KECK HIRES SPECTROSCOPY OF M92 SUBGIANTS - SURPRISING ABUNDANCES NEARTHE TURNOFF

Citation
Jr. King et al., KECK HIRES SPECTROSCOPY OF M92 SUBGIANTS - SURPRISING ABUNDANCES NEARTHE TURNOFF, The Astronomical journal, 115(2), 1998, pp. 666-684
Citations number
89
Categorie Soggetti
Astronomy & Astrophysics
Journal title
ISSN journal
00046256
Volume
115
Issue
2
Year of publication
1998
Pages
666 - 684
Database
ISI
SICI code
0004-6256(1998)115:2<666:KHSOMS>2.0.ZU;2-X
Abstract
Using high-resolution, moderate signal-to-noise ratio spectroscopy obt ained with the 10 m Keck I Telescope and efficient HIRES echelle spect rograph, we derive abundances of several elements in subgiants near th e 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 m any previous determinations. However, our M92 value ([Fe/H] = -2.52) i s a factor of 2 lower than the abundance derived from its red giant me mbers. Differences in model atmospheres, gf-values, and instrumental e ffects 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 un certainties mean that this conclusion is not definitive; the significa nt difference in interstellar Na I line strengths in the M92 and HD 14 0283 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 estimate s 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 similar to 4 Gyr. The adjustment to the M92 distance mod ulus required for a similarly reduced turnoff age that is luminosity-b ased can be accommodated by increases in extinction and alterations to the metal-poor field star distance scale recently inferred from Hippa rcos Cepheid and subdwarf data. Our M92 subgiants demonstrate [Cr/Fe], [Ca/Fe], and [Ti/Fe] ratios that are unremarkable and essentially ide ntical 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. These diff erences (and indeed nearly all our abundance ratios) seem immune to va rious data, analysis, and parameter errors. If real, this striking abu ndance pattern is suggestive of material in our M92 stars' photosphere s that has undergone Ne --> Na and ME --> Al cycling like that inferre d for red giants in M92 and other clusters. While this is generally be lieved to be an in situ process in cluster giants, the presence of abu ndant Li in our M92 objects suggests a polluting source acting either primordially or via accretion after cluster star formation. This may b e consistent with CN and Na variations on the 47 Tucanae main sequence , recently reported Ba and Eu variations in M15 red giants, possible c luster-to-cluster n-capture abundance differences, and very low [O/Fe] ratios observed near the base of the M13 giant branch. We thus sugges t that a polluting source of light-element alteration, in addition to the in situ source for more evolved stars, may be required for M92. Co mparison of our M92 subgiant abundance ratios with those of M92 red gi ants may indicate that pollution occurred after the present generation of cluster stars formed, but until the cause or causes of the subgian t versus giant Fe abundance discrepancy are definitively identified, t his conclusion is uncertain. A polluting source of our Na and Mg anoma lies produced via processing in a previous stellar generation also has complications; namely, how the Mg and Na anomalies arise without appa rently any net influence on our subgiants' Li abundances and on the C abundances of other M92 subgiants. A similar quandary may exist in som e 47 Tuc turnoff stars. An understanding of cluster abundance variatio ns (by whatever mechanisms) and their behavior with evolutionary state may be needed for a complete understanding of absolute and relative g lobular clusters ages, and for derivation of the primordial Li abundan ce.