• Volume 1, Issue 2

      December 1980,   pages  97-196

    • Comments on the source function equality in (Zeeman)-multiplets

      L. G. Stenholm R. Wehrse

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      The conditions for the source functions of a multiplet to be equal are studied for plasmas with and without magnetic fields. It is found that source function equality holds—in addition to the case of collisional predominance—only when the redistribution functions are all identical and no interlocking with other lines occurs. When magnetic fields are present, the assumption of source function equality leads to a violation of the invariance conditions of the scattering matrix and should therefore not be made.

    • An iterative simultaneous solution of the equations of statistical equilibrium and radiative transfer in the comoving frame

      A. Peraiah

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      A direct iteration has been performed to obtain a simultaneous solution of the equations of line transfer in an expanding spherically symmetric atmosphere in the comoving frame with statistical equilibrium for a non-LTE, two-level atom. The solution converges in three or four iterations to an accuracy of 1 per cent of the ratio of the population densities of the two levels. As initial values, the upper level population was set equal to zero or to the LTE densities. The final solution on convergence indicates enhanced population of these levels over the initial values assumed, Large velocity gradients enhance this effect whereas large geometrical sizes of the atmospheres tend to reduce it.

    • Effects of high velocities on photoionization lines

      A. Peraiah G. Raghunath

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      We have treated formation of spectral lines in a commoving frame where photoionization is predominant over collisional processes. We have assumed that the radiation field for causing photoionization is a function of Planck function. We have also considered the situation in which the continuum contributes to the radiation in the line. In all the models the quantityB/A (ratio of outer to inner radii) is kept equal to 10 and the total optical depth is taken to be 103. The velocity is assumed to be varying according to the lawdV/dτ ∼ < 1/τ whereτ is the optical depth (τ > 0) in the given shell. The velocities at the innermost radius (r =A) are set equal to 0 and at the outermost radius (r =B), the maximum velocities are taken to be 0, 1, 3 and 10 Doppler units. The calculated line profiles are those seen by an observer at infinity.P Cygni-type profiles are observed in the case of a medium with no continuum absorption. For a medium with continuum absorption double peaked asymmetric profiles are noticed when the velocities are small; the two emission peaks merge into a single asymmetric peak for larger velocities.

    • The structure of integrated pulse profiles

      M. Vivekanand V. Radhakrishnan

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      We offer two possible explanations to account for the characteristics of integrated pulse profiles, in particular their degree of complexity, their variation from pulsar to pulsar, their stability, and the tendency of complex profiles to be associated with older pulsars.

      It is proposed that the pulse structure could be a reflection of surface irregularities at the polar caps, and it is shown how the surface relief can affect the number of positrons released into the magnetosphere which are subsequently responsible for the observed radio radiation. The electrons produced in the vacuum break-down in the gap carry enough energy to allow creating such a surface relief in ∼ 106 years, and one way in which this could be achieved is discussed.

      Alternatively, the presence of multipole components in the magnetic fields of older pulsars could lead to significant variations in the curvature of the field lines across the gap, and hence to structure in the integrated pulse profiles. An assessment of the two hypotheses from observed pulse profiles seems to favour the polar cap relief picture.

    • Central regions of Sérsic-Pastoriza galaxies: A photographic study

      T. P. Prabhu

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      A classification scheme is proposed for the central regions of Sérsic-Pastoriza galaxies based on high resolution photographs of 50 objects in the integrated light (4000 Å-8700 Å). Structures of two different linear scales are recognized: (1) nucleus(≲1 kpc) and (2) perinuclear formation (∼l.5 kpc). The perinuclear formation is weak in classκ while the nucleus is too faint to detect in class ι. In the intermediate classesε andσ both the components are bright. Classε has an elliptical perinuclear formation with little gas while the classσ consists of bright H II complexes and dust. Observations of a few galaxies in the infrared and the blue ends of the image tube response show that the nucleus is generally redder than its surroundings. Equal intensity contours and the luminosity profiles are presented for the central regions of 27 galaxies. A comparison of their axial ratios with those of the parent galaxies indicates that the perinuclear formations are prolate or barlike. The dependence of the peak surface brightness of the central formation on the size of the bar is investigated as also the dependence of the central surface brightness of the bar on the size of the bar. The following major conclusions are drawn:

      1. The peak central surface brightness of the perinuclear formation varies as the square of the bar length. This relation implies that the bar induces the infall of gas from the bar-disk region.

      2. The formations of classσ move towards classε as star formation ceases and the massive stars die.

      3. The classι differs from classσ in the intensity of the burst of star formation. Low luminosity of the parent galaxies in classι implies less infall of gas and higher domination of the bar potential on the perinuclear formation. Thus the classι structures are more prolate than those of classσ.

      4. The central brightness of the bars varies directly as the length of the bar.

    • Supernovae and the Ap phenomenon

      R. Rajamohan Ashok K. Pati

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      We put forward evidence that relates the phenomenon of the chemically peculiar stars of the upper main sequence to Supernovae explosions in young clusters. In the Upper Scorpius region we find that a supernova shell has interacted with dense clouds and that the peculiar stars lie close to or along the edges of interaction.

      We argue that the stars at or near the cloud faces are capable of acquiring this enriched material which is sufficiently slowed down. The magnetic accretion process of Havnes and Conti provides the mechanism of acquisition. This process with the associated magnetic braking accounts for the build-up in abundance anomalies and the slowing down of rotation with age.

    • Convective instability in the solar envelope

      D. Narasimha S. K. Pandey S. M. Chitre

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      The characteristics of the most unstable fundamental mode and the first harmonic excited in the convection zone of a variety of solar envelope models are shown to be in reasonable agreement with the observed features of granulation and supergranulation.

    • NGC 4650 A: A nearly edge-on ring galaxy?

      S. Laustsen R. M. West

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      The peculiar galaxy NGC 4650 A (α=12h 42m. 1; = δ—40° 26′; 1950·0) has been studied by means of direct and spectral observations with the ESO 3·6-m telescope. It is interpreted as a prolate, elliptical galaxy surrounded by a warped ring of H II regions, dust and stars. The distance is 47 Mpc (H0=55 km s−1 Mpc−1). The ring is seen nearly edge-on (inclination 85°) and it rotates. It has a diameter of about 21 kpc and is bluer than the elliptical galaxy for which the (M/Lv) ratio is ∼12 in solar units. The observed configuration may be the result of interaction with the nearby galaxy, NGC 4650.

    • No detectable supernova remnant near the pulsar PSR 1930 + 22

      W. M. Goss D. Morris

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      No supernova remnant has been found near the third youngest pulsar PSR 1930+22 down to a -limiting brightness temperature of 1·4 K at 610 MHz. This is 6–8 times less than expected of a typical remnant whose age is that of the pulsar (3·60×104 years).

    • A new spectroscopic facility at millimetre wavelengths

      A. Baudry J. Brillet J. M. Desbats J. Lacroix G. Montignac P. Encrenaz R. Lucas G. Beaudin P. Dierich A. Germont P. Landry G. Rerat

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      A new millimeter-wave facility is in operation at the Bordeaux Observatory for spectroscopic observations of interstellar and stratospheric molecules. A cooled receiver has been installed on a 2·5m radio telescope. The overall system temperature is in the range 400 to 600 K (single side band) in the operating frequency range 75 to 115 GHz. The relatively broad beam of the telescope (∼ 5 arcmin) combined with a sensitive receiver will permit studies of extended molecular cloud complexes.

  • 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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