Keck HIRES Spectroscopy of M92 Subgiants: Surprising Abundances near the Turnoff

Using high-resolution, moderate SNR spectroscopy obtained with the 10 m Keck I Telescope and efficient HIRES echelle spectrograph, we derive abundances of several elements in subgiants near the M92 turnoff. The analysis suggests that the M92 reddening may be 0.04-0.05 mag greater than canonical values. It is concluded that either the (Fe/H) of M92 has been significantly overestimated from red giants or current reddening/photometry estimates are too small /red. If the reddening/photometry were in error by this amount, turnoff color based ages for M92 could be reduced by 4 Gyr. The adjustment to the M92 distance modulus required for a similarly reduced turnoff age that is luminosity-based can be accommodated by increases in extinction and alterations to the metal-poor field star distance scale recently inferred from Hipparcos Cepheid and subdwarf data. Our M92 subgiants demonstrate (Cr/Fe), (Ca/Fe), and (Ti/Fe) ratios that are unremarkable and essentially identical to the values for HD 140283. (Ba/Fe) is 0.45 dex larger for the M92 subgiants than for HD 140283. Surprisingly, we find (Mg/Fe) to be 0.55 dex lower in our M92 subgiants than in HD 140283, and (Na/Fe) to be 0.76 dex larger in our M92 subgiants than in HD 140283. If real, this striking abundance pattern is suggestive of material in our M92 stars' photospheres that has undergone Ne-Na and Mg-Al cycling like that inferred for red giants in M92 and other clusters. (Author)