• Volume 15, Issue 2

      June 1994,   pages  103-200

    • Preface

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    • XLVIII. The density of white dwarf stars

      S. Chandrasekhar

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      The density of the white dwarf stars is reconsidered from the point of view of the theory of the poly tropic gas spheres, and gives for themean density of a white dwarf (under ideal conditions) the formula ρ=2.162 × 106 × (M/⊙)2.

      The above formula is derived on considerations which are a much nearer approximation to the conditionsactually existent in a white dwarf than the previous calculations of Stoner based on uniform density distribution in the star and which gave for the limiting density the formula ρ=3.977 × 106 × (M/⊙)2.

    • The maximum mass of ideal white dwarfs

      S. Chandrasekhar

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      The theory of thepolytropic gas spheres in conjunction with the equation of state of arelativislically degenerate electrongas leads to aunique value for the mass of a star built on this model. This mass (=0.9I⊙) is interpreted as representing the upper limit to the mass of an ideal white dwarf.

    • Some remarks on the state of matter in the interior of stars

      S. Chandrasekhar

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      It is shown that forall stars for which the radiation-pressure is greater than a tenth of the total pressure, an appeal to theFebmi-Dirac statistics to avoid the central singularity which arises in the discussions of the centrally condensed and the collapsed stars cannot be made. The bearing of this result on the possible state of matter in the interior of stars is indicated.

    • Stellar configurations with degenerate cores

      S. Chandrasekhar

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    • Seismology of the solar convection zone

      Sarbani Basu H. M. Antia

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      An attempt is made to infer the structure of the solar convection zone from observedp-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to find the sound speed in the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the convective flux give significantly better agreement with observations than models constructed using the mixing length formalism. This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in the CM envelope model is within 0.2% of that in the Sun except in the region withr > 0.99R. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observations.

    • Observations of the bright-rimmed molecular clouds near the Cepheus OB2 association

      C. Indrani T. K. Sridharan

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      Massive stars have significant influence on the evolution of the interstellar medium. Bright rims, cometary morphology of clouds, as well as their motion are some examples of the influence of massive stars on nearby molecular clouds. The cometary clouds in the Gum-Vela region are very good examples. In an attempt to understand the kinematics of the clouds in such regions we have carried out CO line observations towards bright-rimmed clouds near the OB Association Cep OB2. The radial velocities of the clouds are consistent with an expansion of the system at ≈ 4kms−1 away from the dominant O6.5V star in the association, HD206267. We find the rocket mechanism to be the most likely cause for expansion as found for both the Gum-Vela and the Rosette globules. We conclude that such expanding motions are quite common in regions near massive stars and make a brief comparison of the Cepheus system with the Gum-Vela system.

    • Orbital elements and evolutionary nature of the long period RS CVn type eclipsing binary RZ eridani

      P. Vivekananda Rao M. B. K. Sarma B. V. N. S. Prakash Rao

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      Light curves of the long period RS CVn type eclipsing binary RZ Eri, obtained during the period 1976–1979 with the 1.2 m telescope of the Japal-Rangapur Observatory are analysed, using Wilson-Devinney method, by fixing the two parametersTh (7400°K) andq(0.963), resulting in the following absolute elements:A = 72.5 ± 1.4R,Rh = 2.84 ± 0.12R,Rc = 6.94 ± 0.20R,Mbol,h = 1.35 ± 0.28,Mbol,c= 1.41 ± 0.28,mh = 1.69 ± 0.6m andmc= 1.63 ± 0.13m. The presence of humps and dips of varying amplitudes at a few phases in the normal UBV light curves is explained as due to residual distortion wave. The derived (B-V) and (U-B) colours of both the components appear to have been reddened to an extent of 0m.20 in (B-V) and 0m.16 in (U-B) colours. This reddening is attributed to the presence of an envelope around the system, the material of which might have come from the loss of mass experienced by the evolving cooler component. Taking into consideration the dereddened colours and temperatures of the components, spectral types ofF0 IV for the primary and G 5–8 III–IV for the secondary component were derived. The fractional radii of 0.039 and 0.096 of the two components, when compared with the radii of their critical Roche lobes of 0.378 and 0.372 suggest that these components are well within their critical sizes. From the position of the components on the. isochrones and the evolutionary tracks of stars of Pop I composition computed by Maeder & Meynet, it is concluded that the evolution of the components of RZ Eri is abnormal. This system is found to be situated at a distance of 185 pc, with an age of about 2.5 × 109 yrs.

    • Probable spectral steepening of UHEγ-rays of nuclear origin and its consequences

      Abhas Mitra

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      Recent accelerator data based parameterization of the inclusive cross section (cs) forπ0 production in hadronic collisions and an explicit incorporation of the finiteness of the relevant projectile hadron spectrum suggest a significant steepening in the spectrum (by as much as 0.4 in the spectral index) of the secondaryγ-ray towards the end of the spectrum. We emphasize here that this spectral steepening in conjunction with the possibility that in the bright X-ray binaries the maximum energy to which theγ-ray producing progenitor protons may be accelerated is only ∼ l0 PeV, may imply an effective efficiency forγ-ray production,ε, as reckoned by the PeV arrays, one or two orders smaller than the previous estimates. To explain the genesis of a given PeV photon flux from an X-ray binary, one, therefore, has to. accordingly consider a much higher value of the progenitor proton beam luminosity,Lp. This requirement may raise further questions regarding the actual genesis of PeVγ-rays in X-ray binaries, or alternatively, on the veracity of the high values of the PeV photon fluxes reported by earlier experiments.

    • Test of evolutionary models for stellar population studies

      N. Srivastva R. K. Gulati N. C. Rana

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      Using the Andersen (1991) data on the mass, absolute visual magnitude, the effective surface temperature and the bolometric corrections, new calibrations for the mass vs. absolute visual magnitude and for the bolometric correction vs. effective surface temperature have been derived. Comparison between the latest evolutionary models with the empirical data has been made to test the evolutionary models for the purpose of stellar population studies.

    • Mass luminosity ratio of nearby stars and its relation to Baryonic dark matter problem

      N. C. Rana Mridula Chandola

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      In view of the recent report on the discovery of low mass halo stars for the candidates of MACHOs, a calculation has been made for the possible enhancement ofM/L ratio for populations of stars of varying mass domains taking the input data from the latest present day mass function (PDMF) of stars. It is seen that there is good scope for explaining dark matter problem where the dark matter is mostly in the form of low mass stars.

    • Metallicity distribution of F-dwarf stars in the solar neighbourhood

      N. C. Rana Atish D. Jana

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      A volume-limited sample of F-dwarfs is chosen from Knude (1989) with low radial velocity as well as distance from the sun and compared with the expected model metallicity [Fe/H] distributions, taking the possible chemical inhomogeneity into account. There is found to be an agreement between the two within the error limits of small sampling.

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