Showing papers in "Journal of Astrophysics and Astronomy in 1990"
TL;DR: In this article, a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′N).
Abstract: This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping. This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ- 14°. 1). The sensitivity of the survey is 5 Jy/beam (1σ). Special care has been taken to ensure that the antenna responds to all angular scale structures and is suitable for studies of both point sources and extended objects
35 citations
TL;DR: In this paper, the authors present results of model calculations of the evolution of surface brightness and morphology of supernova remnants containing pulsars, and discuss how the morphology of a supernova remnant will depend on the velocity of expansion, the density of the ambient medium into which it is expanding, and the initial period and magnetic field strength of the central pulsar.
Abstract: One of the intriguing aspects of supernova remnants is their morphology. While the majority of them look like hollow shells, a few, called plerions, are centrally filled like the Crab nebula, and some have a shell-plerion combination morphology. The centrally-filled component in these remnants is believed to be powered by a central pulsar. In this paper we present results of model calculations of the evolution of surface brightness and morphology of supernova remnants containing pulsars. We discuss how the morphology of a supernova remnant will depend on the velocity of expansion, the density of the ambient medium into which it is expanding, and the initial period and magnetic field strength of the central pulsar
28 citations
TL;DR: A 128-channel digital correlation receiver has been built for the GEETEE 1, the low-frequency radio telescope situated at Gauribidanur, South India, (latitude 13°36′12′ N) as mentioned in this paper.
Abstract: A 128-channel digital correlation receiver has been built for the GEETEE 1, the low-frequency radio telescope situated at Gauribidanur, South India, (latitude 13°36′12′′ N). The receiver uses a modified doublesideband (DSB) technique. The quadrature samples required for a DSB system are obtained by sampling the digitized intermediate frequency (I.F.) signals by two clocks which are separated in time by one quarter of the period of the I.F. The visibilities required for one-dimensional synthesis are measured using one-bit correlators. A technique to measure amplitude information for the signal using a threshold detector and a one-bit correlator has been developed. The receiver has been successfully used for continuum, spectral-line and pulsar observations. The antenna system of GEETEE and its configuration for one dimensional synthesis are also described in this paper
8 citations
TL;DR: In this paper, the potential energy curves for the X 2 Σ+ and B 2 ǫ+ states of CO+ have been constructed by the Rydberg-Klein-Rees (RKR) method as modified by van der Sliceet al. The dissociation energy is estimated to be 7.70± 0.19 eV by the method of curve fitting using the five parameter Hulburt-Hirschfelder's function.
Abstract: The potential energy curves for theX 2 Σ+ andB 2 Σ+ states of CO+ have been constructed by the Rydberg-Klein-Rees (RKR) method as modified by van der Sliceet al. The dissociation energy is estimated to be 7.70±0.19 eV by the method of curve fitting using the five parameter Hulburt-Hirschfelder’s function. The estimated value is in good agreement with the value (7.839 eV) given by Misraet al. Carefull observation of the results reveals that accurateD 0 value for CO+ is 8.33 eV
8 citations
TL;DR: A comprehensive survey of bright composite-spectrum binaries in the northern sky has provided so many radial-velocity data that orbits can now be determined for many objects whose orbits were hitherto unknown or else insecure or actually erroneous as mentioned in this paper.
Abstract: A comprehensive survey of bright composite-spectrum binaries in the northern sky has provided so many radial-velocity data that orbits can now be determined for many objects whose orbits were hitherto unknown or else insecure or actually erroneous. Elements are given for the orbits of 30 such objects, thereby more than doubling the number of composite-spectrum binaries with known orbits
7 citations
TL;DR: In this paper, the effect of the eccentricity of a planet's orbit on the stability of the orbits of its satellites is studied in the elliptic hill case of the planar restricted three-body problem.
Abstract: The effect of the eccentricity of a planet’s orbit on the stability of the orbits of its satellites is studied. The model used is the elliptic Hill case of the planar restricted three-body problem. The linear stability of all the known families of periodic orbits of the problem is computed. No stable orbits are found, the majority of them possessing one or two pairs of real eigenvalues of the monodromy matrix, while a part of a family with complex instability is found. Two families of periodic orbits, bifurcating from the Lagrangian points L1, L2 of the corresponding circular case are found analytically. These orbits are very unstable and the determination of their stability coefficients is not accurate, so we compute the largest Liapunov exponent in their vicinity. In all cases these exponents are positive, indicating the existence of chaotic motions
7 citations
TL;DR: In this paper, the times of minimum derived from photometry obtained in 1963, 1967, 1978, and 1979 are presented, with these and previously published times, the period is studied.
Abstract: Times of minimum derived from photometry obtained in 1963, 1967, 1978, and 1979 are presented. With these and previously published times, the period is studied. A constant period increase (quadratic ephemeris) represents most of the times but there was a period decrease in 1934 and possibly in 1984. A 50-year magnetic cycle is discussed.
7 citations
TL;DR: In this paper, moderate-resolution spectra of the C2 Swan 0-1 bandhead, the Na I D lines and the KI resonance lines near 7660 A obtained at minimum light during the 1988-1989 decline of R CrB are discussed and interpreted in terms of a popular model for r CrB declines.
Abstract: Moderate-resolution spectra of the C2 Swan 0-1 bandhead, the Na I D lines and the KI resonance lines near 7660 A obtained at minimum light during the 1988–1989 decline of R CrB are discussed and interpreted in terms of a popular model for R CrB declines. High-resolution spectra obtained at maximum light show blue-shifted chromospheric emission in the cores of the Na I D and the Sc π 4246.8 A lines
6 citations
TL;DR: In this paper, the in-plane motion of a Geosynchronous satellite under the gravitational effects of the sun, the moon and the oblate earth has been studied and the radial deviation (Δr) and tangential deviation (r cΔθ) have been determined.
Abstract: The in-plane motion of a Geosynchronous satellite under the gravitational effects of the sun, the moon and the oblate earth has been studied. The radial deviation (Δr) and the tangential deviation (r cΔθ) have been determined. Herer c represents the synchronous altitude. It has been seen that the sum of the oscillatory terms in Δr for different inclinations is a small finite quantity whereas the sum of the oscillatory terms inr cΔθ for different inclinations is quite large due to the presence of the low-frequency terms in the denominator
6 citations
TL;DR: In this paper, it was shown that the spectrum between 1600 and 1800 ∢ is consistent with its arising from a late-A main-sequence companion and that the orbit is of mild eccentricity and has a period of 562 days.
Abstract: 6 Dra has long been known to show small variations in radial velocity, and there is photometric and spectroscopic evidence that its spectrum is composite. We show, largely on the basis of a generous number of photoelectric radial velocities mainly obtained at Cambridge and Fick observatories, that the orbit is of mild eccentricity and has a period of 562 days and a semi-amplitude of 7 km s−1.IUE observations show that the spectrum between 1600 and 1800 ∢ is consistent with its arising from a late-A main-sequence companion
5 citations
TL;DR: In this paper, the authors considered the equilibrium of a magnetofluid disc in Schwarzschild background with an external magnetic field, having the azimuthal and the radial components of the flow velocity nonzero.
Abstract: In this paper we consider the equilibrium of a magnetofluid disc in Schwarzschild background with an external magnetic field, having the azimuthal and the radial components of the flow velocity nonzero. The electrical conductivityσ of the fluid is taken to be finite and thus the solution for the electromagnetic field is required to satisfy the Ohm’s law too with the four-current having onlyJ ϕ andJ t nonzero. The various physical parameters that have to correlate for possible equilibrium configurations are identified and their respective magnitudes estimated. It is found that for a given angular momentum distribution the inner edge of the disc can reach well within the usual6m limit only when the surface magnetic field of the central object is not too high when the matter density at the outer edge of the disc and the accretion rate are taken with reasonable limits
TL;DR: In this paper, an open cluster NGC 2818 containing a planetary nebula has been observed in VRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope.
Abstract: The open cluster NGC 2818 containing a planetary nebula has been observed in VRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to stars V ~ 21 magnitude establishes the distance modulus as (m — M)0 = 12.9 + 0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1) × 10 8 years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor mass of the planetary nebula on the main sequence is 2.5M.
TL;DR: In this paper, the authors discuss the equilibrium configuration of a plasma disc of infinite conductivity around a slowly rotating compact object, and obtain the pressure profiles, and the structure of magnetic field lines for co and counter-rotating discs.
Abstract: In this paper we discuss the equilibrium configuration of a plasma disc of infinite conductivity around a slowly rotating compact object, and obtain the pressure profiles, and the structure of magnetic field lines for co and counter-rotating discs.
TL;DR: In this article, the propagation delay suffered by the HF time signals has been estimated by monitoring the clock error with respect to time signals from five broadcast stations located at distances in the range 500-3700 km from Gulmarg.
Abstract: This paper presents a novel technique to derive the absolute time of occurrence of an event with an accuracy of < 200 µs by adapting the HF time synchronization technique to correct for the aging rate and the frequency drift of a temperature-controlled quartz frequency standard. The propagation delay suffered by the HF time signals has been estimated by monitoring the clock error with respect to time signals from five broadcast stations located at distances in the range 500–3700 km from Gulmarg. Using simulated data, we further show the viability of this technique in permitting periodicity searches on timescales of ≥5 milliseconds in cosmic gamma-ray data
TL;DR: In this paper, the properties of the Newtonian forces acting on a test particle in the field of the Kerr black hole geometry were studied and it was shown that the centrifugal force and the Coriolis force reverse signs at several different locations.
Abstract: We study the properties of the ’Newtonian forces’ acting on a test particle in the field of the Kerr black hole geometry. We show that the centrifugal force and the Coriolis force reverse signs at several different locations. We point out the possible relevance of such reversals particularly in the study of the stability properties of the compact rotating stars and the accretion discs in hydrostatic equilibria
TL;DR: In this paper, the authors used the Very Large Array to image a single field in a set of adjacent frequency bands around 333.0 MHz in an attempt to detect 21 cm emission from large scale H I inhomogeneities at a redshift of z = 3.3 with masses ≃3.5 × 1014 M ⊙ in H I gas and space density exceeding (74 Mpc)−3.3.
Abstract: We have used the Very Large Array to image a single field in a set of adjacent frequency bands around 333.0 MHz in an attempt to detect 21 cm emission from large scale H I inhomogeneities at a redshift of z = 3.3. Following the subtraction of continuum radio sources, the absence of any spectral signals apart from that expected due to the system thermal noise has been used to derive constraints on the evolutionary scenario leading to the formation of the present day clusters of galaxies. The observations rule out the existence of H I protoclusters atz = 3.3 with masses ≃3.5 × 1014 M ⊙ in H I gas and space density exceeding (74 Mpc)−3. This indicates that the present day rich clusters of galaxies either formed as gaseous protocluster condensates prior toz = 3.3 or else they formed through the clustering of their constituent galaxies.
TL;DR: HD 201270/1 is a composite-spectrum binary whose components have spectral types close to G8 III and A2 V as discussed by the authors, and they are in a circular orbit with a period of only 40 days.
Abstract: HD 201270/1 is a composite-spectrum binary whose components have spectral types close to G8 III and A2 V. They are in a circular orbit with a period of only 40 days. Photometric variability of RS CVn character is therefore not unlikely
TL;DR: In this paper, photoelectric radial-velocity measurements show that HD 118670 is a double-lined spectroscopic binary in an orbit which is not quite circular and whose period is about 48 days.
Abstract: Photoelectric radial-velocity measurements show that HD 118670 is a double-lined spectroscopic binary in an orbit which is not quite circular and whose period is about 48 days. Spectral types of K0 V and K7 V would satisfy the photometry and the mass ratio; the mass function would then suggest the possibility of eclipses. However, actual spectral classification indicates a luminosity somewhat above the main sequence
TL;DR: In this article, the space velocities of 61 pulsars were calculated from their tangential velocity vectors, assuming that the orientation of a pulsar's velocity vector is parallel to its spin axis.
Abstract: Assuming that the orientation of a pulsar’s velocity vector is parallel to its spin axis, we have calculated the space velocities of 61 pulsars from their tangential velocities. The mean space velocity of the sample is equal to 267 km s-1. The radial velocities and kinematical ages of 20 pulsars are obtained. The decay time of the magnetic field of pulsars is τD= 2 Myr, smaller than previously found
TL;DR: The intrinsicuvby and HΒ indices of member stars of α-Persei, Pleiades and Scorpio-Centaurus association have been analyzed in detail for rotation effects.
Abstract: The intrinsicuvby and HΒ indices of member stars of α-Persei, Pleiades and Scorpio-Centaurus association have been analysed in detail for rotation effects. These stars range in spectral type from B0 to F0 and the observed effects of rotation are found to be in agreement with photometric effects calculated by Collins & Sonneborn (1977) for rigidly rotating B0 to F0 stars
TL;DR: Differential B-band photometry of BV Dra and BW Dra, obtained in 1965, is presented in this paper, where times of primary and secondary minima are derived and combined with published times for a period study.
Abstract: Differential B-band photometry of BV Dra and BW Dra, obtained in 1965, is presented. Times of primary and secondary minima are derived and combined with published times for a period study. Period variations are found in both binaries and magnetic cycles are proposed as an explanation. The cycle length is 19 years in BV Dra and 8 years in BW Dra.
TL;DR: In this article, the Ooty Radio Telescope has been used to detect 21 cm emission from massive neutral hydrogen condensates at a redshift ofz = 3.3.
Abstract: Observations have been conducted using the Ooty Radio Telescope in order to place constraints on the evolutionary scenario leading to the formation of the present day superclusters. The experiment attempted to detect 21 cm emission from massive neutral hydrogen condensates at a redshift ofz = 3.3. In an Einstein de-Sitter universe with baryon density Ω = 0.05, about ten condensates were expected in the volume surveyed if superclusters, having H I masses ≃5 × 1015 M ⊙, were the first objects to separate out of the Hubble expansion. The sensitivity of our experiment rules out the existence of these condensates atz = 3.3 unless their lifetimes are less than one-tenth the dispersion in their epoch of formation or the proto-superclusters subtend angles greater than 6 arcmin. The result indicates that superclusters form at z > 3.3 if indeed they were the first objects to condense out of the Hubble flow.
TL;DR: In this paper, the specific intensity has been expanded by Taylor series with respect to wavelength and the first three terms have been retained in solving the transfer equation, and it is noted that in a medium stratified in plane parallel layers, the multiple Compton scattering redistributes the initial energy over a range of 3 to 5 Compton wavelengths.
Abstract: We have solved the equation of radiative transfer with Compton scattering. The specific intensity has been expanded by Taylor series with respect to wavelength and the first three terms have been retained in solving the transfer equation. It is noted that in a medium stratified in plane parallel layers, the multiple Compton scattering redistributes the initial energy over a range of 3 to 5 Compton wavelengths. A good fraction of the incident radiation is transferred across the layer with redistribution in wavelength, the actual value depending on the optical thickness of the medium
TL;DR: In this paper, the self-consistency of the EIH equations of motion is examined in the limiting case of a three-body problem where two bodies are very close to each other and a third quite far from them.
Abstract: The self-consistency of the Einstein Infeld and Hoffman (EIH) equations of motion is critically examined in the limiting case of a threebody problem where two bodies are very close to each other and a third quite far removed from them
TL;DR: The equation governing the equilibrium of a centrally-symmetric, self-gravitating distribution of matter is obtained by assuming that matter is described by magnetofluid and it is shown that the equation of stellar structure described by an ideal fluid is recovered when magnetic field is vanishing.
Abstract: The equation governing the equilibrium of a centrally-symmetric, self-gravitating distribution of matter is obtained by assuming that matter is described by magnetofluid and it is shown that the equation of stellar structure described by an ideal fluid is recovered when magnetic fieldh is vanishing
TL;DR: In this paper, a simple modification to the conventional CLEAN algorithm is suggested to ensure both stability and speed when CLEAN is applied to maps containing a mixture of point sources and extended structures.
Abstract: A simple but effective modification to the conventional CLEAN algorithm is suggested. This modification ensures both stability and speed when CLEAN is applied to maps containing a mixture of point sources and extended structures. The method has been successfully applied to the recently-completed sky survey at 34.5 MHz (Dwarakanath & Udaya Shankar 1990). This survey was made using the Gauribidanur T array (GEETEE)1 in 1-D aperture synthesis mode. Since in this case the ‘dirty beam’ (point spread function) cannot be directly computed, a method to obtain this is discussed in detail. The results of this deconvolution procedure have been encouraging in terms of reduced computing time and improved dynamic range in our maps. This algorithm should find wider application in deconvolving maps which have both extended structures and point sources
TL;DR: In this paper, parametric decay instability (PDI) has been applied to a homogeneous and unmagnetized plasma, which causes anomalous absorption of strong electromagnetic radiation under specific conditions of energy and momentum conservation and thus cause anomalous heating of the plasma.
Abstract: The coherent plasma process such as parametric decay instability (PDI) has been applied to a homogeneous and unmagnetized plasma. These instabilities cause anomalous absorption of strong electromagnetic radiation under specific conditions of energy and momentum conservation and thus cause anomalous heating of the plasma. The maximum plasma temperatures reached are functions of luminosity of the radio radiation and plasma parameters. We believe that these processes may be taking place in many astrophysical objects. Here, the conditions in the sources 3C 273, 3C 48 and Crab Nebula are shown to be conducive to the excitation of PDI. These processes also contribute towards the absorption of 21cm radiation
TL;DR: In this paper, the profiles of hydrogen Lyman lines in an expanding spherical atmosphere containing dust and gas were calculated and the variation of equivalent widths with velocities of expansion of the atmosphere, together with the amount of dust present in the medium.
Abstract: We have calculated the profiles of hydrogen Lyman a line in an expanding spherical atmosphere containing dust and gas. We have investigated the variation of equivalent widths with velocities of expansion of the atmosphere, together with the amount of dust present in the medium. We have drawn curves of growth for different velocities and dust optical depths
TL;DR: HD 191262 as discussed by the authors is a double-lined spectroscopic binary consisting of two slightly unequal solar-type stars in an orbit with a period of 5.43435 days.
Abstract: HD 191262 is a double-lined spectroscopic binary consisting of two slightly unequal solar-type stars in an orbit’ with a period of 5.43435 days. The system is both synchronized and circularized and is probably at least as old as the Sun. The inclination of the system is about 45°
TL;DR: In this article, the photometric spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30, and micro-densitometer tracings of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra.
Abstract: Photographic spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30. Microdensitometer tracings of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra. This paper presents these data in an atlas format, discusses the reduction procedures in detail, and presents radial velocity measurements of selected lines in the spectra