• Issue front cover thumbnail

      Volume 28, Issue 1

      March 2007,   pages  1-66

    • Oblique Propagation and Dissipation of Alfv´en Waves in Coronal Holes

      A. K. Srivastava B. N. Dwivedi

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      We investigate the effect of viscosity and magnetic diffusivity on the oblique propagation and dissipation of Alfvén waves with respect to the normal outward direction, making use of MHD equations, density, temperature and magnetic field structure in coronal holes and underlying magnetic funnels. We find reduction in the damping length scale, group velocity and energy flux density as the propagation angle of Alfvén waves increases inside the coronal holes. For any propagation angle, the energy flux density and damping length scale also show a decrement in the source region of the solar wind (< 1.05 R) where these may be one of the primary energy sources, which can convert the inflow of the solar wind into the outflow. In the outer region (> 1.21 R), for any propagation angle, the energy flux density peaks match with the peaks of MgX 609.78 Å and 624.78 Å linewidths observed from the Coronal Diagnostic Spectrometer (CDS) on SOHO and the non-thermal velocity derived from these observations, justify the observed spectroscopic signature of the Alfvén wave dissipation.

    • Homotopy Continuation Method of Arbitrary Order of Convergence for Solving the Hyperbolic Form of Kepler’s Equation

      M. A. Sharaf M. A. Banajh A. A. Alshaary

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      In this paper, an efficient iterative method of arbitrary integer order of convergence ≥ 2 has been established for solving the hyperbolic form of Kepler’s equation. The method is of a dynamic nature in the sense that, moving from one iterative scheme to the subsequent one, only additional instruction is needed. Most importantly, the method does not need any prior knowledge of the initial guess. A property which avoids the critical situations between divergent and very slow convergent solutions that may exist in other numerical methods which depend on initial guess. Computational Package for digital implementation of the method is given and is applied to many case studies.

    • Modeling Repulsive Gravity with Creation

      R. G. Vishwakarma J. V. Narlikar

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      There is a growing interest among cosmologists for theories with negative energy scalar fields and creation, in order to model a repulsive gravity. The classical steady state cosmology proposed by Bondi, Gold & Hoyle in 1948, was the first such theory which used a negative kinetic energy creation field to invoke creation of matter. We emphasize that creation plays a very crucial role in cosmology and provides a natural explanation to the various explosive phenomena occurring in local (𝑧 < 0.1) and extra galactic universe.We exemplify this point of view by considering the resurrected version of this theory – the quasi-steady state theory, which tries to relate creation events directly to the large scale dynamics of the universe and supplies more natural explanations of the observed phenomena.

      Although the theory predicts a decelerating universe at the present era, it explains successfully the recent SNe Ia observations (which require an accelerating universe in the standard cosmology), as we show in this paper by performing a Bayesian analysis of the data.

    • Kinematical Diagrams for Conical Relativistic Jets

      Gopal-Krishna Pronoy Sircar Samir Dhurde

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      We present diagrams depicting the expected inter-dependences of two key kinematical parameters of radio knots in the parsec-scale jets of blazars, deduced from VLBI observations. The two parameters are the apparent speed (𝑣app = c𝛽app) and the effective Doppler boosting factor (𝛿eff) of the relativistically moving radio knot. A novel aspect of these analytical computations of 𝛽–𝛿 diagrams is that they are made for parsecscale jets having a conical shape, with modest opening angles (𝜔 up to 10°), in accord with the VLBI observations of the nuclei of the nearest radio galaxies. Another motivating factor is the recent finding that consideration of a conical geometry can have important implications for the interpretation of a variety of radio observations of blazar jets. In addition to uniform jet flows (i.e., those having a uniform bulk Lorentz factor, 𝛤), computational results are also presented for stratified jets where an ultra-relativistic central spine along the jet axis is surrounded by a slower moving sheath, possibly arising from a velocity shear.

    • GMRT and VLA Observations at 49 cm and 20 cm of the HII Region near 𝑙 = 24.8°, 𝑏 = 0.1°

      N. G. Kantharia W. M. Goss D. Anish Roshi Niruj R. Mohan Francois Viallefond

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      We report multi-frequency radio continuum and hydrogen radio recombination line observations of HII regions near 𝑙 = 24.8°, 𝑏 = 0.1° using the Giant Metrewave Radio Telescope(GMRT) at 1.28 GHz (𝑛 = 172), 0.61 GHz (𝑛 = 220) and the Very Large Array (VLA) at 1.42 GHz (𝑛 = 166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at 𝑙 = 24.8° and 𝑏 = 0.1° is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ∼ 550 M with a linear size of 7 pc and an rms electron density of ∼ 110 cm-3 at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5.

      We also report detection of hydrogen recombination lines from the HII region at 𝑙 = 24.8° and 𝑏 = 0.1° at all observed frequencies near 𝑉𝑙𝑠𝑟 = 100 km s-1. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum.We also report detection of hydrogen recombination lines from two other HII regions in the field.

    • Evolution of the Distribution of Neutron Exposures in the Galaxy Disc: An Analytical Model

      Wenyuan Cui Weijuan Zhang Bo Zhang

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      In this work, based on the analytical model with delayed production approximation developed by Pagel & Tautvaišienė (1995) for the Galaxy, the analytic solutions of the distribution of neutron exposures of the Galaxy (hereafter NEG) are obtained. The present results appear to reasonably reproduce the distribution of neutron exposures of the solar system (hereafter NES). The strong component and the main component of the NES are built up in different epochs. Firstly, the strong component is produced by the s-process nucleosynthesis in the metal-poor AGB stars, starting from [Fe/H] ≈ -1.16 to [Fe/H] ≈ -0.66, corresponding to the time interval 1.06 < 𝑡 < 2.6 Gyr. Secondly, the main component is produced by the s-process in the galactic disk AGB stars, starting from [Fe/H] ≈ -0.66 to [Fe/H] ≈ 0, corresponding to the time interval 𝑡 > 2.6 Gyr. The analytic solutions have the advantage of an understanding of the structure and the properties of the NEG. The NEG is believed to be an effective tool to study the s-process element abundance distributions in the Galaxy at different epochs and the galactic chemical evolution of the neutron-capture elements.

  • 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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