New Analyses of Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M5

Abstract

We present a chemical composition analysis of 36 giant stars in the mildly metalpoor (<[Fe/H]> = -1.21) globular cluster M5 (NGC 5904). The analysis makes use of high resolution data acquired for 25 stars at the Keck I telescope, as well as a reanalysis of the high resolution spectra for 13 stars acquired for an earlier study at Lick Observatory. We employed two analysis techniques: one, adopting standard spectroscopic constraints, including setting the surface gravity from the ionization equilibrium of iron, and two, subsequent to investigating alternative approaches, adopting an analysis consistent with the non-LTE precepts as recently described by Thévenin & 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. These variations are well-correlated with the CN bandstrength index S(3839). Surprisingly, in M5 the dependence of the abundance variations on log g is in the opposite sense to that discovered in M13 by the Lick-Texas group where the relationship provided strong evidence in support of the evolutionary scenario. The present analysis of M5 giants does not necessarily rule out an evolutionary scenario, but it provides no support for it either. In comparing the abundances of M5 and M4 (NGC 6121), another mildly metal-poor (<[Fe/H]> = -1.08) 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

Abundances in Stars from the Red Giant Branch Tip to near the Main-Sequence Turnoff in M5

“…The mean abundance ratios of Sc, V, and Ni are consistent with the earlier results of Sneden et al (1992), who analyzed high-resolution spectra of 13 giant stars in M5. Our iron peak abundance ratios are also in agreement with those results obtained by Ivans et al (2001), who analyzed high-resolution spectra of 36 giants in M5. A comparison of our abundance ratios with those of these two previous studies for all elements analyzed by them is given in Table 8, which will be discussed in detail in x 7.6.…”

“…The slope of the fit is (+1.2 AE 0.5)Â 10 À4 dex K À1 , which is essentially identical to the small, not statistically significant slope obtained from the Fe i lines. The mean [Fe/H] weighted by the errors for the 17 M5 stars with detected Fe ii lines is À1.28 AE 0.02, in very good agreement with our result from Fe i lines and earlier determinations (Sneden et al 1992;Shetrone 1996;Ivans et al 2001).…”

“…The mean abundance ratios of Ba (see x 7.6 for details), La, and Eu are consistent with the earlier results of Ivans et al (2001) listed in Table 8. The abundances of Ba, La, and Eu are overabundant relative to Fe, as is seen in other globular clusters such as M4 (Ivans et al 1999) and M15 (Sneden et al 1997 Burris et al (2000), Ba is a neutron capture element synthesized through s-process reactions that occur mainly in low-mass asymptotic giant branch stars, while Eu is exclusively an r-process element.…”

“…We summarize here the comparison of our results with those of Ivans et al (2001), which is the most extensive previous high-dispersion abundance analysis for M5. Recall that Ivans et al (2001) considered only luminous red giants and asymptotic giant branch stars.…”

“…Sandquist et al (1996) discuss the predominantly blue horizontal branch of M5. Previous high-dispersion abundance studies, the most extensive of which is Ivans et al (2001), include only luminous giant and asymptotic giant branch stars. A study of C and N variations among a large sample of M5 stars at the base of the red giant branch is given in Cohen, Briley, & Stetson (2002).…”