THE RELATION BETWEEN U-B AND ABSOLUTE MAGNITUDE FOR F-TYPE STARS

Abstract

This investigation of the relation between the color index U-B and absolute magnitude for F-type stars has been made with the hope of obtaining additional information about the luminosities of main-sequence stars in globular clusters. It has become customary to think of the globular cluster main-sequence stars as subdwarfs, but there are very few data that have bearing on this subject. The only direct data we have are those of Baum1 for M 13 and of Johnson and Sandage2 for M 3. Baum found that the M 13 main sequence falls about 2 magnitudes below the main sequence for the nearby stars, while Johnson and Sandage found that the main sequence of M 3 falls very close to, or slightly above, that of the nearby stars. Both conclusions were based upon the assumption that the RRLyrae variables lie at Mv = 0.0. Since the determination by photometric methods of the distances of galactic clusters has proved to be successful,3'4 one wonders whether the same procedure of fitting cluster main sequences to a standard main sequence might also be applied to the globular clusters. An essential step in the procedure is the setting up of a satisfactory standard main sequence. Unfortunately, the energydistribution curves for the M 3 stars are fundamentally different from those for stars in the solar neighborhood,2 and there is no assurance that the standard main sequence derived from nearby stars has the same relationship between color and luminosity as do the main sequences of globular clusters. This attempt to determine the distance modulus of a globular cluster (M3) by comparing its main-sequence stars with nearby stars yields a distance modulus that does not agree with that obtained by assuming that the RR Lyrae stars are at Mv = 0.0.