• Volume 2, Issue 1

      March 1981,   pages  1-118

    • General relativistic analysis of structure and stability of charged fluid disks around compact objects

      A. R. Prasanna D. K. Chakraborty

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      In this paper we give a detailed general relativistic formulation of the study of structure and stability of charged fluid disks around compact objects like black holes neglecting the self-gravitation of the disk itself. Having presented the general equations for equilibrium as well as for perturbations we solve explicitly the cases of rigidly and differentially rotating thin disks, with constant charge density and zero pressure, confined to the equatorial plane of the black hole. By using normal mode analysis we have analysed the stability of such disks under purely radial perturbations and find that the disks are generally stable.

    • An analysis of the orbital periods of some eclipsing binaries suspected to be in the pre-main sequence contraction phase of evolution

      T. Panchatsaram K. D. Abhyankar

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      An analysis of the available photoelectric times of minima of KO Aql, TV Cas and Z Her, which are suspected to be in pre-main sequence phase of evolution, reveals that KO Aql shows a secular increase in its orbital period at the rate of 4·34 × 10−8 day per cycle while the period of TV Cas has been decreasing at the rate of 4·08 × 10−9 day per cycle. Z Her does not show any period change at all. The orbital period of any binary system which is in the pre-main sequence phase will be systematically affected because of ′ shrinking′ dimensions of the components. A simple formula for the characteristic period change, defined by (P/P), is derived from a consideration of the conservation of total energy and total angular momentum for a binary system whose components are still in the process of contraction or expansion. The derived formula is applied to the above systems to see whether theoretical characteristic period changes agree with the observed values. The systems are assumed to evolve independently in the pre-main sequence phase in accordance with the model calculations of Iben (1965). It is found that there is no agreement between theoretical and observed characteristic period changes. This suggests that KO Aql and TV Cas may not be in the pre-main sequence phase. We do not have sufficient data for Z Her to judge its evolutionary status by the present procedure; this is also true of TT Hya. We suspect that the period changes observed in KO Aql and TV Cas may be due to light-time effect.

    • Extragalactic sources with one-sided radio structure

      Vijay K. Kapahi

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      A list has been compiled of 49 extragalactic sources, most of them identified with quasars, that appear to have a one-sided (D2 type) radio structure characterized by a single outer component displaced from a compact central (nuclear) component coincident with the optical object. The observed properties of a subsample of 28 D2 quasars that have an overall angular size larger than 5 arcsec are briefly discussed and compared with those of normal (D1 type) double quasars. It is found that the central components in most D2 sources account for more than half the total flux density at high frequencies in contrast to the D1 quasars which generally have less than 20 per cent of their total flux density in a central component. This makes it very unlikely that D2 sources are just those D1s in which there is a large intrinsic difference in the flux densities or separations of the two outer components. The observed properties of D2 sources are easier to understand in the relativistic beaming interpretation in which their axes are inclined at smaller angles with the line of sight compared to D1 sources.

    • Pulsating radio emission at decametre wavelengths from the sun

      Ch. V. Sastry V. Krishan K. R. Subramanian

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      Observations on the pulsation pattern in the time profile of short duration solar radio bursts at decametre wavelengths are presented. The pulsations are found to be present predominantly in the saturation phase of the burst. A tentative physical model based on the non-linear development of the waves interacting in a turbulent medium is invoked to explain the origin of the pulsations.

    • Luminosity-HI velocity width relation for spirals

      N. Visvanathan

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      Multicolour and multiaperture photometry of 22 late-type Virgo cluster galaxies in a newV (5500 Å),r (6738 Å),IV (10500 Å) system confirm the previously known correlations between the luminosity and H1 velocity width, and show them to be strongly wavelength dependent with the slope of the relation reaching a maximum value of ∼ − 10 at 10500 Å and remaining constant at longer wavelengths. The scatter in the luminosity-H1 width relation is nearly the same, whether we use (V)c−0.5, (r)c−0.5, (IV)c-0.5, or (H)c-0.5 magnitudes. The error in the determination of the corrected magnitudes is much less than the scatter in the luminosity-H1 width relations as evidenced by the fact that the residuals of individual galaxies inV, r, IV, andH are correlated with one another. An attempt to use a ‘kinematic magnitude’ instead of an isophotal magnitude shows the slope of the luminosity-H1 width relations to be reduced substantially.

      Observations in theV, r, andIV system for fifteen field galaxies and three galaxies in the Cancer and Zw 74-23 clusters have been obtained and combined with the H1 velocity width to derive their differences in distance modulus, between the galaxy and the Virgo cluster, through the luminosity-H1 width relations of the Virgo cluster galaxies. The three independent differences in the distance modulus of each galaxy agree with one another indicating that the relations usingV, r andIV magnitudes have the same zero point in absolute magnitude, independent of the wavelength of observations. The distance modulus difference from the Virgo cluster to the three galaxies U 4334, U 8942 and U 8944, which are outside the Local Supercluster, are +1·50, +3·45 and +2·81 mag respectively and are in agreement with those of +1·75, +3·23 and +2·46 mag derived for the same galaxies by Aaronsonet al. (1980) throughH magnitudes.

      The scatter in the velocity distance relation of the field galaxies compares well with the mean error derived in the luminosity-HI width relations and hence is intrinsic.

    • Photon escape probabilities in expanding atmospheres

      A. Peraiah K. E. Rangarajan D. Mohan Rao

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      A comparison of mean number of scatterings and escape probabilities has been made in isotropic scattering and dipole scattering by using the angle-averaged partial frequency redistribution functionRI. We have solved the equations of radiative transfer and statistical equilibrium simultaneously in a spherically symmetric expanding atmosphere. Two cases of atmospheric extension (i.e.)B/A=3 and 10 (whereB andA are the outer and inner radii of the atmosphere) have been treated.

      We find that the partial frequency redistribution gives a larger mean number of scatterings compared to that given by complete redistribution. Velocities tend to reduce the mean number of scatterings and in crease the mean escape probabilities.

    • On the relationship between pulsars and supernova remnants

      R. K. Kochhar

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      We propose that single stars in the mass range 4–6·5M, that explode as Supernovae of Type I, are totally disrupted by the explosion and form shell-type remnants. More massive single stars which explode as Supernovae of Type II also give rise to shell-type remnants, but in this case a neutron star or a black hole is left behind. The first supernova explosion in a close binary also gives rise to a shell-type supernova remnant. The Crab-like filled-centre supernova remnants are formed by the second supernova explosion in a close binary. The hybrid supernova remnants, consisting of a filled centre surrounded by a shell, are formed if there is an active neutron star inside the shell.

    • Intrinsic properties of carbon stars. I. Effective temperature scale of N-type carbon stars

      Takashi Tsuji

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      It is shown that the infrared flux method for determining stellar effective temperatures (Blackwell and Shallis 1977; Blackwell, Petford and Shallis 1980) can be applied to cool carbon stars. Although the spectra of cool carbon stars are highly line blanketed, the spectral region between 3 and 4 μm (L-band in the infrared photometry system) is found to be relatively free from strong line absorption. The ratioRL of bolometric flux toL flux can then be used as a measure of effective temperature. On the basis of the predicted line-blanketed flux based on model atmospheres, with an empirical correction for the effect of 3 μm absorption due to polyatomic species (HCN, C2H2), it is shown thatRL is roughly proportional to T3eff. The high sensitivity ofRL to Teff makes it a very good measure of effective temperature, and the usual difficulty due to differential line blanketing effect in the analyses of photometric indices of cool carbon stars can be minimized.

      It is found that the majority of N-type carbon stars with small variability (SRb and Lb variables) are confined to the effective temperature range between 2400 and 3200 K, in contrast to M-giant stars (M0 III - M6 III, including SRb and Lb variables) that are confined to the effective temperature range between 3200 and 3900 K. The effective temperatures based on the infrared flux method show good agreement with those derived directly from angular diameter measurements of 5 carbon stars. On the basis of the new effective temperature scale for carbon stars, it is shown that the well known C-classification does not represent a temperature sequence. On the other hand, colour temperatures based on various photometric indices all show good correlations with our derived effective temperatures.

    • A spectroscopic orbit for 26 Comae

      R. F. Griffin

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      Photoelectric radial-velocity measurements show that 26 Comae is a spectroscopic binary with a very eccentric orbit and a period of 972 days.

  • Journal of Astrophysics and Astronomy | News

    • Continuous Article Publication

      Posted on January 27, 2016

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