MASS-LOSS FROM RED GIANT STARS IN M-67 - IS THERE ANY EVIDENCE FOR A METALLICITY DEPENDENCE

Recent claims in the literature, that the rate of mass loss from red g iant branch (RGB) stars in old, open clusters like M 67 and NGC 6791, increases with the metallicity above the value holding for Globular Cl usters and predicted by the classical Reimers (1975) relation, are tho roughly scrutinized by means of the global fit of the color-magnitude diagram (CMD) of these clusters (M 67 in particular) to modern sets of theoretical isochrones. The above claim stems from recognizing that t here is wide agreement in literature on the metallicity (nearly solar) , age (about 5 Gyr), and distance modulus ((m - M)=9.55) of M 67, and from estimating the mass of its giant stars in the red clump to be abo ut 0.7 M.. At the above age, the masses at the turn-off and near the t ip of the RGB are about 1.2 M. and 1.3 M., respectively. This means th at in clusters with nearly solar metallicity, much more mass ought to be lost by the red giant stars than classically predicted. In this pap er we show that in order to be able to derive the mass of the red star s in the clump from their position in the CMD, the color difference De lta(B - V) between the turnoff and the base of the RGB should be known with a precision smaller than delta[Delta(B - V)] = 0.01 mag and, at the same time, the distance modulus should be determined with a precis ion higher than 0.025 mag. The analysis of the problem clarifies in fa ct that in the global isochrone fitting technique, both the distance m odulus and mass of the clump stars depend on the above color differenc e according to the relations partial derivative M(V)(TO)/partial deriv ative Delta(B - V) similar or equal to -9.2 and partial derivative M(H B)/partial derivative Delta(B - V) similar or equal to -22 M./mag. The refore, an uncertainty in Delta(B - V) as small as delta[Delta(B - V)] = 0.01 mag implies an uncertainty in the distance modulus of delta(m - M) = 0.1 mag and, even more important, an uncertainty in the mass of the clump stars of 0.22 M.. Fits of the CMD of M 67 (and NGC 6791) ar e presented, in which the classical value of the mass for the stars in question is recovered. The claim that the rate of mass loss from RGB stars increases with the metallicity is not supported by the present d ay data for M 67. Finally, the claim that in NGC 6791 the metallicity higher than in M 67 is the cause of the occurrence of a few faint, blu e stars, and in turn that mass loss plays the key role in generating s tars that are potential candidates as sources of UV flux in elliptical galaxies and bulges, are also not supported by current data.