• Volume 3, Issue 3

      September 1982,   pages  217-361

    • M. K. Vainu Bappu (1927-1982)

      V. Radhakrishnan

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    • The eclipsing binary V 836 Cygni: Photometric evidence for an early evolutionary status

      R. A. Breinhorst H. W. Duerbeck

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      ThreeUBV light curves of the short-period EB-type eclipsing binary V 836 Cyg, observed in 1971, 1976 and 1980, are presented. The instability of the light curve found by previous authors is confirmed. However, the activity was not very strong between 1971 and 1980. From the observed period variation, previous light-curve analysis data and spectroscopic data it is concluded that V 836 Cyg consists of two main-sequence stars, and will undergo rapid mass exchange before terminating the main sequence (case A evolution).

    • The spectroscopic orbit of the short period eclipsing binary V 836 Cygni

      H. W. Duerbeck J. D. Schumann

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      Spectroscopic elements of the single-lined spectroscopic binary V 836 Cyg are presented.

    • On selection effects in pulsar searches

      M. Vivekanand R. Narayan V. Radhakrishnan

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      Selection effects are a major source of error in statistical studies of pulsar data since the observed sample is a biased subset of the full galactic pulsar population. It is important to identify all selection effects and make a reasonable model before attempting to determine pulsar properties. Here we discuss a hitherto neglected selection effect which is a function of the periodP of the pulsar. We find that short-P pulsars are more difficult to detect, particularly if their dispersion measures are high. We also discuss a declination-dependent selection effect in the II Molonglo Survey (II MS), and find some evidence for the existence of both selection effects in the pulsar data from this survey. We discuss the implications of these additional selection effects for the recently proposed ‘injection’ of pulsars whereby pulsars seem to switch on only at longerP. Using the II MS data we calculate that the observability of pulsars withP between 0.0 s and 0.5 s is about 18 per cent less with the new selection effects than hitherto believed; the mean correction is 6 per cent forP between 0.5 s and 1.0 s. We conclude that injection is not qualitatively affected by these corrections.

    • Eclipse observations of coronal emission lines. I. [Fex] 6374Å profiles at the eclipse of 16 February 1980

      Jagdev Singh M. K. V. Bappu A. K. Saxena

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      Coronal spectra during the total solar eclipse of 1980 February 16, were obtained in the 6374Å [Fex] line using a multislit spectrograph. These spectra have a dispersion of 2.5 Å mm-1. The observed line profiles from 1.1 to 1.7 R with a spatial resolution of 10 × 22 arcsec2, give half-widths that vary between 0.6 Å and 2.4Å. A large number of locations have half-widths around 1.3 Å corresponding to a temperature of 4.6 × 106 K. If temperature of the order of 1.3 × 106 K are typical of the regions that emit [Fex], then turbulent velocities of ~ 30 km s-1 need to be invoked for the enhanced line broadening. The line-of-sight velocities measured range between +14 km s-1 to -17 km s-1. Most of the locations have velocities less than ±5 km s-1. From these observations we conclude that corona does not show any localized differential mass motion and that it co-rotates with the photospheric layers deeper down.

    • HD 147010: The Ap star in the reflection nebula vdB 102 in upper Scorpius region

      N. Kameswara Rao R. Rajamohan

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      HD 147010, which is in the reflection nebula vd B 102, has been found to be a hot Ap silicon star rather than a normal A supergiant. From theUBV and JHK photometry of the star, colour excessE(B - V) of 0.29 and the ratio of total-to-selective absorptionR of 4.3 have been obtained. The high value ofR implies bigger grain size and also confirms the association of the star with the nebula. The dereddened colours of the star can be fitted to a theoretical energy distribution with Teff = 13000 ± 500 K and logg = 3.6 ± 0.2. HD 147010 has also been found to be a spectrum variable; in particular, lines of Cr II show large intensity variations.

    • Chromospheric activity of late-type giants and supergiants: Reappearance of dynamo activity in the interior due to the spin-up of the core in evolution

      Yutaka Uchida M. K. V. Bappu

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      The renewal of chromospheric activity in red giants and supergiants is interpreted in terms of the reappearance of dynamo activity in the interior due to the spin-up of the core caused by its contraction in the course of evolution from the main sequence to the giant stage. A region of very high rotational shear (differential rotation) develops between the core, which spins up by a large factor through the drastic contraction, and the envelope, which spins down in contrast by virtue of expansion. Mechanisms of angular momentum transfer may operate to smear this large shear, and bring the inner part of the envelope into sheared rotation. A convective layer, on the other hand, develops in the envelope from the surface inwards, when the envelope expands and the temperature is lowered. A dynamo layer, or a layer in which the sheared rotation co-exists with the convection (the presence of a remnant magnetic field being postulated), will thus reappear in the inner part of the envelope when the envelope-convection reaches down and invades the layer of sheared rotation. Surface chromospheric activity due to the magnetic field is thus renewed when the regenerated magnetic field is brought up to the surface by the envelope-convection. These phenomena occur as the star evolves into the giant stage and hence explain the observed characteristic of gradual revival of chromospheric activity from the subgiant to the giant stage.

    • On the Balmer progression in the expanding shell of Pleione

      A. Peraiah

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      An attempt has been made to explain the Balmer progression in velocities seen in Pleione and other shell stars. Equations of conservation of mass and momentum are simultaneously solved with assumptions which simplify the calculations of the solution. We have considered the radiation pressure due to lines and continuum. It is found that the high-excitation lines are formed where the velocity gradients are high.

    • Intensity and polarization line profiles in a semi-infinite Rayleigh-scattering planetary atmosphere. I. Integrated flux

      R. K. Bhatia K. D. Abhyankar

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      Absorption and polarization line profiles as well as the curves of growth in the integrated light of a planet over the whole range of phase angles have been computed assuming a semi-infinite atmosphere scattering according to Rayleigh’s phase-matrix which takes polarization into account. The relative change in line depth and equivalent widths qualitatively agree with the observations of the CO2 bands in Venus reported by Young, Schorn and Young (1980). It is pointed out that the bands might be formed in a part of the atmosphere which is different from that where continuum polarization originates.

    • Exact relativistic model for a superdense star

      P. C. Vaidya Ramesh Tikekar

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      Assuming that the physical 3-spacet = const in a superdense star is spheroidal, a static spherically symmetric model based on an exact solution of Einstein’s equations is given which will permit densities of the order of 2 × 1014 gm cm-3, radii of the order of a few kilometers and masses up to about four times the solar mass.

    • Further observations of radio sources from the BG survey. II. Mainly extragalactic

      F. Mantovani M. Nanni C. J. Salter P. Tomasi

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      New observations of six fields from the low galactic latitude BG survey are presented. The radio sources within each of these fields appear to be of a predominantly extragalactic nature. However, one object is in close positional proximity to the pulsar 1922 + 20 and the possibility of their association is considered. The extended source BGE 0323 + 65 is shown to be unlikely to exist.

    • A new look at the birthrate of supernova remnan

      G. Srinivasan K. S. Dwarakanath

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      We have reanalysed a homogeneous catalogue of shell-type supernova remnants and we find that the radio data are consistent with a birthrate of one in 22±3 yr. Our approach is based on the secular decrease of surface brightness of the historical remnants whose ages are precisely known. The abovementioned birthrate is significantly higher than most previous estimates which range from one in 50–150 yr, and is consistent with the supernova rate in our galaxy derived from historical observations, as well as with recent estimates of the pulsar birthrate.

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