• Volume 8, Issue 2

      June 1987,   pages  79-209

    • Preface

      G. Cayrel de Strobel M. Parthasarathy

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    • Abundances of the very light elements D, He, Li and their cosmological implications

      Jean Audouze

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      The abundances of very light elements D,3He,4He and7Li in low-mass stars and their cosmological implications are discussed.

    • Li production in the big bang

      Douglas K. Duncan L. M. Hobbs

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      Li abundance is determined for 23 halo subdwarfs. About half of the stars show [Fe/H] < −1.4 and a space velocityV > 160 km s−1 Li appears to be present in all our halo stars, with an abundance within about ± 0.2 dex of the value logn (Li) = 2.0 found by Spite & Spite (1982). Thus our results provide confirmation of the main conclusion of Spite & Spite.

    • Recent work on CNO in dwarfs and subdwarfs

      Robert P. Kraft

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      The recent results on CNO in dwarfs and subdwarfs are discussed. The value of [O/Fe] ≃ 0.0 at solar abundances but gradually increases with decreasing [Fe/H]. The value of [C/Fe] is constant at ≃ 0.0−2+1 as [Fe/H] declines from + 0.5 to −2.0; for [Fe/H] < − 2.0, [C/Fe] increases somewhat reaching a mean value near + 0.2 or + 0.3 dex. All the investigations agree that N has a strong primary component.

    • Nitrogen enhancement in metal-poor dwarfs: From inside or outside?

      F. Spite M. Spite

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      The five known metal-poor dwarfs with an enhanced N/Fe ratio have been observed spectroscopically. Two of these dwarfs have no lithium line; the absence of lithium is most probably accounted for by the usual convective destruction. The three other dwarfs have the same lithium abundance as the normal metal-poor dwarfs (Spite, Maillard & Spite 1984). This excludes the deep mixing process as the general source of nitrogen enhancement, since lithium is destroyed in deep (hot) layers. Deep mixing had been previously found unlikely in metal-poor dwarfs (Da Costa & Demarque 1982). The discussion stresses the remarkable uniformity of the lithium abundance in metal-poor dwarfs, and shows that the N-rich contaminating matter has a high N/H ratio. Finally, the Al abundance is not greatly enhanced in these five stars.

    • Relative elemental abundances in extremely metal-deficient stars

      M. S. Bessell John Norris

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      The relative elemental abundances in an extremely metaldeficient star CD-38° 245 with [Fe/H] ∼ −4.5 and in similar stars are discussed.

    • Chemical evolution of the galaxy: Abundances of the light elements (sodium to calcium)

      David L. Lambert

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      The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also25Mg and26Mg).

      The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3 with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical.

      For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show [Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and [Fe/H] remains uncertain.

    • Past and present metal abundance gradient in the galactic disc

      M. Grenon

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      A critical discussion of some determinations of metal abundance gradient in the galactic disc is presented with an attempt to homogenize the results related to local and large-scale gradients. A new estimation of the palaeogradient in intermediate and old disc population is presented. The gradient is found to depend on galactocentric distance, and to be nearly constant with time during the lifetime of the galactic disc.

    • On a possible metallicity gradient in SMR stars

      G. Cayrel de Strubel

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      The possibility of existence of a metallicity gradient in super-metal-rich (SMR) stars is discussed. It is concluded that the SMR phenomenon was more active in the past than it is now.

    • Chemical evolution of galaxies and abundances in low-mass stars

      Jean Audouze

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      The chemical evolution of galaxies is discussed with reference to elemental abundances in low- and intermediate-mass stars.

    • A new interpretation of the metallicity histogram of globular clusters

      R. Cayrel

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      A new interpretation is given to the low metallicity peak of the bimodal metallicity histogram of galactic globular clusters. It is proposed that these globular clusters are primordial,i.e., formed out of big-bang matter. Their nonvanishing metallicity is attributed to pollution by supermassive stars like R 136a. The first stellar generation is formed out of the ‘dirty’ primordial matter.

    • Concluding remarks

      B. Gustafsson

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    • The role of a strong far-infrared radiation field in line excitation and cooling of photodissociation regions and molecular clouds

      Abdul Qaiyum S. M. R. Ansari

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      We have theoretically studied the influence of a far-infrared radiation (FIR) field from Hπ region on the cooling by C and O atoms, C+ ion and CO molecule in a photodissociation region, and a molecular cloud associated with Hπ region (hereinafter referred as HI region) at low temperatures (Tk≤200 K). Comparisons have been made for cooling with and without FIR for two extreme abundances (10−4 and 10−7) of the mentioned species for temperatures ranging between 10 and 200K and an hydrogen particle density range 10 cm−3no≤ 107 cm3.

      The cooling by the species with low line-splitting (CI, Cπ and CO) is significantly influenced by the radiation field for temperaturesTk < 100 K while the effect of radiation field on cooling by OI is significant even at higher temperatures (Tk > 100 K). The effect of FIR field on the cooling of CO from low rotational transitions is negligibly small, whereas it is considerable for higher transitions. In general, the cooling terms related to the short-wavelength transitions are more affected by FIR than those related to longer wavelengths.

      It is also demonstrated here that in the determination of thermal structure of an HI region the dust grains play an important role in the heating of gas only through photoelectron emission following irradiation by far-ultraviolet (FUV) radiation, as the infrared radiation from the dust is too small to have substantial effect on the cooling. It is found that in the Hπ /HI interface the FIR field from grains in the Hπ region is not capable of modifying the temperature of the warmest regions but does so in the inner part where the temperature is low enough.

    • The He II line λ4686 in WN binaries

      B. S. Shylaja

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      The behaviour of the λ4686 line of He II in various WN binaries is studied for variations in total flux as well as radial velocities. It appears that the true motion of the WN component is not depicted by this line especially for short-period binaries. A qualitative explanation is sought in terms of a large, extended line-emitting region and multiple-component line profile.

    • Evolution of the optical spectrum of SN 1987a in the large magellanic cloud

      B. N. Ashoka G. C. Anupama T. P. Prabhu S. Giridhar K. K. Ghosh S. K. Jain A. K. Pati N. Kameswara Rao

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      The evolution of the spectrum of SN1987a is traced from 1987 February 26 to March 31. Based on the low-resolution spectroscopic data we identify the lines of H, He I, Na I, Fe II, Sc II, Ca II which are known to be present in Type II Supernovae, and also present evidence for the existence of lines of Mg I, CaI, O I, and N I. We discuss the evolution of the Hα profile, and draw attention to its complex structure around March 30. Close to the rest wavelength of Ha a double-peaked structure appeared in the profile with a peak-to-peak separation of ∼ 1400 km s−1, suggestive of an expanding shell or disc of gas.

      Using the available broadband photometric information, we also trace the evolution of the photosphere of SN1987a assuming that it radiates like a supergiant.

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