Robert P. Kraft

A Globular Cluster Metallicity Scale Based on the Abundance of Fe ii

Assuming that in the atmospheres of low-mass, metal-poor red giant stars, one-dimensional models based on local thermodynamic equilibrium accurately predict the abundance of iron from Fe ii, we derive a globular cluster metallicity scale based on the equivalent widths of Fe ii lines measured from high-resolution spectra of giants in 16 key clusters lying in the abundance range 2.4 ! (Fe/H)II ! 0.7. We base the scale largely on the analysis of spectra of 149 giant stars in 11 clusters by the Lick-Texas group supplemented by high-resolution studies of giants in five other clusters. We also derive ab initio the true distance moduli for certain key clusters (M5, M3, M13, M92, and M15) as a means of setting stellar surface gravities. Allowances are made for changes in the abundance scale if one employs (1) Kurucz models with and without convective overshooting to represent giant star atmospheres in place of MARCS models and (2) the Houdashelt et al. color-temperature scale in place of the Alonso et al. scale. We find that (Fe/H)II is correlated linearly with , the reduced strength of the near-infrared Ca ii triplet defined � W

Star-to-Star Abundance Variations among Bright Giants in the Mildly Metal-poor Globular Cluster M4

We present a chemical composition analysis of 36 giants in the nearby mildly metal-poor ( = -1.18) CN-bimodal globular cluster M4. The stars were observed at the Lick and McDonald Observatories using high-resolution echelle spectrographs and at the Cerro Tololo Inter-American Observatory using the multiobject spectrometer. Confronted with a cluster having interstellar extinction that is large and variable across the cluster face, we combined traditional spectroscopic abundance methods with modifications to the line depth ratio technique pioneered by Gray to determine the atmospheric parameters of our stars. We derive a total-to-selective extinction ratio of 3.4 ± 0.4 and an average E(B-V) reddening of 0.33 ± 0.01, which is significantly lower than that estimated by using the dust maps made by Schlegel and coworkers. We determine abundance ratios typical of halo field and cluster stars for scandium, titanium, vanadium, nickel, and europium with star-to-star variations in these elements of less than ±0.1. Silicon, aluminum, barium, and lanthanum are overabundant with respect to what is seen in other globular clusters of similar metallicity. These overabundances confirm the results of an earlier study by Brown & Wallerstein based on a much smaller sample of M4 giants. Superposed on the primordial abundance distribution is evidence for the existence of proton capture synthesis of carbon, oxygen, neon, and magnesium. We recover some of the C, N, O, Na, Mg, and Al abundance swings and correlations found in other more metal-poor globular clusters, but the range of variation is muted. In the case of Mg and Al, this is compatible with the idea that the Al enhancements are derived from the destruction of 25,26Mg, not 24Mg. We determine that the C + N + O abundance sum is constant to within the observational errors and agrees with the C + N + O total that might be expected for M4 stars at birth. The asymptotic giant branch (AGB) stars in M4 have C, N, and O abundances that show less evidence for proton capture nucleosynthesis than is found in the less evolved stars of the red giant branch (RGB). Deeply mixed stars of the RGB, subsequent to the helium core flash, might take up residence on the blue end of the horizontal branch and thus fail to evolve back to the AGB, but reasons for skepticism concerning this scenario are noted.

Oxygen abundances in halo giants. II : Giants in the globular clusters M13 and M3 and the intermediately metal-poor halo field

Oxygen, sodium, iron, vanadium, and scandium abundances are derived for giants in the intermediately metal-poor globular clusters M 3 and M 13 and for giants of comparable metallicity in the local halo field. The data are obtained from Lick Observatory Hamilton Echelle spectra centered on the [O I] doublet. For M13 we derive = -1.51 ± 0.01 from 13 giants with a range of 500 K in effective temperature. For M3, we find = -1.47 ± 0,01, from seven stars with a range of 300 K in effective temperature. There is no compelling evidence for star-to-star variations in [Fe/H] in either cluster

THE ABUNDANCES OF NEUTRON-CAPTURE SPECIES IN THE VERY METAL-POOR GLOBULAR CLUSTER M15: A UNIFORM ANALYSIS OF RED GIANT BRANCH AND RED HORIZONTAL BRANCH STARS

The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Δ(RHB – RGB) ≈ 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.

Oxygen abundances in halo giants. I : giants in the very metal-poor globular clusters M92 and M15 and the metal-poor halo field

We derive oxygen, iron, vanadium, and scandium abundances for very metal-poor giants in the globular clusters M92 and M15, and giants of comparable metallicity in the local halo field. We analyze the [O I] doublet (λλ 6300, 6363) and nearby metallic lines in spectra obtained with a TI CCD detector and the Hamilton echelle spectrograph at the Lick Observatory 3.0 m telescope, using line analysis and spectral synthesis codes developed at the University of Texas.

The two metallicity groups of the globular cluster M 22: a chemical perspective

We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, , Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a di erent content in iron, neutron capture elements Y, Zr and Ba, and element Ca. The principal results of this study are: (i) substantial star-to-star metallicity scatter ( 2.0 . [Fe/H] . 1.6); (ii) enhancement of s-process/r-process neutron-capture abundance ratios in a fraction of giants, positively correlated with metallicity; (iii) sharp separation between the s-process-rich and s-process-poor groups by [La/Eu] ratio; (iv) possible increase of [Cu/Fe] ratios with increasing [Fe/H], suggesting that this element also has a significant s-process component; and (v) presence of Na-O and C-N anticorrelations in both the stellar groups.

The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3

We report new chemical abundances of 23 bright red giant members of the globular cluster M3, based on high-resolution (R ~ 45,000) spectra obtained with the Keck I telescope. The observations, which involve the use of multislits in the HIRES Keck I spectrograph, are described in detail. Combining these data with a previously reported small sample of M3 giants obtained with the Lick 3 m telescope, we compare metallicities and [X/Fe] ratios for 28 M3 giants with a 35-star sample in the similar-metallicity cluster M13, and with Galactic halo field stars having [Fe/H] < -1. For elements having atomic number A ? A(Si), we derive little difference in [X/Fe] ratios in the M3, M13, or halo field samples. All three groups exhibit C depletion with advancing evolutionary state beginning at the level of the red giant branch bump, but the overall depletion of about 0.7?0.9 dex seen in the clusters is larger than that associated with the field stars. The behaviors of O, Na, Mg, and Al are distinctively different among the three stellar samples. Field halo giants and subdwarfs have a positive correlation of Na with Mg, as predicted from explosive or hydrostatic carbon burning in Type II supernova sites. Both M3 and M13 show evidence of high-temperature proton-capture synthesis from the ON, NeNa, and MgAl cycles, while there is no evidence for such synthesis among halo field stars. But the degree of such extreme proton-capture synthesis in M3 is smaller than it is in M13: the M3 giants exhibit only modest deficiencies of O and corresponding enhancements of Na, less extreme overabundances of Al, fewer stars with low Mg and correspondingly high Na, and no indication that O depletions are a function of advancing evolutionary state, as has been claimed for M13. We have also considered NGC 6752, for which Mg isotopic abundances have been reported by Yong et al. Giants in NGC 6752 and M13 satisfy the same anticorrelation of O abundances with the ratio (25Mg + 26Mg)/24Mg, which measures the relative contribution of rare to abundant isotopes of Mg. This points to a scenario in which these abundance ratios arose in the ejected material of 3?6 M? cluster stars, material that was then used to form the atmospheres of the presently evolving low-mass cluster stars. It also suggests that the low oxygen abundance seen among the most evolved M13 giants arose in hot bottom O-to-N processing in these same intermediate-mass cluster stars. Thus, mixing is required by the dependence of some abundance ratios on luminosity, but an earlier nucleosynthesis process in a hotter environment than giants or main-sequence stars is required by the variations previously seen in stars near the main sequence. The nature and the site of the earlier process is constrained but not pinpointed by the observed Mg isotopic ratio.

The abundance spread among giants and subgiants in the globular cluster omega centauri

We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster omega Centauri based on the CaII infrared triplet. Two samples of stars were observed: 234 stars at M_V = 1.25 on the lower giant branch at radial distances between 8 and 23arcmin, and 145 stars at M_V = -1.3 at radial distances between 3 and 22arcmin. Previous metallicity studies found a non-gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are significantly narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of --1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which omega Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, omega Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that omega Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and may have sunk to the center by dynamical mass segregation over the lifetime of the cluster.

Carbon and nitrogen abundances in giant stars of the metal-poor globular cluster M92

Zinn in 1973 and 1977 and Norris and Zinn in 1977 showed that in M92 and several other metal-poor globular clusters the G bands (mostly due to CH) in the spectra of asymptotic giant branch (AGB) stars are systematically weaker than those found in the less highly evolved subgiant branch (SGB) stars. If carbon is depleted in the atmospheres of evolved stars because material at the base of the envelope, processed through the CN cycle, has been mixed with the material above, then the atmospheric nitrogen abundance should be correspondingly increased. In this paper we test the hypothesis that C and N abundances in M92 giants are negatively correlated as the evolutionary state becomes more advanced. We find that this simple hypothesis is not adequate to describe the complex behavior of C and N in the cluster giants.

Oxygen abundances in halo giants. III : Giants in the mildly metal-poor globular cluster M5

We derive abundances of oxygen, sodium, iron-peak, and several traditional «α-elements» for giants in the mildly metal-poor globular cluster M5. The data are obtained from Lick Observatory Hamilton Echelle spectra centered on the [O I] doublet. We derive =−1.17±0.01 from 13 giants with a range of 350 K in effective temperature. There is no evidence for star-to-star variations in [Fe/H] in this cluster. The range of oxygen abundances is similar to that found earlier in M3 and M92. The largest values of [O/Fe] are near +0.35 and the smallest are near −0.25