• Volume 2, Issue 2

      June 1981,   pages  119-213

    • A model of the Algol type close binary TT hydrae

      A. G. Kulkarni K. D. Abhyankar

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      UBV photometric observations and elements of TT Hydrae obtained by Kulkarni and Abhyankar (1980) are combined with the radial velocity curve of Popper (1979, personal communication) to derive the absolute dimensions and. a model of this important Algol system. While the photometric ratios of radii inV andB are in agreement givingk = 0.3812 for a limb darkening coefficient ofx = 0.6, application of Irwin’s (1947) method givesx = 0.4 forU. The primary is found to be a main sequence Al V star of mass 2.61M and radius 2.01 R, and the secondary is classified as a Kl III star of mass 0.70M and radius 5.33R. The observed Fourier coefficients for the light outside the eclipse agree with those calculated from theory for the reflection and ellipticity effects. The system shows an ultraviolet excess of 0.5 to 0.6 magnitudes during primary eclipse, which is attributed to an asymmetric circumstellar distribution of matter around the primary. The evolutionary status of the secondary, which does not appear to fill its Roche lobe completely, is discussed.

    • Time-dependent interaction of granules with magnetic flux tubes

      P. Venkatakrishnan S. S. Hasan

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      The time-dependent interaction of the granulation velocity field with a magnetic flux tube is investigated here. It is seen that when a magnetic field line is displaced normal to itself so as to simulate thebuffeting action of granules, a flow of gas is initiated along the field. By choosing a lateral velocity field which is consistent with observations of granules, it is found that the resulting gas motion is a downward flow with a velocity compatible with the observed downflow in isolated photospheric flux tubes. It is therefore proposed that the observed photospheric downflow is a manifestation of the interaction of granules with flux tubes.

    • Photographic spectrophotometry of selected bright B and Be Stars

      Elaine M. Hendry J. D. R. Bahng

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      Coudé spectra taken in the red region of 49 bright B and Be stars were examined for the behaviour of Hα and the He I λλ 5876 and 6678 line profiles. 51 per cent of the stars were found to show Hα emission to some degree and 22 per cent showed helium emission. Evidence is presented for the existence of drastic changes in the line profile and the radial velocity in some of the stars on a time scale of fractions of a day.

    • Anomalous giants in Magellanic Cloud clusters

      P. W. Hodge

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      Many of the populous intermediate-age star clusters (theblue globulars) of the Magellanic Clouds have a few stars that occupy a “forbidden zone” in the colour-magnitude diagrams of the clusters, where stellar evolutionary theory does not predict that stars should lie. The properties of these stars in 13 clusters are gathered in Table 1, where it can be seen that the absolute magnitudes of the stars correlate with the cluster age, lying at a mean value of 2.7 mag above the tip of the main sequence. Several hypotheses regarding their nature are discussed, and it is concluded tentatively that they represent an unpredicted advanced stage of evolution from the giant branch, though no theoretical evidence supports such a conclusion. Considerably more observational data are needed before a more positive statement can be made.

    • How high can a mountain be?

      P. A. G. Scheuer

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      The possible height of a mountain on a solid self-gravitating object such as a planet or a neutron star is limited by the strength of the rock. Estimates of the limiting height and conditions for their validity are discussed.

    • Current rate of nucleosynthesis and its implications

      D. C. V. Mallik

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      The current rate of nucleosynthesis in the solar neighbourhood is re-evaluated on the basis of Arnett’s (1978) stellar yields, the mass loss models of Chiosi, Nasi and Sreenivasan (1978) and the initial mass function determined by Lequeux (1978). If massive stars are held responsible for most of the metals we observe, a higher birthrate of these stars in the past is indicated in view of the low current rate of nucleosynthesis. The intermediate mass stars may not supply the bulk of the metals unless total disruption of their carbon core takes place.

      While a declining birthrate is in conflict with the result obtained from the age-metallicity relation of stars, it is supported by some galactic evolution models which interpret successfully the white dwarf mass distribution data. If the constraint of a nearly time-invariant birthrate were strictly accepted, then models of the prompt initial enrichment type are required to explain the observed abundances in terms of nucleosynthesis in massive stars.

    • Intersteller masers: The influence of the geometrical shape on the radiation properties

      Erich Bettwieser

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      The relation between the minimum and the maximum of the energy density of the microwave field is important in determining the internal physical conditions in a maser source and is directly connected with the size of the emission spot. This relation is investigated for models of homogeneous maser clouds for three different geometries: a thin tube, a thin disk and a sphere.

      For substantial degrees of saturation, an approximate analytical calculation scheme is presented. The radiation properties found satisfy the transfer and the rate equations well. As expected, the analysis supports the long standing view that the active medium is 10 to 100 times larger than the interferometer size. Here the real purpose is to provide some insight into the dependence of the radiation characteristics on the geometrical shape of the emission region. The luminosity is only slightly affected by the shape of the emitting region (for the same volume). However, the angular variation of the intensity and the peak intensity of the rays reflect sensitively on the geometrical shape.

    • Long term variations in solar flare activity

      J. N. Goswami R. Jha D. Lal

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      A statistical analysis of the contemporary (1954-1975) solar flare particle events has been made for the parametersF (integrated, proton fluence in cm-2 in an event with kinetic energy above 10 MeV) andR0 (the characteristic rigidity). These data are compared with the long-term averaged values determined from stable- and radio-nuclide measurements of lunar samples. The analysis shows that the ancient solar flare proton spectrum was harder (higher R0 values) compared to that observed in contemporary flares. A similar analysis can not be made for the mean long-term averaged flux (¯J, cm-2 S-1), since the contemporary averages suffer from an uncertainty due to the statistics of a single event. However, the average flux estimates for time durations 〈T〉 exceeding 103 yr, are free from such uncertainties. The long-term averaged ¯J values obtained over different time scales (104 - 106 yr) suggest a possible periodic variation in solar flare activity, with enhanced flux level during the last 105 yr. The available data rule out the occurrence of giant flares, with proton fluence exceeding 1015 cm-2 during the last million years.

    • A spectroscopic orbit for 26 Comae

      R. F. Griffin

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