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

By I. Ivans, R. P. Kraft, C. Sneden, Graeme H. Smith, R. Rich, M. Shetrone. 2001

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We present a chemical composition analysis of 36 giant stars in the mildly metal-poor (⟨[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: (1) adopting standard spectroscopic constraints, including setting the surface gravity from the ionization equilibrium of iron, and (2) 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 band strength 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 that 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 anticorrelations observed in M5 are similar to those found in more metal-poor clusters, M3, M10, and M13 (⟨[Fe/H]⟩ = -1.5 to -1.6), whereas the behavior in M4 is more like that of the more metal-rich globular cluster M71 (⟨[Fe/H]⟩ ∼ -0.8). We conclude that among stars in Galactic globular clusters there is no definitive "single" value of [X/Fe] at a given [Fe/H] for at least some α-capture, odd-Z, and slow neutron-capture process elements, in this case, silicon, aluminum, barium, and lanthanum.

Subfields: Astronomy and Astrophysics, Instrumentation

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Probing the Neutron‐Capture Nucleosynthesis History of Galactic Matter (2002, background): “To date, there have been only a few detailed abundance studies of the heavy neutron capture elements in globular clusters (see, e.g. Ivans et al. 1999; 2001).”
Abundance Variations within Globular Clusters (2004, background): “Ivans et al. (2001) noted in their study of M5 that this cluster does not behave like M13, and the same result was found by Carretta et al. (2003) in examining the Na distributions found by Sneden et al. (2000a) for M15 and M92.”
A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies (2005, background): “The ratios from Sneden et al. (1992) are not used because the stars were reanalyzed by Ivans et al. (2001).”
MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. II. CATALOG OF STARS IN MILKY WAY DWARF SATELLITE GALAXIES (2010, methodology): “Fe I lines vary little with log g. Ideally, we would use only Fe II lines because they are not subject to overionization problems (Thévenin & Idiart 1999; Ivans et al. 2001).”
Abundances in giant stars of the globular cluster NGC 6752 (2005, unknown): “The line lists were compiled from Kurucz & Bell (1995), Ivans et al. (2001), Ramírez & Cohen (2002), and Reddy et al. (2003) and are presented in Table 2.”
Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta (2010, background): “From all the above, it is apparent that, in order to explain the GC chemical patterns, it is necessary to introduce a form of dilution of the AGB ejecta, either with pristine gas, or with gas not showing the peculiar abundance patterns of the hot–CNO processed matter.”
Multiple populations in globular clusters (2012, background): “Neither will we insist on the early work on the Na-O (and related Mg-Al) anticorrelations conducted mainly by the Lick-Texas group (e.g., Kraft et al. 1993; 1997; Ivans et al. 2001; Sneden et al. 2004, to name just a few).”
Spectroscopic Binaries, Velocity Jitter, and Rotation in Field Metal-poor Red Giant and Red Horizontal-Branch Stars (2003, background): “…and even between magnesium and aluminum in some cases (see, e.g., Sneden et al. 1991; Kraft et al. 1992; Sneden et al. 1992; Kraft et al. 1993; Sneden et al. 1994; Kraft et al. 1995; Pilachowski et al. 1996b; Kraft et al. 1997; Sneden et al. 1997; Kraft et al. 1998; Ivans et al. 1999, 2001).”
Properties of stellar generations in globular clusters and relations with global parameters (2010, background): “This is shown by the close comparison between M 4 and M 5 (NGC 6121 and NGC 5904), performed by Ivans et al. (2001), which is fully confirmed by our analysis (Carretta et al. 2009b).”
Abundances of Baade’s Window Giants from Keck HIRES Spectra. II. The Alpha and Light Odd Elements (2006, background): “Also plotted are the 12 Fulbright et al. same of M4 stars from Ivans et al. (1999) and M5 stars from Ivans et al. (2001).”
CHEMICAL TAGGING CAN WORK: IDENTIFICATION OF STELLAR PHASE-SPACE STRUCTURES PURELY BY CHEMICAL-ABUNDANCE SIMILARITY (2016, result): “N—also agree well with other studies (Ivans et al. 2001; Mészáros et al. 2015).”
Abundances in Stars from the Red Giant Branch Tip to Near the Main-Sequence Turnoff in M71. III. Abundance Ratios (2001, background): “…metal poor globular clusters, 47 Tuc (Brown et al. 1990; Brown &Wallerstein 1992; Norris & Da Costa 1995), M71 (Sneden et al. 1994; this paper), M5 (Ivans et al. 2001; Shetrone 1996; Sneden et al. 1992), M4 (Ivans et al. 1999), NGC 6752 (Gratton et al. 2001), M3 (Kraft et al. 1993), M10 (Kraft et…”
The two metallicity groups of the globular cluster M 22: a chemical perspective (2011, background): “However the wellstudied M5 (Ivans et al. 2001, Ramı́rez & Cohen 2003) and M4 (Ivans et al. 1999, Marino et al. 2008) have relative abundance mixes that resemble those of the lower and higher metallicity M22 groups, respectively.”
Near-Ultraviolet Observations of HD 221170: New Insights into the Nature of r-Process-rich Stars (2006, background): “Supplementing these lines are features we have employed in other high resolution abundance studies of globular cluster red giant stars (e.g., Ivans et al. 2001; Sneden et al. 2004; Johnson, Ivans & Stetson 2005), rprocess-enriched metal-poor stars such as BD+17◦3248 and CS 22892-052 (e.g., Cowan et…”
A Globular Cluster Metallicity Scale Based on the Abundance of Feii (2002, methodology): “Adoption of the Houdashelt et al. Teff-Scale Ivans et al. (2001), in their study of 36 giants in M5, compared the values of Teff obtained from the Houdashelt et al. (2000) and Alonso et al. (1999) color calibrations and found an average offset of +43K ± 20K, in the sense Houdashelt et al. minus…”
Na-O anticorrelation and HB - VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra (2009, result): “Some cases are obvious, like the pair of clusters M 5 (NGC 5904) and M 4 (NGC 6121), for which such a difference has already been found in previous analyses (Carney 1996; Ivans et al. 1999, 2001).”