• Volume 14, Issue 2

      June 1993,   pages  53-114

    • The bolometric light curve of SN 1993J and the nature of its progenitor

      A. Ray K. P. Singh F. K. Sutaria

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      We have constructed the bolometric light curve of SN 1993J based on UBVRI(JHK) photometric data obtained from various sources and assumingAV = 0 and a distance modulus of 27.6. Effective temperatures and photosphere radius at various times have been obtained from detailed blackbody fits. The bolometric light curve shows two maxima. The short rise time to the second maximum, and the luminosities at the minimum and the second maximum are used to constrain the properties of the progenitor star. The total mass of the hydrogen envelope MH, in the star is found to be ≲ 0.2 M at the time of explosion, and the explosion ejected about 0.05 M of Ni56. Thin hydrogen envelope combined with a sufficient presupernova luminosity suggest that the exploding star was in a binary with a probable period range of 5yr ≤Porb 11yr.

    • Bipolar jets in planetary nebulae: An analytical model

      G. Pascoli

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      The various and hitherto partially unsolved problems relative to the origin of bipolar jets or highly collimated fast outflows in planetary nebulae are reviewed within the framework of a stationary magnetohydrodynamic model.

      In order to explain the observations of high polar velocities and the presence of polar blobs or knots in planetary nebulae, theoretical models are proposed taking into account both a large scale azimuthal magnetic field and an anisotropic turbulent velocity field.

      The models predict equatorial-to-polar density ratios which are rather small, in the range 2 to 3. Conversely, the polar-to-equatorial velocity contrasts are higher, with typical values upto 10. Thus thead hoc hypothesis implicit in the literature that the density contrast is varying in inverse ratio to the velocity one, does not seem well adapted to the bipolar jet phenomenon in planetary nebulae.

      We point out, therefore, that the bipolar jets have to be considered as a transient aspect of a very complex phenomenon. The model can be applied to objects such as He 2–104 or Mz3, M2–9.

    • Chemical evolution of the high redshift galaxies

      Pushpa Khare N. C. Rana

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      We have tried to determine the rate of chemical evolution of high redshift galaxies from the observed redshift distribution of the heavy element absorption systems in the spectra of QSOs, taking into account the evolution in the intensity of the metagalactic UV ionizing radiation background, the radius and/or the co-moving number density of, and the fraction of mass in the form of gas in, the absorbers. The data for both the Lyman limit systems and the C IV systems have been fitted simultaneously. It seems that the abundance of carbon has possibly increased by about a factor of 5 to 20 from the cosmic time corresponding to the redshift ≃ 4 to 2. The data also suggest that either the radius or the co-moving number density of the galaxies increased with redshift up to z = 2.0 and decreased slowly thereafter. The total mass of the halo gas was higher in the past, almost equal to the entire mass of the galaxy at z = 4. The hydrogen column density distribution for Lyman limit systems predicted by the model is in agreement with the observed distribution.

    • Differential evolution of Lyman alpha lines

      M. Acharya Pushpa Khare

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      We have analysed a large, homogeneous sample of Lyman alpha lines, observed at intermediate resolution, using the maximum likelihood method. The analysis shows that the evolutionary index γ, is a function of rest equivalent width with stronger lines evolving faster. The mean equivalent width increases with redshift. This behaviour is similar to that exhibited by the heavy element absorption lines in the quasar spectra.

    • Plasma instability at the inner edge of the accretion disk—I

      S. C. Tripathy C. B. Dwivedi A. C. Das A. R. Prasanna

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      In this paper, the analytical and numerical results of the stability analysis of the accretion disk at the inner boundary is presented. Including the effect of finite conductivity in the disk dynamics, a simple calculation considering only the radial perturbation has been carried out. Within local approximation, it is concluded that the disk is stable to Kelvin-Helmholtz and resistive electromagnetic modes whereas the magnetosonic mode can destabilise the disk structure.

  • 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.

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