Keck HIRES Spectroscopy of M92 Subgiants: Surprising Abundances Near the TurnoffThe Astrophysical Journal
PublisherThe American Astronomical Society
AbstractUsing high-resolution, moderate signal-to-noise ratio 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. As a consistency check, we also analyze the metal-poor field star HD 140283 and find an Fe abundance in fine agreement with many previous determinations. However, our M92 value ([Fe/H] = -2.52) is a factor of 2 lower than the abundance derived from its red giant members. Differences in model atmospheres, gf-values, and instrumental effects might account for this difference, but whether they in fact do so is unclear. We note possible evidence for [Fe/H] differences within M92. Our spectroscopic analysis suggests that the M92 reddening, E(B-V), may be 0.04–0.05 mag greater than canonical values, but various uncertainties mean that this conclusion is not definitive; the significant difference in interstellar Na I line strengths in the M92 and HD 140283 spectra may be consistent with an increased reddening. Regardless, the conclusion that either the [Fe/H] of M92 has been significantly overestimated from red giants or current reddening/photometry estimates are too small/red is not easily escaped. 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.
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