Mass loss from red giant stars in M67 : is there any evidence for a metallicity dependence?

Abstract

Recent claims in the literature, that the rate of mass loss from red giant branch (RGB) stars in old, open clusters like M 67 and NGC 6791, increases with the metallicity above the value holding for Globular Clusters and predicted by the classical Reimers (1975) relation, are thoroughly 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 there 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 about 0.7 MΘ . At the above age, the masses at the turn-off and near the tip of the RGB are about 1.2 MΘ and 1.3 MΘ, respectively. This means that in clusters with nearly solar metallicity, much more mass ought to be lost by the red giant stars than classically predicted. In this paper we show that in order to be able to derive the mass of the red stars in the clump from their position in the CMD, the color difference Δ(B – V) between the turnoff and the base of the RGB should be known with a precision smaller than δ[Δ(B – V)] = 0.01 mag and, at the same time, the distance modulus should be determined with a precision higher than 0.025 mag. The analysis of the problem clarifies in fact that in the global isochrone fitting technique, both the distance modulus and mass of the clump stars depend on the above color difference according to the relations ðMvto/ðΔ(B – V) ~- –9.2 and ð MHB/ðΔ(B – V) ~- –22 MΘ/mag. Therefore, an uncertainty in Δ(B – V) as small as δ[Δ(B – V)] = 0.01 mag implies an uncertainty in the distance modulus of δ(m – M) = 0.1 mag and, even more important, an uncertainty in the mass of the clump stars of 0.22 MΘ . Fits of the CMD of M 67 (and NGC 6791) are 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 day 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, blue stars, and in turn that mass loss plays the key role in generating stars that are potential candidates as sources of UV flux in elliptical galaxies and bulges, are also not supported by current data.