Christopher Sneden

The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

High dispersion spectra (R > 40 000) for a quite large number of stars at the main sequence turn-o and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The (Fe/H) values we found are 2:03 0:02 0:04 and 1:42 0:02 0:04 for NGC 6397 and NGC 6752 respectively, where the rst error bars refer to internal and the second ones to systematic errors (within the abundance scale dened by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the (Fe/H)s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The (O=Fe) = 0:21 0:05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the rst time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very dicult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

Galactic evolution of Sr, Y, and Zr : a multiplicity of nucleosynthetic processes

In this paper we follow the Galactic enrichment of three easily observed light n-capture elements: Sr, Y, and Zr. Input stellar yields have been first separated into their respective main and weak s-process components and r-process component. The s-process yields from asymptotic giant branch (AGB) stars of low to intermediate mass are computed, exploring a wide range of efficiencies of the major neutron source, 13 C, and covering both disk and halo metallicities. AGB stars have been shown to reproduce the mains-component in the solar system, i.e., the s-process isotopic distribution of allheavy isotopes with atomic mass number A > 90, with a minor contribution to the light s-process isotopes up to A � 90. The concurrent weak s-process, which accounts for the major fraction of the light s-process isotopes in the solar system and occurs in massive stars by the operation of the 22 Ne neutron source, is discussed in detail. Neither the main s -n or the weaks-components are shown to contribute significantly to the neutron-capture element abundances observed in unevolved halo stars. Knowing the s-process distribution at the epoch of the solar system formation, we first employed the r-process residuals method to infer the isotopic distribution of the r-process. We assumed a primary r-process production in the Galaxy from moderately massive Type II supernovae that best reproduces the observational Galactic trend of metallicity versus Eu, an almost pure r-process element. We present a detailed analysis of a large published database of spectroscopic observations of Sr, Y, Zr, Ba, and Eu for Galactic stars at various metallicities, showing that the observed trends versus metallicity can be understood in light of a multiplicity of stellar neutron-capture components. Spectroscopic observations of the Sr, Y, and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of neutron-capture processes forming Sr, Y, and Zr. In particular, the observed [Sr, Y, Zr/Ba, Eu] ratio is clearly not flat at low metallicities, as we would expect if Ba, Eu and Sr, Y, Zr all had the same r-process nucleosynthetic origin. We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr. Making use of the very r-process‐rich and very metal-poor stars like CS 22892� 052 and CS 31082� 001, we find hints and discuss the possibility of a primary process in low-metallicity massive stars, different from the ‘‘classical s-process’’ and from the ‘‘classical r-process’’ that we tentatively define LEPP (lighter element primary process). This allows us to revise the estimates of the r-process contributions to the solar Sr, Y, and Zr abundances, as well as of the contribution to the s-only isotopes 86 Sr, 87 Sr, and 96 Mo. Subject headings: Galaxy: abundances — Galaxy: evolution — nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB

The Extremely Metal-poor, Neutron Capture-rich Star CS 22892-052: A Comprehensive Abundance Analysis*

High-resolution spectra obtained with three ground-based facilities and the Hubble Space Telescope (HST) have been combined to produce a new abundance analysis of CS 22892-052, an extremely metal-poor giant with large relative enhancements of neutron capture elements. A revised model stellar atmosphere has been derived with the aid of a large number of Fe peak transitions, including both neutral and ionized species of six elements. Several elements, including Mo, Lu, Au, Pt, and Pb, have been detected for the first time in CS 22892-052, and significant upper limits have been placed on the abundances of Ga, Ge, Cd, Sn, and U in this star. In total, abundance measurements or upper limits have been determined for 57 elements, far more than previously possible. New Be and Li detections in CS 22892-052 indicate that the abundances of both these elements are significantly depleted compared to unevolved main-sequence turnoff stars of similar metallicity. Abundance comparisons show an excellent agreement between the heaviest n-capture elements (Z ≥ 56) and scaled solar system r-process abundances, confirming earlier results for CS 22892-052 and other metal-poor stars. New theoretical r-process calculations also show good agreement with CS 22892-052 abundances and the solar r-process abundance components. The abundances of lighter elements (40 ≤ Z ≤ 50), however, deviate from the same scaled abundance curves that match the heavier elements, suggesting different synthesis conditions or sites for the low-mass and high-mass ends of the abundance distribution. The detection of Th and the upper limit on the U abundance together imply a lower limit of 10.4 Gyr on the age of CS 22892-052, quite consistent with the Th/Eu age estimate of 12.8± 3 Gyr. An average of several chronometric ratios yields an age 14.2± 3 Gyr.

THE HIGH-RESOLUTION CROSS-DISPERSED ECHELLE WHITE PUPIL SPECTROMETER OF THE MCDONALD OBSERVATORY 2.7-M TELESCOPE

A new high-resolution cross-dispersed echelle spectrometer has been installed at the coude focus of the McDonald Observatory 2.7-m telescope. Its primary goal was to simultaneously gather spectra over as much of the spectral range 3400A to 1 micron as practical, at a resolution R = lambda/delta-lambda 60,000 with signal-to-noise ratio of ~100 for stars down to magnitude 11, using 1-hour exposures. In the instrument as built, two exposures are all that are needed to cover the full range. Featuring a white-pupil design, fused silica prism ross disperser, and folded Schmidt camera with a Tektronix 2048 X 2048 CCD used at either of two foci, it has been in regularly-scheduled operation since April 1992. Design details and performance will be described.

Improved Laboratory Transition Parameters forEu II and Application to the Solar Europium Elemental and Isotopic Composition

New radiative lifetime measurements using time-resolved laser-induced fluorescence are reported for the lowest six even-parity levels of Eu II. Branching fractions, measured from Fourier transform spectra, are combined with these lifetimes to determine atomic transition probabilities for the strongest blue-UV lines and additional yellow-red lines of Eu II. These results are compared with published data, and generally good agreement is found. Recommended hyperfine structure constants and isotopic shifts for these lines are also assembled from the literature and supplemented, as needed, using results from nonlinear least-squares fits of line profiles in Fourier transform spectra. These laboratory data are applied in a new determination of the solar Eu elemental abundance, yielding log10 e(Eu) = 0.52 ± 0.01, with ±0.04 estimated for each of internal (scatter) and external (systematic) uncertainties. From analysis of the profiles of three Eu II lines, primarily λ4129, isotopic fractions of 151Eu and 153Eu are shown to be consistent with their values in meteoritic material.

Experimental Radiative Lifetimes, Branching Fractions, and Oscillator Strengths for La II and a New Determination of the Solar Lanthanum Abundance

Radiative lifetimes, accurate in most cases to ±5%, from time-resolved laser-induced fluorescence measurements on a slow beam of lanthanum ions are reported for 31 odd-parity levels of La II. Experimental branching fractions for La II from emission spectra covering the near-ultraviolet to the near-infrared are also reported. The spectra were recorded using the US National Solar Observatory 1.0 m Fourier transform spectrometer. The branching fractions are combined with the radiative lifetimes to produce 84 experimentally determined transition probabilities or oscillator strengths, generally accurate to ±10%, for La II. These new experimental results are compared to older experimental and theoretical results. These data are applied to determine a new value for the solar photospheric lanthanum abundance, (La) = 1.13, with estimated internal errors of ±0.03 and external errors of ±0.03.

The Rise of the s-Process in the Galaxy

From newly obtained high-resolution, high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3 < [Fe/H] < +0.3 in 159 giant and dwarf stars. Lanthanum is predominantly made by the s-process in the solar system, while Eu owes most of its solar system abundance to the r-process. The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix. Large s-process abundances can be the result of mass transfer from very evolved stars, so to identify these cases we also report carbon abundances in our metal-poor stars. Results indicate that the s-process may be active as early as [Fe/H] = -2.6, although we also find that some stars as metal-rich as [Fe/H] = -1 show no strong indication of s-process enrichment. There is a significant spread in the level of s-process enrichment even at solar metallicity.

The Chemical Composition and Age of the Metal-poor Halo Star BD +17°3248*

We have combined new high-resolution spectra obtained with the Hubble Space Telescope (HST )a nd ground-based facilities to make a comprehensive new abundance analysis of the metal-poor, halo star BD +17 � 3248. We have detected the third r-process peak elements osmium, platinum, and (for the first time in a metal-poor star) gold, elements whose abundances can only be reliably determined using HST. Our observations illustrate a pattern seen in other similar halo stars with the abundances of the heavier neutron capture elements, including the third r-process peak elements, consistent with a scaled solar system r-process distribution. The abundances of the lighter neutron capture elements, including germanium and silver, fall below that same scaled solar r-process curve, a result similar to that seen in the ultra–metal-poor star CS 22892-052. A single site with two regimes or sets of conditions, or perhaps two different sites for the lighter and heavier neutron capture elements, might explain the abundance pattern seen in this star. In addition, we have derived a

The r-process-enriched low-metallicity giant HD 115444

New high-resolution, very high signal-to-noise spectra of ultra-metal-poor (UMP) giant stars HD 115444 and HD 122563 have been gathered with the High-Resolution Echelle Spectrometer of the McDonald Observatory 2.7 m telescope. With these spectra, line identification and model atmosphere analyses have been conducted, emphasizing the neutron-capture elements. Twenty elements with Z > 30 have been identified in the spectrum of HD 115444. This star is known to have overabundances of the neutron-capture elements, but it has lacked a detailed analysis necessary to compare with nucleosynthesis predictions. The new study features a line-by-line differential abundance comparison of HD 115444 with the bright, well-studied halo giant HD 122563. For HD 115444, the overall metallicity is [Fe/H] -3.0. The abundances of the light and iron-peak elements generally show the same pattern as other UMP stars (e.g., overdeficiencies of manganese and chromium, overabundances of cobalt), but the differential analysis indicates several nucleosynthesis signatures that are unique to each star. Synthetic spectrum analyses reveal substantial overabundances of the heavier neutron-capture elements (Z ≥ 56; elements barium and beyond) in HD 115444. Thus with [Eu/Fe] +0.9, for example, HD 115444 is a moderate version of the extremely neutron-capture-rich UMP giant CS 22892-052 ([Fe/H] -3.1, [Eu/Fe] +1.7). The abundance pattern of the heavier neutron-capture elements is consistent with scaled solar system r-process-only abundances (with little contribution from the s-process). In HD 115444, [Ba/Eu] = -0.73, while in CS 22892-052 this ratio is -0.79. Thus HD 115444 becomes the second UMP r-process-rich halo giant unambiguously identified from a very detailed abundance analysis. Abundances of the lighter neutron-capture elements strontium, yttrium, and zirconium are, however, nearly identical in HD 115444 and HD 122563. Along with the heavier neutron-capture elements, the 4019 A line of Th II has been detected in HD 115444, yielding log e(Th) = -2.23 ± 0.07. Comparing the observed thorium abundance in HD 115444, along with CS 22892-052, with other theoretical estimates of the time-zero abundance suggests an age for both of these UMP stars of 15.6 ± 4 Gyr, consistent with previous radioactive age estimates for CS 22892-052 and other Galactic and cosmological age determinations.

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.