An Empirical Hγ - Luminosity Calibration

Abstract

High signal-to-noise Reticon spectra for 87 members of 8 open clusters and associations together with 37 stars having reliable parallaxes (early A-type stars with reliable trigonometric parallaxes, eclipsing binaries, and visual binaries) have been used to calibrate the W(Hγ)-Mv relation for spectral types 0 to early A of luminosity classes III-V. The new calibration has a mean probable dispersion of ±0.28 mag. The distance modulus of the Pleiades is 5.54 ± 0.06 mag, which is in excellent agreement with other, recent determinations, as are the distance moduli for all the calibrating clusters. The use of visual-binary parallaxes implies a Hyades distance modulus of about 3.0 which is significantly smaller than the Hanson (1980) value of 3.30 mag. Although no spectral-type corrections are necessary, stellar evolution probably affects the construction of the new calibration and special care should be taken when determining distance moduli from slightly evolved cluster sequences or for individual stars. Systematic departures from the calibration may be present for stars with Vsin i ≥ 220–250 km/sec. Significant residuals are found between our values of W(Hγ) and those of Petrie in the range 1–13 Å equivalent width, which are due in part to systematic errors in Petrie's W(Hγ) measures. Our distance modulus of 11.11 mag for NGC 2244 is in excellent agreement with the photometric distance. The new calibration is compared to other early type star calibrations for main sequence stars. It is 1.2 mag brighter than Petrie's (1965) Hγ calibration at spectral type 06 and 0.7 mag brighter at A3. For types B1 and earlier the new calibration averages 0.4 mag brighter than the Balona and Crampton (1974) Hγ calibration. There is generally good agreement with the Blaauw (1963) MK calibration although the latter is 0.4 mag brighter at spectral type BO. The Crawford (1978) Hβ calibration is up to 0.5 mag brighter for the earlier spectral types and 0.4 mag fainter for later types. More complete discussions of the Hγ-luminosity calibration are available in Millward and Walker (1984, 1985).