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Oxygen abundances in halo giants. I - Giants in the very metal-poor globular clusters M92 and M15 and the metal-poor halo field
Oxygen, iron, vanadium, and scandium abundances are derived for verymetal-poor giants in the globular clusters M92 and M15, and giants ofcomparable metallicity in the local halo field. The forbidden O I linedublet (6300, 6363) and nearby metallic lines in spectra are analyzedusing line analysis and spectral synthesis codes. The Fe/H abundance forM92 is estimated at -2.25 +/-0.02 based on nine giants with a range of500 K in effective temperature. No evidence for star-to-star variationsin the Fe/H abundance was found. O-rich and O-poor stars appearintermixed in the H-R diagram. O - N nuclear synthesis and mixing to thesurface are proposed as the best explanation for the low-oxygen giants.The nitrogen abundances obtained earlier for nine of the ten halo fieldgiants in this sample are incompatible with the very large nitrogenabundances expected of the O/Fe abundance of about + 1.2 in halo fieldsubdwarfs, as found by Abia and Rebolo (1989), and not more than 0.6 inhalo giants, as found in this and other studies.
| Positional reference stars in the Magellanic Clouds
The equatorial coordinates are determined of 926 stars (mainly ofgalactic origin) in the direction of the Magellanic Clouds at the meanepoch T = 1978.4 with an overall accuracy characterized by the meanvalues of the O-C coordinates, Sa = 0.35 arcsec and Sd = 0.38 arcsec,calculated from the coordinates of the Perth reference stars. Thesevalues are larger than the accuracy expected for primary standard stars.They allow the new positions to be considered as those of reliablesecondary standard stars. The published positions correspond to anunquestionable improvement of the quality of the coordinates provided inthe current catalogs. This study represents an 'astrometric step' in thestarting of a 'Durchmusterung' of the Magellanic Clouds organized by deBoer (1988, 1989).
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