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A catalogue of QSOs and link stars for the HST link of the HIPPARCOS reference frame
A list of bright (less than 17 mag) southern QSOs and bright (less than11 mag) stars that may be suitable for the Hubble Space Telescope linkbetween the Hipparcos astrometric reference frame and the VLBIextragalactic frame is presented. The QSOs have been selected fromvarious lists of radio objects and identifications. The stars have beenselected from the Strasbourg (CDS) data base and from the PreliminarySecond Cape Photographic Catalog, and supplemented with stars measuredfrom the SERC IIIa-J Schmidt survey. These QSOs and stars have beenincluded in the Hipparcos and HST schedule of observations.
| The chemical evolution of the solar neighborhood. I - A bias-free reduction technique and data sample
The possible ways of measuring the age-metallicity relation for thegalactic disk in the neighborhood of the sun are discussed. It is shownthat the use of a field star sample chosen on the basis of effectivetemperature introduces a bias which results in a monotonic increase inthe metal abundance of the disk with time. However, if theage-metallicity relation for the disk can be shown to satisfy certaincriteria, the bias introduced in such a sample can be neglected: thegalactic disk apparently satisfies the criteria. It is concluded that asample analyzed through the use of uvby and H(beta) photometry inconjunction with a self-consistent set of theoretical isochronesprovides the least biased, most accurate estimate of the age-metallicityrelation for the disk.
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Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Durbun |
| Sað Açýklýk: | 19h16m29.39s |
| Yükselim: | -54°58'58.6" |
| Görünürdeki Parlaklýk: | 8.053 |
| Uzaklýk: | 255.102 parsek |
| özdevim Sað Açýklýk: | -2.8 |
| özdevim Yükselim: | -9.3 |
| B-T magnitude: | 8.378 |
| V-T magnitude: | 8.08 |
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