No abstract

We study the non-linear evolution of a dust ellipsoid, embedded in a Friedmann flat background universe, in order to determine the evolution of the density of the ellipsoid as the perturbation to it related detaches from general expansion and begins to collapse. We show that while the growth rate of the density contrast of a mass element is enhanced by the shear in agreement with Hoffman (1986a), the angular momentum acquired by the ellipsoid has the right magnitude to counterbalance the effect of the shear. The result confirms the previrialization conjecture (Peebles & Groth 1976;Davis & Peebles 1977;Peebles 1990) by showing that initial asphericities and tidal interactions begin to slow down the collapse after the system has broken away from the general expansion.

Context. 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. Aims. To locate the origin of gamma rays based on the X-ray data of the shell of Cas A. We also aim to explain the GeV−TeV gamma-ray data in the context of both leptonic and hadronic scenario. Methods. We modeled the multi-wavelength spectrum of Cas A. We use synchrotron emission process to explain the observed nonthermal 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. Results. Based on this analysis, it has been shown that the southern part of the remnant is bright in TeV gamma rays. We also show that, 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, lepto-hadronic model provides the best fit to the data.

We present here the results of the observation of CTB 37A obtained with the Xray Imaging Spectrometer onboard the Suzaku satellite. The X-ray spectrum of CTB 37A is well fitted by two components, a single-temperature ionization equilibrium component (VMEKAL) with solar abundances, an electron temperature of kT e ∼ 0.6 keV, absorbing column density of N H ∼ 3 × 10 22 cm −2 and a power-law component with photon index of Γ ∼ 1.6. The X-ray spectrum of CTB 37A is characterized by clearly detected K-shell emission lines of Mg, Si, S, and Ar. The plasma with solar abundances supports the idea that the X-ray emission originates from the shocked interstellar material. The ambient gas density, and age of the remnant are estimated to be ∼1f −1/2 cm −3 and ∼3×10 4 f 1/2 yr, respectively. The center-filling X-ray emission surrounded by a shell-like radio structure and other X-ray properties indicate that this remnant would be a new member of mixed-morphology supernova remnant class.

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 $10^{-4}-0.01 M_{\odot}$, 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 $a$ 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