Annu. Rev. Astron. Astrophys. 1997. 35: 503-556 Copyright © 1997 by Annual Reviews. All rights reserved

3. SUPER METAL-RICH STARS

The existence of super metal-rich (SMR) stars was first claimed by Spinrad & Taylor (1969), based on low-resolution spectra. The term SMR is generally meant to signify that a star is more metal-rich than the sun by an amount that cannot be explained as simple measurement error. The existence of SMR stars is, historically, a controversial subject; the main question is whether SMR stars are really metal-rich or just appear so because of some kind of measurement dispersion or systematic error. Perhaps the notion of SMR stars became more acceptable with claims that the Galactic bulge red giant stars are on average more metal-rich than the sun (e.g. Whitford & Rich 1983, Frogel & Whitford 1987, Rich 1988). McWilliam & Rich (1994) showed that the average bulge [Fe/H] is the same as in the solar neighborhood, but that the most metal-rich bulge giant, BW IV-167, at [Fe/H] = +0.44 is almost identical to µ Leo, a metal-rich disk giant. Taylor (1996) has reviewed abundance estimates for SMR stars, including low- and high-resolution results, and concluded that true SMR stars do not exist.

Given the controversy and the potential significance for chemical evolution, it seems important to establish whether any firm cases of SMR stars exist at all. In the Galactic disk, the most well-studied SMR candidate is the K giant star µ Leo. High-resolution high-S/N model atmosphere abundance analyses of µ Leo have been performed by several groups: Gustafsson et al (1974), Branch et al (1978), Brown et al (1989), Gratton & Sneden (1990), McWilliam & Rich (1994), Castro et al (1996) all found values near [Fe/H] = +0.45 for a solar scale of (Fe) = 7.52; on the other hand, Lambert & Ries (1981), McWilliam (1990), Luck & Challener (1995) found [Fe/H] from +0.1 to +0.2 dex.

Metal-rich stars in the McWilliam (1990) study (e.g. µ Leo) were affected by two systematic problems: CN blanketing depressed most of the µ Leo continuum regions in the two small 100-Å portions of the spectrum observed (found by McWilliam & Rich 1994), which resulted in smaller equivalent widths; second, McWilliam (1990) did not have access to metal-rich model atmospheres, which caused underestimation of the [Fe/H] for metal-rich stars (~ 0.1-dex underestimate for µ Leo). Both of these effects decreased the measured µ Leo [Fe/H] in the McWilliam (1990) work; accounting for the model atmosphere correction alone would increase [Fe/H] to +0.30 dex.

The Luck & Challener (1995) study concluded that their sample of strong-lined stars showed only small iron abundance enhancements at [Fe/H] ~ +0.1 dex; in the case of µ Leo they found [Fe/H] = +0.20 dex. Luck & Challener (1995) chose not to use a SMR model atmosphere for µ Leo, thus artificially lowering the computed [Fe/H] by ~ 0.08 dex (Castro et al 1996). Castro et al (1996) showed that the low Luck & Challener [Fe/H] must result from differences in analysis because of the good agreement between equivalent widths of lines in common. Furthermore, Luck & Challener confused the [A/H] = 0.0 of the Bell et al (1976) atmosphere grid with a solar iron abundance of (Fe) = 7.67 (from Anders & Grevesse 1989), whereas the models were actually calculated with (Fe) = 7.50. Castro et al noted that when these two problems are taken into account the Luck & Challener result for µ Leo becomes [Fe/H] = +0.43, assuming the solar (Fe) = 7.52.

Thus the most recent high resolution abundance studies of µ Leo that are discordant with the notion of [Fe/H] = +0.45 can be readily resolved, and it appears that there is a convergence of the µ Leo iron abundance near [Fe/H] = +0.45 dex with the assumed low value for the solar iron abundance. I do not have an explanation for the Lambert & Ries (1981) low [Fe/H], although it seems possible that the heavy line blanketing and limited spectral coverage may have affected the continuum placement.

Studies with the highest S/N spectra, and the most detailed abundance analyses (e.g. Gratton & Sneden 1990, Branch et al 1978, Castro et al 1996, McWilliam & Rich 1994), consistently find [Fe/H] ~ +0.4 dex for µ Leo. In conclusion, the high dispersion abundance analyses confirm at least one case of super metallicity.

High-resolution abundance analyses of SMR stars have also been carried out by Edvardsson et al (1993), who found F dwarf stars up to [Fe/H] = +0.26 dex; Feltzing (1995), who extended the Edvardsson sample to find stars between [Fe/H] of -0.08 and +0.42 dex; and Castro et al (1997), who studied a subset of the sample identified by Grenon (1989) and found [Fe/H] ranging from +0.10 to +0.50 dex. McWilliam & Rich (1994) found two SMR Galactic bulge giants, BW IV-167 and BW IV-025, with [Fe/H] of +0.44 and +0.37 dex, respectively. It appears that high-resolution abundance studies do find SMR stars with [Fe/H] up to approximately 0.4-0.5 dex.