Abstract

The ultraviolet features in the 33o(} 4o A region of 117 A grating spectra of A and F stars are studied for the purpose of improving the relation between MK spectral type and b - y color. The uvby system is used to define temperature sequences among dwarfs of similar line strengths. The temperature criteria are presented in tables showing the relative waxing and waning of ratios of spectral features as a function of spectral type. Luminosity criteria obtained from the ultraviolet spectra of the MK luminosity standards are tabulated for each 0.5 spectral type. A description is given of the differences in the ultraviolet spectra which correlate with differences in the mi index for stars of similar spectral type and luminosity. More than 160 stars are classified according to temperature, luminosity, and line strength. The resulting relation between spectral type and color is an improvement over that of the MK standards and the Hyades main-sequence stars classified by Morgan and Hiltner. The new inminosity criteria permit the use of more luminosity subclasses than are possible with the standard MK criteria. Photometric methods for identifying Am and X Bootis stars are suggested. While the photometry of spectroscopically identified weak-line F stars indicates low luminosity and low metallic-line strength, the photometry of the fast rotating spectroscopically identified weak-line A stars indicates luminosity slightly higher than normal and low metallic-line strength. Evidence is presented indicating the need for improvement in the definition of the Am stars.