Color Magnitude Diagrams ( CMDs ) of individual stars in 8 Globular Clusters ( GCs ) in M31 down to about 1 mag fainter ( V \sim 26.5 ) than the Horizontal Branch ( HB ) have been obtained with the Hubble Space Telescope -HST .

In particular , we observed G280 and G351 with the FOC ( f / 96 + F 430 W and f / 96 + F 480 LP ) 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 with sufficiently accurate ( to \sim \pm 0.1 mag ) measures of the mean 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 ) _ { o } = 24.43 , this has allowed us to yield a direct calibration for the mean absolute magnitude of the HB at the instability strip –M _ { V } ^ { HB } ~ { } – with varying [ Fe/H ] :

M _ { V } ^ { HB } = ( 0.13 \pm 0.07 ) ~ { } [ Fe / H ] ~ { } + ( 0.95 \pm 0.09 ) where the associated errors result from the adopted global errors in the measure and best fitting procedures .

The slope of the derived relation is fully consistent with that predicted by the standard and canonical models ( \sim 0.15 ) and obtained by various ground-based observations , while it is only marginally compatible with higher values ( \sim 0.30 ) , also obtained in the past .

The zero-point , which is crucial to absolute age determinations , depends on the adopted distance to M31 and is moreover affected by an additional error due to the residual uncertainties in the HST photometric zero-points ( \sim 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 the oldest Galactic globulars and fairly small age spread among those having a constant magnitude difference between the Main-Sequence Turnoff and the HB .