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Debris Disks around Nearby Stars with Circumstellar Gas
We conducted a survey for infrared excess emission from 16 nearbymain-sequence shell stars using the Multiband Imaging Photometer forSpitzer (MIPS) on the Spitzer Space Telescope. Shell stars areearly-type stars with narrow absorption lines in their spectra thatappear to arise from circumstellar (CS) gas. Four of the 16 stars in oursurvey showed excess emission at 24 and 70 μm characteristic of coolCS dust and are likely to be edge-on debris disks. Including previouslyknown disks, it appears that the fraction of protoplanetary and debrisdisks among the main-sequence shell stars is at least 48%+/-14%. Whiledust in debris disks has been extensively studied, relatively little isknown about their gas content. In the case of β Pictoris, extensiveobservations of gaseous species have provided insights into the dynamicsof the CS material and surprises about the composition of the CS gascoming from young planetesimals. To understand the coevolution of gasand dust through the terrestrial planet formation phase, we need tostudy the gas in additional debris disks. The new debris disk candidatesfrom this Spitzer survey double the number of systems in which the gascan be observed right now with sensitive line-of-sight absorptionspectroscopy.
| A search for previously unrecognized metal-poor subdwarfs in the Hipparcos astrometric catalogue
We have identified 317 stars included in the Hipparcos astrometriccatalogue that have parallaxes measured to a precision of better than 15per cent, and the location of which in the(MV,(B-V)T) diagram implies a metallicitycomparable to or less than that of the intermediate-abundance globularcluster M5. We have undertaken an extensive literature search to locateStrömgren, Johnson/Cousins and Walraven photometry for over 120stars. In addition, we present new UBV(RI)C photometry of 201of these candidate halo stars, together with similar data for a further14 known metal-poor subdwarfs. These observations provide the firstextensive data set of RCIC photometry ofmetal-poor, main-sequence stars with well-determined trigonometricparallaxes. Finally, we have obtained intermediate-resolution opticalspectroscopy of 175 stars. 47 stars still lack sufficient supplementaryobservations for population classification; however, we are able toestimate abundances for 270 stars, or over 80 per cent of the sample.The overwhelming majority have near-solar abundance, with theirinclusion in the present sample stemming from errors in the colourslisted in the Hipparcos catalogue. Only 44 stars show consistentevidence of abundances below [Fe/H]=-1.0. Nine are additions to thesmall sample of metal-poor subdwarfs with accurate photometry. Weconsider briefly the implication of these results for clustermain-sequence fitting.
| A-shell stars in the Geneva system
Among the various kinds of A stars having a peculiar spectrum, we findthe A-shell stars. Many questions are still open concerning these stars,including their evolutionary status. In the present study we have useddata from the Hipparcos catalogue to examine this point. We have foundthat the majority of A-shell stars are well above the main sequence. Nodifferences could be established between A-shell stars in luminosityclasses III and I and those in luminosity class V as regardsvariability, duplicity, or the importance of the shell feature.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
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