Volume 8, Issue 1
March 1987, pages 1-78
pp 1-16 March 1987
In a model galaxy composed of a relativistically active nucleus, a main body, and a halo, all three components considered as homogeneous prolate ellipsoids, we explore the probable association of the internal characteristics of the nucleus and the observed orbits of the stars near the surface of the main body. Using the authors’ theoretical framework of post-Newtonian general relativistic galactic dynamics, proposed earlier, we prove that a fast-rotating and possibly expanding or contracting nucleus affects the distribution of the box-type orbits near the surface of the main body resulting in a flattening of the main body. The nuclear rotation always results in a flattening, and the contraction contributes less to the flattening than the expansion. However, the contributions of a rotating and changing nucleus are not additive. The study of the post-Newtonian effects in the nucleus on the stellar orbits in the main body, and the consequent modifications of the corresponding non-relativistic results, could in principle provide useful information concerning the kinematical and dynamical characteristics of the nuclei of the elliptical galaxies. The explanation (of at least the post-Newtonian part) of the flattening of elliptical galaxies attempted here seems to be the first theoretical one proposed in the literature.
pp 17-31 March 1987
N-Body simulations have been performed to study the tidal effects of a primary stellar system on a secondary stellar system of density close to the Roche density. Two hyperbolic, one parabolic and one elliptic encounters have been simulated. The changes in energy, angular momentum, mass distribution, and shape of the secondary system have been determined in each case. The inner region containing about 40 per cent of the mass was found to be practically unchanged and the mass exterior to the tidal radius was found to escape. The intermediate region showed tidal distension. The thickness of this region decreased as we went from hyperbolic encounters to the elliptic encounter keeping the distance of closest approach constant. The numerical results for the fractional change in energy have been compared with the predictions of the available analytic formulae and the usefulness and limitations of the formulae have been discussed.
pp 33-50 March 1987
The interpretation of the observed relation between median angular sizes (θm) of extragalactic radio sources and flux density at 408 MHz has been examined. The predictedθm-S relations based on well-observed strong sources in parent samples selected at 178 and 1400 MHz, and existing models of the evolving radio luminosity function can be made to fit the observed relation only by invoking cosmological evolution in linear sizes even for theq0 = 0 universe. Predictions based on a parent sample at 2.7 GHz are shown to overestimate the contribution of steep-spectrum, compact (SSC) sources in low-frequency samples unless the downward curvature in the spectra of such sources is taken into account. When approximate corrections are made for this effect, predictions based on the 2.7 GHz parent sample cannot obviate the need for linear size evolution as claimed in the literature.
pp 51-56 March 1987
One hundred and twentythree radial velocities for α Cyg are derived between December 1977 and October 1982. These photospheric velocities are derived from NI lines near 8700 å. Semiregular variations in radial velocities are present with periods of 7 to 20 days. The range of variation of 14.3 kms−1 observed in the present radial velocities of α Cyg is close to the sum of the amplitudes (10.44 kms−1) of all the pulsation periods from 7 to 101 days (Lucy 1976a) and is also approximately equal to micro and macro-turbulent velocities.
pp 57-68 March 1987
Using a first approximation to the Hubble space telescope point-spread function, the problems concerning observation and reduction of globular clusters in the Andromeda nebula are analysed through a simulation procedure in order to study the feasibility of accurate photometry of individual stars in the clusters.
pp 69-78 March 1987
The Ooty data on VHE (> 600 GeV) γ-rays from the Crab pulsar have been used to look at possible emission on three different timescales,viz. years, minutes and hours. When averaged over three years of observation, there is no significant time-averaged emission. But interesting aspects are revealed when the data are subdivided into miniruns of one-minute duration. Minutes with moderate γ-ray activity were isolated with a κ2 analysis. The summed phasogram of such minutes shows two strong peaks coinciding with the radio main pulse and the interpulse respectively. The probability that these are due to chance is small. The phasogram has a bimodal distribution which indicates some emission from the middle region between the two phases. In general, there is no significant emission on the timescale of a few hours. However, the main pulse, as well as the bimodal distribution, were seen also in the total data of two nights of simultaneous observation at two sites during 1984–1985.
Volume 41, 2020
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