periods equal to or less than 0.4 s, and
therefore capable of being detected by an interferometric gravitational antenna
like VIRGO, is of the order of 5100-7800. With integration times lasting
between 2 and 3 yr, our simulations suggest that about two detections should be
possible, if the mean equatorial ellipticity of the pulsars is
=10. A mean ellipticity an order of magnitude higher increases the
expected number of detections to 12-18, whereas for , no
detections are expectedComment: accepted for publication in A&A, 9 pages, 8 figure
Ages of Elliptical Galaxies: Single versus Multi Population Interpretation
New calibrations of spectrophotometric indices of elliptical galaxies as
functions of spectrophotometric indices are presented, permitting estimates of
mean stellar population ages and metallicities. These calibrations are based on
evolutionary models including a two-phase interstellar medium, infall and a
galactic wind.Free parameters were fixed by requiring that models reproduce the
mean trend of data in the color-magnitude diagram as well as in the plane of
indices Hbeta-Mg2 and Mg2-. To improve the location of faint ellipticals(MB
> -20) in the Hbeta-Mg2 diagram, down-sizing was introduced. An application of
our calibrations to a sample of ellipticals and a comparison with results
derived from single stellar population models is given. Our models indicate
that mean population ages span an interval of 7-12 Gyr and are correlated with
metallicities, which range from approximately half up to three times solar.Comment: 10 pages and 6 figures. Accepted for publication in Monthly Notices
of the Royal Astronomical Society Main Journa
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