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تسجيل مستخدم جديدChemical Abundance Variations in Globular Clusters: Recent Results from Mildly Metal-Poor M5
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Inese I. Ivans
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We present a chemical composition analysis of 36 giant stars in mildly metal-poor globular cluster M5. In comparing the M5 results to those obtained in M4, a cluster previously considered to be a ``twin in age, metallicity and chemical composition, we find large star-to-star variations in the abundances of elements sensitive to proton-capture nucleosynthesis, similar [Fe/H] values, but factor of two differences in some alpha-capture, odd-Z and slow neutron-capture process elements. Among stars in globular clusters, apparently there are no definitive ``single values of [el/Fe] at a given [Fe/H] for many important elements.
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We present a chemical composition analysis of 36 giant stars in the mildly metal-poor globular cluster M5 (NGC 5904). The analysis makes use of high resolution data acquired at the Keck I telescope as well as a re-analysis of high resolution spectra
acquired for an earlier study at Lick Observatory. We employed two analysis techniques: one, adopting standard spectroscopic constraints, and two, adopting an analysis consistent with the non-LTE precepts as recently described by Thevenin & Idiart. The abundance ratios we derive for magnesium, silicon, calcium, scandium, titanium, vanadium, nickel, barium and europium in M5 show no significant abundance variations and the ratios are comparable to those of halo field stars. However, large variations are seen in the abundances of oxygen, sodium and aluminum, the elements that are sensitive to proton-capture nucleosynthesis. In comparing the abundances of M5 and M4 (NGC 6121), another mildly metal-poor globular cluster, we find that silicon, aluminum, barium and lanthanum are overabundant in M4 with respect to what is seen in M5, confirming and expanding the results of previous studies. In comparing the abundances between these two clusters and others having comparable metallicities, we find that the anti-correlations observed in M5 are similar to those found in more metal-poor clusters, M3, M10 and M13, whereas the behavior in M4 is more like that of the more metal-rich globular cluster M71. We conclude that among stars in Galactic globular clusters, there is no definitive ``single value of [el/Fe] at a given [Fe/H] for at least some alpha-capture, odd-Z and slow neutron-capture process elements, in this case, silicon, aluminum, barium and lanthanum.
We present a chemical composition analysis of 36 giants in the nearby mildy metal-poor (<[Fe/H]> = -1.18) CN-bimodal globular cluster M4. Confronted with a cluster that has large and variable interstellar extinction across the cluster face, we combin
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in order to check whether the K abundances are uniform within a cluster and to investigate the behavior of [K/Fe] ratio at the relevant metallicity range of -2.5 <[Fe/H] < -1. We confirmed that [K/H] (as well as [Fe/H]) is almost homogeneous within each cluster to a precision of < ~0.1 dex, though dubiously large deviations are exceptionally seen for two peculiar stars showing signs of considerably increased turbulence in the upper atmosphere. The resulting [K/Fe] ratios are mildly supersolar by a few tenths of dex for three clusters, tending to gradually increase from ~+0.1-0.2 at [Fe/H] ~-1 to ~+0.3 at [Fe/H] ~ -2.5. This result connects reasonably well with the [K/Fe] trend of disk stars (-1 < [Fe/H]) and that of extremely metal-poor stars (-4 <[Fe/H] < -2.5). That is, [K/Fe] appears to continue a gradual increase from [Fe/H]~0 toward a lower metallicity regime down to [Fe/H]~-3, where a broad maximum of [K/Fe]~+0.3-0.4 is attained, possibly followed by a slight downturn at [Fe/H]<~-3.
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Heavy elements, those produced by neutron-capture reactions, have traditionally shown no star-to-star dispersion in all but a handful of metal-poor globular clusters (GCs). Recent detections of low [Pb/Eu] ratios or upper limits in several metal-poor
GCs indicate that the heavy elements in these GCs were produced exclusively by an r-process. Reexamining GC heavy element abundances from the literature, we find unmistakable correlations between the [La/Fe] and [Eu/Fe] ratios in 4 metal-poor GCs (M5, M15, M92, and NGC 3201), only 2 of which were known previously. This indicates that the total r-process abundances vary star-to-star (by factors of 2-6) relative to Fe within each GC. We also identify potential dispersion in two other GCs (M3 and M13). Several GCs (M12, M80, and NGC 6752) show no evidence of r-process dispersion. The r-process dispersion is not correlated with the well-known light element dispersion, indicating it was present in the gas throughout the duration of star formation. The observations available at present suggest that star-to-star r-process dispersion within metal-poor GCs may be a common but not ubiquitous phenomenon that is neither predicted by nor accounted for in current models of GC formation and evolution.
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