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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).
| A sample of solar-type stars of known age
A sample of field F and G dwarfs of known ages is presented. All thedwarfs are secondaries of visual binaries in which the temperature andgravity of the hotter primary has been estimated from Stromgrenphotometry, and its age derived from reference to isochrones. This ageis taken to apply to the secondary. Even in the case of F-typesecondaries, which themselves have measured Stromgren indices, it ismuch better to estimate ages from the primaries as it is demonstratedthat use of isochrones to determine age of stars not far evolved fromthe main sequence can lead to significant systematic overestimates ofages when the photometric measurements have typical random errors. Anumber of systems in which photometry is available for both componentshave primaries which appear much younger than the secondaries,suggesting a need for further investigation.
| Uvby-beta photometry of stars in the Michigan Spectral Catalogue. I - The Delta Del stars
uvby-beta photometry of 74 stars classified as Delta Del in Volume 1 ofthe Michigan Spectral Catalogue is presented. The photometry is inagreement with the spectral classification in that it indicates DeltaDel stars are metal-line-enhanced stars with equivalent-temperaturetypes of A9 + or - 2 subtypes. There is a discrepancy between theluminosity implied from the delta c1 index and the spectralclassification. It is argued that c1 is an inadequateluminosity indicator for some late A- to early F-type stars. Several ofthe Delta Del stars are known Delta Scuti pulsators, and some commentson the metallicism-pulsation relation are made.
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