907 research outputs found
The Effects of Shear and Rotation Anisotropy Upon the Process of Gravitational Instability
In this paper, we study the role of shear fields on the evolution of density
perturbations embedded in a Friedmann flat background universe, by studying the
evolution of a homogeneous ellipsoid model. In this context, we show that while
the effect of the shear is that of increasing the growth rate of the density
contrast of a mass element, the angular momentum acquired by the ellipsoid has
the right magnitude to counterbalance the shear. Finally, our result show that
initial asphericities and tidal interaction induce a slowing down of the
collapse after the system has broken away from the general expansion, in
perfect agreement with the previrialization conjecture (Peebles & Groth 1976;
Davis & Peebles 1977)
Origin of gamma-ray emission in the shell of Cassiopeia A
Non-thermal X-ray emission from the shell of Cassiopeia A (Cas A) has been an
interesting subject of study, as it provides information about relativistic
electrons and their acceleration mechanisms in the shocks. Chandra X-ray
observatory revealed the detailed spectral and spatial structure of this SNR in
X-rays. The spectral analysis of Chandra X-ray data of Cas A shows unequal flux
levels for different regions of the shell, which can be attributed to different
magnetic fields in those regions. Additionally, the GeV gamma-ray emission
observed by Large Area Telescope on board Fermi Gamma Ray Space Telescope
showed that the hadronic processes are dominating in Cas A, a clear signature
of acceleration of protons. In this paper we aim to explain the GeV-TeV
gamma-ray data in the context of both leptonic and hadronic scenario. We
modeled the multi-wavelength spectrum of Cas A. We use synchrotron emission
process to explain the observed non-thermal X-ray fluxes from different regions
of the shell. These result in estimation of the model parameters, which are
then used to explain TeV gamma-ray emission spectrum. We also use hadronic
scenario to explain both GeV and TeV fluxes simultaneously. We show that a
leptonic model alone cannot explain the GeV-TeV data. Therefore, we need to
invoke a hadronic model to explain the observed GeV-TeV fluxes. We found that
although pure hadronic model is able to explain the GeV-TeV data, a
lepto-hadronic model provides the best fit to the data.Comment: Accepted in A&
Migration of giant planets in planetesimal discs
Planets orbiting a planetesimal circumstellar disc can migrate inward from
their initial positions because of dynamical friction between planets and
planetesimals. The migration rate depends on the disc mass and on its time
evolution. Planets that are embedded in long-lived planetesimal discs, having
total mass of , can migrate inward a large distance and
can survive only if the inner disc is truncated or because of tidal interaction
with the star. In this case the semi-major axis, a, of the planetary orbit is
less than 0.1 AU. Orbits with larger are obtained for smaller value of the
disc mass or for a rapid evolution (depletion) of the disc. This model may
explain several of the orbital features of the giant planets that were
discovered in last years orbiting nearby stars as well as the metallicity
enhancement found in several stars associated with short-period planets.Comment: 21 pages; 6 encapsulated figures. Accepted by MNRA
Constraints to cosmological parameters through clusters evolution
In this paper, I revisit the constraints obtained by several authors
(Reichart et al. 1999; Eke et al. 1998; Henry 2000) on the estimated values of
Omegam, n and sigma_8 in the light of recent theoretical developments: 1) new
theoretical mass functions (Sheth & Tormen 1999, Sheth, Mo & Tormen 2001, Del
Popolo 2002b); 2) a more accurate mass-temperature relation, also determined
for arbitrary Omega_m and Omega_Lambda (Del Popolo 2002a).Comment: 3 pages; 3 encapsulated figures. Proceedings of Iau Coll. 19
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