• Volume 24, Issue 3-4

      September 2003,   pages  51-113

    • Large scale magnetic fields: Density power spectrum in redshift space

      Rajesh Gopal Shiv K. Sethi

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      We compute the density redshift-space power spectrum in the presence of tangled magnetic fields and compare it with existing observations. Our analysis shows that if these magnetic fields originated in the early universe then it is possible to construct models for which the shape of the power spectrum agrees with the large scale slope of the observed power spectrum. However requiring compatibility with observed CMBR anisotropies, the normalization of the power spectrum is too low for magnetic fields to have significant impact on the large scale structure at present. Magnetic fields of a more recent origin generically give density power spectrumαk4which doesn’t agree with the shape of the observed power spectrum at any scale. Magnetic fields generate curl modes of the velocity field which increase both the quadrupole and hexadecapole of the redshift space power spectrum. For curl modes, the hexadecapole dominates over quadrupole. So the presence of curl modes could be indicated by an anomalously large hexadecapole, which has not yet been computed from observation.

      It appears difficult to construct models in which tangled magnetic fields could have played a major role in shaping the large scale structure in the present epoch. However if they did, one of the best ways to infer their presence would be from the redshift space effects in the density power spectrum.

    • Studies of clump structure of photodissociation regions at millimeter and sub-millimeter wavelengths

      Abdul Qaiyum Syed Salman Ali

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      To interpret the millimeter and sub-millimeter line emissions of atomic and molecular species from galactic and extragalactic photodissociation regions, warm gas components and molecular clouds, generally, escape probability formalism of Tielens & Hollenbach (herein referred as TH) are employed which is based on the assumption of plane parallel geometry of infinite slab allowing photons to escape only from the front. Contrary to the assumption observationally it is found that these lines are optically thin except OI(63μ m) and low rotational transitions of CO and some other molecules. This observational evidence led us to assume that emitting regions are finite parallel plane slab in which photons are allowed to escape from both the surfaces (back and front). Therefore, in the present study escape of radiations from both sides of the homogeneous and also clumpy PDR/molecular clouds are taken into consideration for calculating the line intensities at millimeter and sub-millimeter wavelengths (hereinafter referred as QA). Results are compared with that of the TH model. It is found that thermal and chemical structures of the regions are almost similar in both the formalisms. But line intensities are modified by differing factors. Particularly at low density and low kinetic temperature and also for optically thin lines line intensities calculated from TH and QA model differ substantially. But at density higher than the critical density and also for optically thick lines TH and QA models converge to almost same values. An attempt has been made to study the physical conditions of the M17 region employing the present formalism.

    • Comparing maps to symplectic integrators in a galactic type Hamiltonian

      N. D. Caranicolas N. J. Papadopoulos

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      We obtain thex - pxPoincare phase plane for a two dimensional, resonant, galactic type Hamiltonian using conventional numerical integration, a second order symplectic integrator and a map based on the averaged Hamiltonian. It is found that all three methods give good results, for small values of the perturbation parameter, while the symplectic integrator does a better job than the mapping, for large perturbations. The dynamical spectra are used to distinguish between regular and chaotic motion.

    • The mass/eccentricity limit in double star astronomy

      J. Dommanget

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      A research that we conducted in 1963 on the evolution of the binaries based on the available orbital data to obtain a philosophical degree, led to the establishment of an interesting and new diagram between the logarithm of the total mass and a particular parameterX, bound to the areal constant. This appeared to have a real physical significance but the basic observational material was insufficiently extended to assure its undeniable existence. In 1981, a new research based on a more extended orbital material, has confirmed this diagram. Presently, another important increase in the orbital material and the availability of highly accurate trigonometric parallaxes produced by the Hipparcos satellite, gave us the opportunity to confirm once more the stability of this diagram. This last research is here described.

    • Author Index

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    • Subject Index

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  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

      Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

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