• Volume 10, Issue 2

      June 1989,   pages  139-235

    • CF octantis, an RS CVn-type variable with a prominent activity cycle

      K. R. Pollard J. B. Hearnshaw A. C. Gilmore P. M. Kilmartin

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      Photoelectric photometry has been obtained for CF Octantis on 39 nights. The object is a member of the RS CVn class and has a highly active chromosphere. A 20.15 ±0.06 d period has been found from theB andV magnitudes, which is interpreted as the rotational modulation of the light from a non-uniformly spotted star. The amplitude of this spot wave is observed to vary slowly between ΔV about 0m.l and 0m.3, which may be evidence for an activity cycle of 9 ±3 years.

    • [OIII] Electron temperatures in planetary nebulae

      F. P. Keenan K. M. Aggarwal

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      Relative level populations for O III, derived using electron impact excitation rates calculated with the R-matrix code, are used to deduce the electron-temperature-sensitive emission-line ratioR=I(2s22p21D–2s22p21S)/I(2s2 2p23P1,2–2s22p21D) =I(4363 Å)/I(4959 + 5007 Å) for a range ofTe = (7500–20000 K) applicable to planetary nebulae. Electron temperatures deduced from the observed values ofR in several planetary nebulae are in excellent agreement with those determined fromTe-sensitive line ratios in other species, including CIII]/C [II], [NII] and [ArIII], which provides support for the accuracy of the atomic data adopted in the level population calculations.

    • The reclassification of the suspected R coronae borealis star SY hyi as a semiregular variable

      W. A. Lawson P. L. Cottrell A. C. Gilmore P. M. Kilmartin

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      SY Hyi is classified as a suspected R Coronae Borealis-type variable star. Photometric and spectroscopic observations of SY Hyi lead to the star’s reclassification as a semiregular variable of spectral type M5-6.

    • Bond dissociation energies and bond orders for some astrophysical molecules

      R. R. Reddy R. Viswanath

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      The bond dissociation energies for astrophysically important diatomic molecules have been estimated based on the derived relation DAB = DAB + 32.058 Δχ where DAB = (DAA DBB)1/2, Δχ represents Pauling electronegativity difference χA – χB) Based on the formula suggested by Reddyet al., bond orders are estimated. The ambiguity arising from Parr & Borkman relation is discussed. The present study supports the view of Politzer thatq/(0.5re)2 is the correct definition of bond order. The estimated bond energies are in reasonably good agreement with the values in the literature. The bond energies estimated with the relation we suggested, for the molecules in the present study, give an error of 8.0 per cent. The corresponding error associated with Pauling’s equation is 26.8 per cent.

    • Thick radio shell in supernova remnant DA 495 (G 65.7+ 1.2)?

      T. Velusamy R. H. Becker W. M. Goss D. J. Helfand

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      High resolution (11.7 to 64 arcsec) multifrequency radio observations of DA 495 (G 65.7 +1.2) at 20, 49 and 92 cm wavelengths with the VLA, WSRT and OSRT are presented. A map of infrared emission in the surrounding region is also presented. Although the remnant has a minimum near the centre, its brightness distribution does not show limb brightening. Hence it has characteristics of both shell-type SNRs and Crab-type SNR. Alternatively it has an unusually thick shell with inner and outer radii of 125 and 500 arcsec respectively. The volume emissivity within the shell decreases radially outward. Its integrated flux density spectrum is characteristic of a shell-type remnant. The role of the reverse shock in the formation of a thick shell is discussed. The association of the open star cluster NGC6834 with DA 495 is also noted. The parameters of twenty-nine small diameter sources detected around DA 495 are also presented. The radio source 1949 + 291 is associated with an IRAS source. 1948 + 292 may be a Crablike pulsar-driven synchrotron nebula. The radio spectrum of the planetary nebula NGC 6842 is optically thick at 49 cm.

    • Radial distribution of the integrated light and photometric colours in open star clusters

      Ram Sagar Harish C. Bhatt

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      Mass segregation in the form of preferential concentration of more massive stars in the central regions of a number of open star clusters has been known for some time. In this paper, integratedUBV colours in concentric zones have been estimated for 12 nearby open clusters using the observations of individual cluster members. It is found that the clusters showing pronounced mass segregation also show significant radial variations in the integrated colours. However, the effects of stochastic fluctuations around the massive portion of the mass distribution function on the integrated colours should be taken into consideration, if they are present.

    • Giant CP Stars?

      L. O. Lodén A. Sundman

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      This study is part of an investigation of the possibility of using chemically peculiar (CP) stars to map local galactic structure. Correct luminosities of these stars are therefore crucial. CP stars are generally regarded as main-sequence or near-main-sequence objects. However, some CP stars have been classified as giants.

      A selection of stars, classified in literature as CP giants, are compared to normal stars in the same effective temperature interval and to ordinary ‘non giant’ CP stars. We find no clear confirmation of a higher luminosity for ‘CP giants’, than for CP stars in general. In addition, CP characteristics seem to be individual properties not repeated in a component star or other cluster members.

    • Some properties of an intrinsically degenerate Hamiltonian system

      N. Caranicolas

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      Using theoretical arguments we study some properties of a time independent, two-dimensional Hamiltonian system in the 1:1 resonance case. In particular we find the frequency of the oscillation near the elliptic points, the ratio of the semiaxes of the ellipses surrounding them and the inclination angle of the asymptotes of the hyperbolic orbits in the original variablesx–px. The results are in satisfactory agreement with those given by numerical integration of the equations of motion. Some possible applications for the motion of stars in elliptical galaxies are also discussed.

    • Extragalactic sources with very asymmetric radio structure: VLA and MERLIN observations

      D. J. Saikia P. Shastri T. J. Cornwell W. Junor T. W. B. Muxlow

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      As part of our study to understand the nature of extragalactic radio sources which are very asymmetric in the surface brightness of the two lobes, often with radio emission on only one side of the nucleus, we have observed a large number of them with high angular resolution and good surface brightness sensitivity at radio frequencies. In this paper we present VLA and MERLIN observations of 15 such sources. We discuss their observed structures and spectra, and possible explanations for their morphologies. We report evidence of a possible correlation between the hot-spot brightness ratio and the degree of core prominence, used as a Statistical measure of source orientation, suggesting that relativistic beaming of the hot-spot emission does play a significant role in the observed brightness asymmetry. To explain the apparently one-sided sources within the relativistic beaming framework, the velocities required are in the range of 0.2 to 0.8c. We discuss the possibility that the lobe which is seen to the south of the jet in 3C273 is the counter-lobe seen in projection. We also draw-attention to a number of one-sided sources with very weak cores, and discuss their possible nature.

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    • Continuous Article Publication

      Posted on January 27, 2016

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