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      Volume 34, Issue 4

      December 2013,   pages  297-440

    • Inhomogeneous Chemical Evolution of the Galaxy in the Solar Neighbourhood

      S. Sahijpal

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      𝑁-body numerical simulations of an inhomogeneous Galactic Chemical Evolution (GCE) of the solar neighbourhood with a high temporal resolution are presented. The solar annular ring is divided into distinct spatial grids of area ∼ 1–2 kpc2. Each grid evolves distinctly in terms of star formation and nucleosynthetic yields from numerous generations of stars. The evolution of the galaxy is simulated by considering discrete episodes of star formation. Subsequent to the evolution of the simulated stars within each grid the stellar nucleosynthetic yields are homogenized within the grid rather than the traditionally adopted criteria of homogenizing over the entire solar annular ring. This provides a natural mechanism of generating heterogeneities in the elemental abundance distribution of stars. A complex chemical evolutionary history is inferred that registers episodes of time-dependent contributions from SN II+Ib/c with respect to SN Ia. It was observed that heterogeneities can remerge even after episodes of large scale homogenizations on scales larger than the grid size. However, a comparison of the deduced heterogeneities with the observed scatter in the elemental abundances of the dwarf stars suggest only a partial match, specifically, for [Fe/H] > -0.5. The deduced heterogeneities in the case of carbon, oxygen, magnesium, silicon, sulphur, calcium and titanium can explain the observed heterogeneities for [Fe/H] < -0.5. It may not be possible to explain the entire observed spread exclusively on the basis of the inhomogeneous GCE.

    • Measurements of Narrow Mg II Associated Absorption Doublets with Two Observations

      Zhi-Fu Chen Cai-Juan Pan Guo-Qiang Li Wei-Rong Huang Mu-Sheng Li

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      The measurement of the variations of absorption lines over time is a good method to study the physical conditions of absorbers. In this paper, we measure the variations of the line strength of 36 narrow Mg II𝜆𝜆2796, 2803 associated absorption doublets, which are imprinted on 31 quasar spectra with two observations of the Sloan Digital Sky Survey (SDSS). The timescales of these quasar span 1.1–5.5 years at the quasar rest-frame. On these timescales, we find that these narrow Mg II associated absorption doublets are stable, with no one 𝜆2796 line showing strength variation beyond 2 times error (2𝜎).

    • Light Curve Stability and Period Behavior of the Contact Binary TZ Boo

      M. M. Elkhateeb M. I. Nouh

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      New CCD observations of the eclipsing binary TZ Boo in BVR bands were carried out in 2006 (presented three new minima) and used together with all published minima to study and update the orbital period of the system TZ Boo by means of an (O–C) diagram. The period variation from 1926 to 2011 is represented by polynomial of eighth degree and indicates period variation of about 9.752 × 10-10 days/yr. We studied light curve stability over 85 yr covering all published observations in the V band and confirm the cyclic light curve variations.

    • Satellite Orbital Precessions Caused by the Octupolar Mass Moment of a Non-Spherical Body Arbitrarily Oriented in Space

      G. Renzetti

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      I consider a satellite moving around a non-spherical body of mass 𝑀 and equatorial radius 𝑅, and calculate its orbital precessions caused by the body’s octupolar mass moment 𝐽4. I consider only the effects averaged over one orbital period 𝑇 of the satellite. I give exact formulas, not restricted to any special values of either the eccentricity 𝑒 or the inclination 𝑖 of the satellite’s orbit. I do not assume any preferential orientation for the body’s spin axis $\hat{\mathbf{k}}$ because in many cases of potential interest (exoplanets, neutron stars, black holes) it is poorly known or unknown at all.

    • Ion–Cyclotron Resonance Frequency Interval Dependence on the O VI Ion Number Density in the North Polar Coronal Hole 1.5𝑅–3𝑅 Region

      Özgür Gültekin Emine Rızaoǧlu K. Gediz Akdeniz

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      The frequency intervals in which O VI ions get in resonance with ion–cyclotron waves are calculated using the kinetic model, for the latest six values found in literature on O VI ion number densities in the 1.5𝑅–3𝑅 region of the NPCH. It is found that the common resonance interval is 1.5 kHz to 3 kHz. The 𝑅-variations of wave numbers necessary for the above calculations are evaluated numerically, solving the cubic dispersion relation with the dielectric response derived from the quasi-linear Vlasov equation for the left-circularly polarized ion-cyclotron waves.

    • Distribution of Doppler Redshifts of Associated Absorbers of SDSS Quasars

      Cai-Juan Pan Zhi-Fu Chen

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      Doppler redshifts of a sample of Mg II associated absorbers of SDSS DR7 quasars are analysed. We find that there might be three Gaussian components in the distribution of the Doppler redshift. The first Gaussian component, with the peak being located at 𝑧Dopp = -0.0074, probably arises from absorbers with outflow histories observed in the direction close to jets of quasars. The second Gaussian component, with the peak being located at 𝑧Dopp = -0.0017, possibly arises from absorbers with outflow histories observed in the direction far away from jets of quasars. Whereas, the third Gaussian component, with the peak being located at 𝑧Dopp = -0.0004, might arise from the random motion of absorbers with respect to quasars.

    • Puzzling Origin of CEMP-r/s Stars: An Interpretation of Abundance and Enrichment of s- and r-Process Elements from Asymptotic Giant Branch Supernovae

      Jiang Zhang Fang Zhao Yanping Chen Wenyuan Cui Bo Zhang

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      CEMP-r/s stars at low metallicity are known as double-enhanced stars that show enhancements of both r-process and s-process elements. The chemical abundances of these very metal-poor stars provide us a lot of information for putting new restraints on models of neutron-capture processes. In this article, we put forward an accreted scenario in which the double enrichment of r-process and s-process elements is caused by a former intermediate-mass Asymptotic Giant Branch (AGB) companion in a detached binary system. As the AGB superwind is only present at the ultimate phase of AGB stars, there is thus a lot of potential that the degenerate-core mass of an intermediate-mass AGB star reaches the Chandrasekhar limit before the AGB superwind. In these circumstances, both s-process elements produced in the AGB shell and r-process elements synthesized in the subsequent explosion would be sprayed contemporaneously and accreted by its companion. Despite similarity to physical conditions of a core-collapse supernova, a major focus in this scenario is the degenerate C–O core surrounded by an envelope of a former intermediate-mass AGB donor that may collapse and explode. Due to the existence of an outer envelope, r-process nucleosynthesis is expected to occur. Hypothesizing the material-rich europium (Eu) accreted by the secondary via the wind from the supernova to be in proportion to the geometric fraction of the companion with respect to the exploding donor star, we find that the estimated yield of Eu (as representative of r-process elements) per AGB supernova event is about 1 × 10-9𝑀 ∼ 5 × 10-9𝑀. Using the yields of Eu, the overabundance of r-process elements in CEMP-r/s stars can be accounted for. The calculated results show that the value of parameter 𝑓, standing for efficiency of wind pollution from the AGB supernova, will reach about 104, which means that the enhanced factor is much larger than unity due to the impact of gravity of the donor and the result of the gravitational focusing effect of the companion.

    • X-ray Observations of Eight Young Open Star Clusters: I. Membership and X-ray Luminosity

      Himali Bhatt J. C. Pandey K. P. Singh Ram Sagar Brijesh Kumar

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      We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-NEWTON. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (< 2𝑀), 36 intermediate mass (2-10𝑀) and 16 massive (> 10𝑀) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature < 2 keV, and mean 𝐿𝑋/𝐿bol of 10-6.9. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ∼ 1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ∼ 4–14 Myr. These observed values of 𝐿𝑋/𝐿bol are found to have a mean value of 10-3.6 ± 0.4, which is below the X-ray saturation level. The 𝐿𝑋/𝐿bol values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their 𝐿𝑋/𝐿bol values, however have been found to be significantly different from each other with a confidence level greater than 99.999% and the strength of X-ray activity in the intermediate mass stars is found to be lower compared to the low mass stars. However, the possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out.

    • Acknowledgements

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    • Author Index

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    • Subject Index

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      Posted on January 27, 2016

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