Ultraviolet Photometry from the Orbiting Astronomical Observatory. IV. Photometry of Late-Type Stars

Broad-band interference-filter nhotometry obtained with OAO-2 at four ultraviolet wavelengths is presented for 57 normal, bright stars of spectral type A2 to M. Relative digital counting rates are given for selected filter-photometer combinations with effective wavelengths of 3320, 2980, 2460, and 1910 A. Errors in the net stellar counts depend principally on variations in instrument sensitivity and dark counts due to effects of the Earth's radiation belt. For the cooler stars, uncertainties in sky background can be significant. Corrections for identified secondary stars in the wide (10 arc minute) field are estimated for one-third of the pointings. Internal consistency and low dispersion in olor-color diagrams indicate that errors of + 2 percent at the longer wavelengths and + 10 percent at shorter wavelengths are typical. Color-color diagrams show that there is remarkably little intrinsic variation in ultraviolet properties among stars with the same visual color and luminosity class, if class I supergiants are excepted. The correlation between 2460 - V and 2980 - 3320 is particularly good and, moreover, independent of luminosity, again with the exception of class I stars. Generally, A and F supergiants are fainter than main-sequence stars of the same type by about two magnitudes at 2460 A and 1910 A, relative to the visual. K and M supergiants are brighter than giants by about the same amount. OAO and ground observations of UMa (A7 V), 24 UMa (G4 IV), and a Ari (K2 III) from 1910 A to 1 are compared with blanketed models from the Smithsonian grid. Good agreement is possible for all wavelengths considered except 2460 A, which is weaker than predicted. This discrepancy increases from 0.5 mag at A7 to about 2 mag for a Ari.