A Search for Metal-Deficient Stars

Objective-prism plates taken with the Curtis Schmidt telescope were searched for previously unknown metal-deficient stars, with the purposes of increasing the number of such objects known and of investigating the extent to which the properties of the previously known sample were due to proper- motion selection effects. The relatively high dispersion of the objective- rism spectrograms, 108 mm-1 A at H , allows the effects of very low metal content to be easily discernewhen the plates are taken under good seeing conditions. Four-color photometry was obtained for over 250 seventh- to ninth-magnitude suspected weak-lined stars found on the objective-prism plates. This photometric system separates the effects of differing stellar metal contents, temperatures, and luminosities with an accuracy somewhat higher than that attained by visual classification of low-dispersion slit spectrograms, especially for stars with very weak metallic lines. Among the newly discovered metal-deficient stars are: several extremely low-metal red giants similar to HD 122563; giants showing large but somewhat less extreme heavy-element deficiencies; "subdwarfs"; stars possibly showing evidence of nucleosynthesis in their interiors; a possible very metal-poor supergiant; and a group of early F-type metal-poor dwarfs that may be a mixture of young X Bootis stars and much older "blue stragglers." The frequency with which very metal-poor objects occur among the field stars is very low, about one in 10 for stars in the spectral range F5G5 with Am1 > 0.10. This suggests that massive objects played an important role in the earliest stellar generations. Most of the metal-deficient stars brighter than a given apparent magnitude are evolved giants, in agreement with prediction. The space motions of metal-deficient stars selected on the basis of their spectra alone are much lower than for those discovered as a consequence of their large proper motions. Indeed, it now appears that, at least for stars with more than one-tenth the solar metal content, there is little or no correlation between galactic orbital eccentricity and chemical composition. The rapid collapse of the proto-Galaxy, proposed by Eggen, Lynden-Bell, and Sandage, is still required to explain the motions of the extremely metal-poor stars. However, the collapse must have taken place before the synthesis of more than 90 percent of the heavy-element content of the Population I objects.