Spectroscopic Evidence for Small Metallicity Variations among M92 Giants

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Abstract
We present spectroscopic evidence for small star-to-star variations in the abundance of the atoms from Ca to Ni (primarily Fe) among three bright giants in the metal-poor globular cluster M92: V-45, XI-19, and XII-8. Although all three have very similar B - V colors and visual magnitudes, weak "iron peak" absorption lines are typically about 15%–30% stronger in the spectrum of star XI-19. A line-by-line analysis shows that the abundances of the "iron peak" elements are very similar in stars V-45 and XII-8; the (logarithmic) mean difference is only 0.01 ± 0.01 dex for 106 lines. The average abundance of the "iron peak" elements is 0.18 ± 0.01 dex larger in XI-19 (121 lines). The very small "internal" error in the mean difference can be traced to (1) comparing the abundances one line at a time in order to avoid errors introduced by uncertain gf-values, (2) averaging equivalent widths for two spectra for both XI-19 and the comparison stars to reduce random errors in the equivalent widths, and (3) using a large number of lines to drive down the error in the mean. The "external" errors must also be small: the very small differences in the positions of the stars in the (V, B - V) color-magnitude diagram imply such similar values of Teff and log g that the line-strength differences must be due to real abundance differences. Comparison of small "iron peak" abundance variations with the larger variations that have been observed in the abundances of the light elements C, N, O, Na, Mg, and Al or the n-capture elements Ba and Eu in metal-poor Galactic globular clusters appears to be possible and may shed some light on the origins of both in the future.