THE M(V)(HB) VERSUS [FE H] CALIBRATION .1. HST COLOR-MAGNITUDE DIAGRAMS OF 8 GLOBULAR-CLUSTERS IN M31/

Color magnitude diagrams (CMDs) of individual stars in 8 globular clus ters (GCs) in M31 down to about 1 mag fainter (V similar to 26.5) than the horizontal branch (HE) have been obtained with the Hubble Space T elcscope-HST. In particular, we observed G280 and G351 with the FOC (f /96+F430W and f/96 + F480LP) while the WFPC2 (F555W, F814W) frames for G1, G58, G105, G108, G219 + Bo468 were retrieved from the HST archive . The cluster metallicities-[Fe/H]-range from -1.8 to -0.4. Coupled wi th sufficiently accurate (to similar to +/- 0.1 mag) measures of the m ean brightness of the HB-V-HB-, appropriate estimates of reddening for each cluster, and the adoption of a distance modulus to M31 of (m - M )(0) = 24.43, this has allowed us to yield a direct calibration for th e mean absolute magnitude of the HE at the instability strip-M(V)(HB)- with varying [Fe/H]:M(V)(HB) = (0.13 +/- 0.07)[Fe/H] +/- (0.95 +/- 0.0 9), where the associated errors result from the adopted global errors in the measure and best fitting procedures. The slope of the derived r elation is fully consistent with that predicted by the standard and ca nonical models (similar to 0.15) and obtained by various ground-based observations, while it is only marginally compatible with higher value s (similar to 0.30), also obtained in the past. The zero-point, which is crucial to absolute age determinations, depends on the adopted dist ance to M31 and is moreover affected by an additional error due to the residual uncertainties in the HST photometric zero-points (similar to 0.05 mag, at least). If confirmed, such a calibration of the M(V)(HB) vs [Fe/H] relationship would imply old absolute ages (>16 Gyr) for th e oldest Galactic globulars and fairly small age spread among those ha ving a constant magnitude difference between the main-sequence turnoff and the HE. (C) 1996 American Insitute of Physics.