On the metallicity of the Hyades cluster

As a by-product of our study on Hyades G stars, it would be appropriate to touch briefly upon some topics that are closely related to the subject of metal-rich stars.

The metallicity ([Fe/H]) of the Hyades cluster is of profound importance, since it is involved with the definition of super-metal-rich (SMR) stars. While its values reported so far have a scatter of between +0.1 and +0.2 (Figure 32.8(a)), our result suggests A(Fe) ~ 7.7 (Figure 32.6(a)) or [Fe/H] ~ +0.2 (the solar Fe abundance is 7.5). In order to confirm this, we carried out a complete differential analysis relative to the Sun by applying the technique described in Takeda (2005) to 16 near-solar-type stars, which were selected from our sample of 37 Hyades G-type stars by application of the criterion that the difference of Teff from the solar value (5,780 K) had to be within ±250 K. The results are displayed in Figure 32.9, from which we derived the average [Fe/H] to be +0.19 (a = 0.05). Therefore, we would lend support to the "high" scale of [Fe/H] ~ +0.2 as the metallicity of Hyades cluster, which means that the widely adopted criterion of [Fe/H] > 0.2 is surely reasonable for the definition of SMR stars.

Incidentally, the fact that the resulting standard deviation of 0.05 is larger than the formally estimated errors may reflect the intrinsic metallicity dispersion within the Hyades-cluster stars. From this viewpoint, the apparent abundance dispersion of 0.05-0.06 dex for five heavy elements recently derived by De Silva et al. (2006) in their analysis of Hyades-cluster F-K stars (from which they concluded that there is little or no intrinsic scatter within the Hyades, though) might be real. Further follow-up studies should be awaited.

Figure 32.9. The differential metallicity (relative to the Sun) plotted as a function of the differential effective temperature (5Teff; relative to the Sun), on the basis of results of the complete differential analysis for 16 selected near-Solar-type Hyades G dwarfs.

Figure 32.9. The differential metallicity (relative to the Sun) plotted as a function of the differential effective temperature (5Teff; relative to the Sun), on the basis of results of the complete differential analysis for 16 selected near-Solar-type Hyades G dwarfs.

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