Photoelectric measures of Hydrogen-line absorption in early-type stars

Abstract

Photoelectric determinations of H absorption line intensity have been made of stars in selected clusters and associations as well as in the general galactic field. Interference filters having a width at half intensity of 45 A were used to isolate spectral regions centred on 4z80 A, H and 4410 A. The F indices derived, which represent a measure of the Hy absorption, can be used in conjunction with unreddened values of either (U-B) or (B - V) for spectral and luininosity class determinations of stars in the spectral range O6 to Ao. The separation of age loci is shown to exist on F, (U-B)0 diagrams of clusters and associations having different ages. A F, Mv calibration for the young, or less evolved stars, has been derived. Tests of such a calibration have been made on the Orion Association and NGC z362. The distance moduli thus obtained are in good agreement with the values derived from the colour magnitude arrays.