Abundances in a Large Sample of Stars in M3 and M13

By J. Cohen, Jorge Meléndez. 2004

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We have carried out a detailed abundance analysis for 21 elements in a sample of 25 stars with a wide range in luminosity from luminous giants to stars near the main-sequence turnoff in the globular cluster M13 ([Fe/H] = -1.50 dex) and in a sample of 13 stars distributed from the tip to the base of the red giant branch (RGB) in the globular cluster M3 ([Fe/H] = -1.39 dex). The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R = λ/Δλ = 35,000). Most elements, including Fe, show no trend with Teff and low scatter around the mean between the top of the RGB and near the main-sequence turnoff, suggesting that at this metallicity, non-LTE effects and gravitationally induced heavy-element diffusion are not important for this set of elements over the range of stellar parameters spanned by our sample. We have detected an anticorrelation between O and Na abundances, observed previously among the most luminous RGB stars in both of these clusters, in both M3 and in M13 over the full range of luminosity of our samples, i.e., in the case of M13 to near the main-sequence turnoff. M13 shows a larger range in both O and Na abundance than does M3 at all luminosities, in particular having a few stars at its RGB tip with unusually strongly depleted O. We detect a correlation between Mg abundance and O abundance among the stars in the M13 sample. We also find a decrease in the mean Mg abundance as one moves toward lower luminosity, which we tentatively suggest is due to our ignoring non-LTE effects in Mg. Although CN burning must be occurring in both M3 and in M13, and ON burning is required for M13, we combine our new O abundances with published C and N abundances to confirm with quite high precision that the sum of C+N+O is constant near the tip of the giant branch, and we extend this down to the bump in the luminosity function. The same holds true for a smaller sample in M3, with somewhat larger variance. Star I-5 in M13 has large excesses of Y and of Ba, with no strong enhancement of Eu, suggesting that an s-process event contributed to its heavy-element abundances. The mean abundance ratios for M3 and for M13 are identical to within the errors. They show the typical pattern for metal-poor globular clusters of scatter among the light elements, with the odd atomic number elements appearing in the mean enhanced. The Fe-peak elements, where the odd atomic number elements are excessively depleted, do not show any detectable star-to-star variations in either cluster. The abundance ratios for 13 Galactic globular clusters with recent detailed abundance analyses, obtained by combining our samples with published data, are compared with those of published large surveys of metal-poor halo field stars. For most elements, the agreement is very good, suggesting a common chemical history for the halo field and cluster stars.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

The second and third parameters of the horizontal branch in globular clusters (2010, methodology): “…et al. (2009a, 2009b); we complement this data set with a few GCs from the literature (Shetrone & Keane 2000 for NGC 362; Sneden et al. 2004 and Cohen & Melendez 2005 for NGC 5272 and NGC 6205; Sneden et al. 1991, 2000 for NGC 6341; Marino et al. 2008 for NGC 6121; Marino et al. 2009 for NGC…”
The Double Subgiant Branch of NGC 1851: The Role of the CNO Abundance (2007, unknown): “As for other GCs with spectroscopic measurements of the CNO sum, Cohen & Meléndez (2005) found for example a total CNO abundance constant within ∼0.12 dex in M13.”
Formation and dynamical evolution of multiple stellar generations in globular clusters (2008, background): “Some studies show that the sum of CNO elements is approximately constant among FG and SG stars (Cohen, Briley & Stetson 2002; Cohen & Meléndez 2005).”
Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca, and Ti (2010, background): “As pointed out by Fulbright et al. (2007), two O-deficient stars (I-264 and IV-203) at [Fe/H] ≈ −1.25 have peculiar abundances similar to the O-Na anti-correlation seen in globular clusters (e.g. Gratton et al. 2004; Cohen & Meléndez 2005; Yong et al.”
An Abundance Analysis of Bright Giants in the Globular Cluster NGC 1851 (2007, background): “Available measurements indicate that the abundances of Fe, O, Na, Al, and neutron-capture elements do not change as a function of evolutionary status from the main sequence to the giant branch in globular clusters (e.g., Gratton et al. 2001; James et al. 2004; Cohen & Mele´ndez 2005).”
The fraction of second generation stars in globular clusters from the analysis of the horizontal branch (2008, background): “The matter must have been processed through the hot CNO cycle, and not, or only marginally, through the helium burning phases, since the sum of CNO elements is the same in the “normal” and in the anomalous stars (e.g. Smith et al. 1996; Ivans et al. 1999; Cohen & Meléndez 2005).”
A double stellar generation in the globular cluster NGC 6656 (M 22): Two stellar groups with different iron and s-process element abundances (2009, background): “…et al. (1998) found a CN bimodality in MS and subgiant branch (SGB) stars in 47 Tuc, and the NaO anti-correlation was observed for the MS turn off (TO) and SGB in M 13 (Cohen & Melendez 2005), NGC 6397 and NGC 6752 (Carretta et al. 2005; Gratton et al. 2001), and NGC 6838 (Ramírez & Cohen 2002).”
NEW OPTICAL REDDENING MAPS OF THE LARGE AND SMALL MAGELLANIC CLOUDS (2011, background): “They assumed one single metallicity for all RR Lyrae stars in the LMC, −1.39 dex (Cohen & Meléndez 2005), which results in a theoretical intrinsic colour of (V −I) = 0.48, with a mean period of 0.57 days for all RR Lyrae in the LMC.”
THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED METHOD ALONG WITH COLOR–MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES (2013, background): “A noteworthy cluster in this regard is NGC 7492, which shows the O–Na anticorrelation (Cohen & Meléndez 2005b) despite having log σ0ve,0 ≈ 3.0.”
A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies (2005, background): “…(0.02) +0.28 (0.01) +0.16 (0.02) +0.31 (0.02) 0.17 (0.04) +0.26 (0.03) +0.14 (0.03) +0.49 (0.03) +0.23 (0.02) +0.26 +0.43 0.66 0.23 0.35 25 stars; Cohen & Meléndez (2005a) 1.57 +0.13 (0.03) +0.31 (0.02) +0.28 (0.01) +0.16 (0.02) +0.31 (0.02) 0.17 (0.04) +0.25 (0.03) +0.13 (0.03) +0.49 (0.03)…”
Atomic Diffusion and Mixing in Old Stars. I. Very Large Telescope FLAMES-UVES Observations of Stars in NGC 6397 (2007, background): “Cohen and Meléndez (2005) studied M 13 at [Fe/H]≈−1.5, but found no abundance trends.”
Abundance patterns of multiple populations in globular clusters: a chemical evolution model based on yields from AGB ejecta (2010, background): “2.4) that the sum of CNO elements varies no more than a factor two among the GC stars (Cohen & Meléndez 2005) (or by a factor three in NGC 1851, see Ventura et al. 2009); this implies that the SG progenitors must be AGB stars with masses larger than ∼5 M⊙ (∼ 4 M⊙ in the latter case) and that the…”
THE SEGUE STELLAR PARAMETER PIPELINE. IV. VALIDATION WITH AN EXTENDED SAMPLE OF GALACTIC GLOBULAR AND OPEN CLUSTERS (2010, background): “Finally, while our value is only barely within 1σ of the estimated iron abundance for M3 from Cohen & Meléndez (2005), who obtained a somewhat higher value of [Fe/H] = −1.39 based on Keck/HIRES spectroscopy, it should be kept in mind that recent results from Cohen and collaborators adopt a…”
ON THE SOURCE OF THE DUST EXTINCTION IN TYPE Ia SUPERNOVAE AND THE DISCOVERY OF ANOMALOUSLY STRONG Na i ABSORPTION (2013, background): “…variations in C, N, O and Na have been observed in stars at or near the mainsequence turn-off (Gratton et al. 2001; Ramı́rez & Cohen 2003; James et al. 2004; Cohen & Meléndez 2005; Cohen et al. 2005), in support of the self-pollution scenario as first proposed by Cottrell & Da Costa (1981).”
Multiple populations in globular clusters (2012, result): “This was later confirmed on rather small samples by Ramirez & Cohen (2002, 2003) for M 71 and M 5, Carretta et al. (2004) for 47 Tuc, Cohen & Melendez (2005) for M 13, and recently by D’Orazi et al. (2010b), Lind et al. (2011), and Monaco et al. (2011) on large samples of stars in 47 Tuc, NGC 6397, ”
Spectroscopic and photometric evidence of two stellar populations in the Galactic globular cluster NGC 6121 (M 4) (2008, background): “Observational evidence for variations in the chemical compsition of light elements in Globular Cluster (GC) stars were know since Cohen (1978), who noted a scatter in Na among stars in M3 and M13.”
Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra (2009, methodology): “…in Figure 10, where we used cluster errors from Table 4 for our sample; for M 3 (NGC 5272) and M 13 (NGC 6205) error bars in [Fe/H] are the quadratic sum of therms scatters quoted in Sneden et al. (2004) and Cohen and Melendez (2005), since no systematic errors are derived in the original studies.”
The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851 (2007, background): “We note that if the CNO enhancement in second-generation stars of NGC 1851 is confirmed, NGC 1851 might differ from other GCs, in which the sum of CNO elements for normal and self-enriched stars seems to be constant (Cohen & Meléndez 2005).”
PROBING THE OUTER GALACTIC HALO WITH RR LYRAE FROM THE CATALINA SURVEYS (2012, background): “Zinn & West 1984; Armosky et al. 1994; Kraft et al. 1992; Sandstrom et al. 2001; Cohen & Melendez 2005; Cacciari et al. 2005).”