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Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
| Planetary nebula carbon yields and the chemical evolution of the Galactic disc
Two sets of observational carbon stellar yields for low-and-intermediate mass stars are computed based on planetary nebulaabundances derived from CIIλ4267 and CIIIλλ1906 +1909 lines, respectively. These observational yields are assumed inchemical evolution models for the solar vicinity and the Galactic disc.C/O values observed in stars in the solar vicinity and Galactic HIIregions are compared with those predicted by chemical evolution modelsfor the Galaxy. It is concluded that the C yields derived from permittedlines are in better agreement with the observational constraints thanare those derived from forbidden lines.
| The β Cephei instability strip
We carried out a series of linear stability analyses of the radial andlow-degree non-radial p modes for stellar models with initial masses of6-30Msolar. The stellar models were computed by usingconvective overshoot distance dover=0.10, 0.25 and0.40HP. Our numerical results show that the β Cepheiinstability strip forms a horn-shaped region pointing upwards near themain sequence on the Hertzsprung-Russell diagram (HRD). The lower partof the instability strip for the radial modes join the zero-agemain-sequence (ZAMS) at M~8.0-10.5Msolar, while the top ofthe instability strip extends up to M~15.5Msolar. Theinstability strip for the non-radial modes is even wider. The overallinstability strip is dominated by the radial and non-radial fundamentalmodes. The first overtone (the radial-order index n=1) is alsopulsationally unstable. We have shown that the β Cephei stabilityis almost independent of the overshoot parameter dover usedfor the stellar models, while it depends critically on the metalabundance. With decreasing metal abundance, the instability regionshrinks and eventually disappears for Z<~0.005.
| Accurate Positions for MCG Galaxies
We have measured accurate celestial coordinates for 4741 extragalacticobjects, primarily drawn from a list of MCG galaxies with no recentlypublished accurate positions. The standard deviations in the newpositions depend slightly on the measurement method but are on the orderof 1.0" to 1.2". Standard deviations in the original MCG positions areconfirmed to be at the 1.5′-2.0′ level. These new positionswere integrated into NED in 1997 December.
| The Southern Sky Redshift Survey
We report redshifts, magnitudes, and morphological classifications for5369 galaxies with m_B <= 15.5 and for 57 galaxies fainter than thislimit, in two regions covering a total of 1.70 sr in the southerncelestial hemisphere. The galaxy catalog is drawn primarily from thelist of nonstellar objects identified in the Hubble Space TelescopeGuide Star Catalog (GSC). The galaxies have positions accurate to ~1"and magnitudes with an rms scatter of ~0.3 mag. We compute magnitudes(m_SSRS2) from the relation between instrumental GSC magnitudes and thephotometry by Lauberts & Valentijn. From a comparison with CCDphotometry, we find that our system is homogeneous across the sky andcorresponds to magnitudes measured at the isophotal level ~26 magarcsec^-2. The precision of the radial velocities is ~40 km s^-1, andthe redshift survey is more than 99% complete to the m_SSRS2 = 15.5 maglimit. This sample is in the direction opposite that of the CfA2; incombination the two surveys provide an important database for studies ofthe properties of galaxies and their large-scale distribution in thenearby universe. Based on observations obtained at Cerro TololoInter-American Observatory, National Optical Astronomy Observatories,operated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation;Complejo Astronomico El Leoncito, operated under agreement between theConsejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; the European Southern Observatory, LaSilla, Chile, partially under the bilateral ESO-ObservatórioNacional agreement; Fred Lawrence Whipple Observatory;Laboratório Nacional de Astrofísica, Brazil; and the SouthAfrican Astronomical Observatory.
| Redshift observations in the Hydra-Centaurus region
The paper reports 406 redshifts for galaxies in the northern galactichemisphere, south of delta = 0 deg. A substantial fraction of theobserved galaxies are located in the equatorial zone between deltavalues of -17.5 and 0 deg. By combining these new data with thoseavailable in the literature, it is possible to extend the original CfAredshift survey of galaxies brighter than m(B(0)) = 14.5 to b = 30 deg,south of delta = 0. New data taken at lower galactic latitudes alsocontribute to the existing surveys of the Hydra-Centaurus complex.
| Extremely energetic planetary nebulae in the Large Magellanic Cloud
Highly energetic planetary nebulae have been identified in the LargeMagellanic Clouds. These have higher expansion velocities than any oftheir galactic counterparts and appear to show directed expansion flowsrather than having a spherically symmetric expanding shell. Comparisonof their properties with similar, but less extreme, galactic planetariessuggests that these objects are highly luminous bipolar type Iplanetaries in which the central star is undergoing continued energeticmass loss, but which is nevertheless insufficient to supply the energyand momentum of the nebula. This class of objects is expected torepresent the upper end of the mass range of precursor stars which areable to evolve to the planetary nebula phase, and circumstantialevidence connecting them with the symbiotic stars is presented.
| Variable stars in clusters.
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