Alex Stephens

Primordial Lithium: Keck Observations in M92 Turnoff Stars

We present new Keck I/HIRES observations at R = 45,000 (=3 pixels) of seven stars near the turnoff of the old, metal-poor globular cluster M92. In three of these stars, we have signal-to-noise ratios (S/Ns) of 40 pixel-1, and in the other four, the S/N is near 20. The Li abundance in star 18 is high compared with the halo field-star plateau and is similar to that in the remarkable Li-rich halo field star BD +23°3912. In addition to the high Li abundance in star 18, there is a dispersion in Li abundance in our seven stars covering the full range of a factor of 3. We have attempted to determine whether the excess Li in star 18 is due to less than average Li depletion in this star from an even higher initial abundance, as predicted by the Yale rotational models, or whether it is due to the extraordinary action of Li production mechanisms in the material that formed this star. We have found no convincing evidence that favors Li production: (1) Stars 18, 21, and 46 have identical Ba abundances, which argues against Li production carrying an s-process signature. (2) These three stars have indistinguishable Ca, Cr, Fe, and Ti, which argues against supernova Li production. (3) We discuss ν-process production of Li and find no convincing observational evidence for this from the strengths of the Mg, Ca, and Fe lines. (4) The similarity in age of these cluster stars argues against cosmic-ray Li production that requires age differences of gigayears. The most likely explanation for the Li dispersion is differential Li depletion from a (possibly significantly) higher primordial Li abundance due to differences in the initial angular momentum in each star followed by spin-down; the most rapid rotators destroy the most Li, whereas the initially slower rotators preserve more Li.

Keck HIRES Spectroscopy of M92 Subgiants: Surprising Abundances near the Turnoff

Using high-resolution, moderate signal-to-noise ratio spectroscopy obtained with the 10 m Keck I Telescope and efficient HIRES echelle spectrograph, we derive abundances of several elements in sub- giants near the M92 turno†. As a consistency check, we also analyze the metal-poor -eld star HD 140283 and -nd an Fe abundance in -ne agreement with many previous determinations. However, our M92 value ((Fe/H) \( 2.52) is a factor of 2 lower than the abundance derived from its red giant members. Di†erences in model atmospheres, gf-values, and instrumental e†ects might account for this di†erence, but whether they in fact do so is unclear. We note possible evidence for (Fe/H) di†erences within M92. Our spectroscopic analysis suggests that the M92 reddening, E(B(V ), may be 0.04E0.05 mag greater than canonical values, but various uncertainties mean that this conclusion is not de-nitive; the signi-cant di†erence in interstellar Na I line strengths in the M92 and HD 140283 spectra may be consistent with an increased reddening. Regardless, the conclusion that either the (Fe/H) of M92 has been signi-cantly overestimated from red giants or current reddening/photometry estimates are too small/red is not easily escaped. If the reddening/photometry were in error by this amount, turno† colorE based ages for M92 could be reduced by D4 Gyr. The adjustment to the M92 distance modulus required for a similarly reduced turno† age that is luminosity-based can be accommodated by increases in extinction and alterations to the metal-poor -eld star distance scale recently inferred from Hipparcos Cepheid and subdwarf data. Our M92 subgiants demonstrate (Cr/Fe), (Ca/Fe), and (Ti/Fe) ratios that are unremarkable and essentially identical to the values for HD 140283. (Ba/Fe) is 0.45 dex larger for the M92 subgiants than for HD 140283. Surprisingly, we -nd (Mg/Fe) to be 0.55 dex lower in our M92 subgiants than in HD 140283, and (Na/Fe) to be 0.76 dex larger in our M92 subgiants than in HD 140283. These di†erences (and indeed nearly all our abundance ratios) seem immune to various data, analysis, and parameter errors. If real, this striking abundance pattern is suggestive of material in our M92 starsI photospheres that has undergone Ne ) Na and Mg ) Al cycling like that inferred for red giants in M92 and other clusters. While this is generally believed to be an in situ process in cluster giants, the presence of abun- dant Li in our M92 objects suggests a polluting source acting either primordially or via accretion after cluster star formation. This may be consistent with CN and Na variations on the 47 Tucanae main sequence, recently reported Ba and Eu variations in M15 red giants, possible cluster-to-cluster n-capture abundance di†erences, and very low (O/Fe) ratios observed near the base of the M13 giant branch. We thus suggest that a polluting source of light-element alteration, in addition to the in situ source for more evolved stars, may be required for M92. Comparison of our M92 subgiant abundance ratios with those of M92 red giants may indicate that pollution occurred after the present generation of cluster stars formed, but until the cause or causes of the subgiant versus giant Fe abundance discrepancy are de-ni- tively identi-ed, this conclusion is uncertain. A polluting source of our Na and Mg anomalies produced via processing in a previous stellar generation also has complications; namely, how the Mg and Na anomalies arise without apparently any net inNuence on our subgiantsI Li abundances and on the C abundances of other M92 subgiants. A similar quandary may exist in some 47 Tuc turno† stars. An understanding of cluster abundance variations (by whatever mechanisms) and their behavior with evolu- tionary state may be needed for a complete understanding of absolute and relative globular clusters ages, and for derivation of the primordial Li abundance.

Boron in Lithium- and Beryllium-deficient F Stars

The Goddard High Resolution Spectrograph (GHRS) has been used with the Hubble Space Telescope (HST) to observe the B I region at 2497 A in nine F and G dwarfs of approximately solar metallicity. The stars were selected because they have a variety of Li and Be deficiencies. Most of the nine stars were newly observed at high spectral resolution and high signal-to-noise ratios at the Keck I 10 m telescope, the Canada-France-Hawaii 3.6 m telescope, and the University of Hawaii 2.2 m telescope at 3131 A for Be II and 6708 A for Li I. With spectrum synthesis we have determined the abundances of B in our nine program stars and in five other stars from the HST archive. The stellar parameters we have used have been determined in a self-consistent way for the program stars and the archive stars. Spectrum synthesis has also been used to determine the Li and Be abundances or upper limits. Corrections to the B and Li abundances due to non-LTE effects have been applied. The stars originate from the region on the ZAMS of the Li (and Be) dip. In spite of large deficiencies in Li and Be, we find a striking uniformity in the B abundances, i.e., there is no B dip. In all cases the Li deficiency is greater than the Be deficiency. For the coolest and most evolved star in our sample, ζ Her A, the B abundance is 0.6 dex lower than the mean for the other stars. This star also has the largest Be deficiency (more than a factor of 80) and the largest Li deficiency (more than a factor of 600). These data, together with other studies of the Li dip, argue strongly against diffusion and mass loss and in favor of slow mixing as the cause of the Li and Be dip and the absence of a B dip. Six stars with [Fe/H] from -0.75 to +0.15 have Be abundances ranging from the maximum of the sample to a factor of 4 below the maximum, yet these stars have a B/Be ratio that is constant to within ±0.10 dex and that is close to the predictions of Galactic cosmic-ray spallation of 10-15. The Be range for four stars with solar metallicity is still a factor of 2, and yet the B/Be ratio is constant to within ±0.03 dex. These results imply that the Galactic cosmic-ray production of B and Be is not uniform relative to the production of elements such as Fe by stellar nucleosynthesis.

The Chemical Composition of Halo Stars on Extreme Orbits

Presented within is a fine spectroscopic analysis of 11 metal-poor (-2.15 < [Fe/H] < -1.00) dwarf stars on orbits that penetrate the outermost regions of the Galactic halo. Abundances for a select group of light metals (Na, Mg, Si, Ca, and Ti), Fe-peak nuclides (Cr, Fe, and Ni), and neutron-capture elements (Y and Ba) were calculated using line strengths measured from high-resolution (R ? 48,000), high signal-to-noise ratio (S/N ? 110 pixel-1) echelle spectra acquired with the Keck I 10 m telescope and HIRES spectrograph. Ten of the stars have apogalactica, a proxy for stellar birthplace, which stretch between 25 and 90 kpc; however, these outer halo stars exhibit strikingly uniform abundances. The average, Fe-normalized abundances?[Mg/Fe] = +0.23 ? 0.09, [Si/Fe] = +0.24 ? 0.10, [Ca/Fe] = +0.22 ? 0.07, [Ti/Fe] = +0.20 ? 0.08, [Cr/Fe] = 0.02 ? 0.07, [Ni/Fe] = -0.09 ? 0.07, and [Ba/Fe] = +0.01 ? 0.12?exhibit little intrinsic scatter; moreover, the evolution of individual ratios (as a function of [Fe/H]) is generally consistent with the predictions of galactic chemical evolution models dominated by the ejecta of core-collapse supernovae. Only [Y/Fe] = -0.13 ? 0.21 exhibits a dispersion larger than observational uncertainties, which suggests a different nucleosynthesis site for this element. It has been conjectured that stars on high-energy orbits?either those that penetrate the remote halo or ones with extreme retrograde velocities?were once associated with a cannibalized satellite galaxy. Such stars, as shown here, are indistinguishable from metal-poor dwarfs of the inner Galactic halo. The uniformity of the abundances, regardless of kinematic properties, suggests that physically, spatially, and temporally distinct star-forming regions within (or near) the growing Milky Way experienced grossly similar chemical evolution histories. Implications for galaxy formation scenarios are discussed.

Lithium and Lithium Depletion in Halo Stars on Extreme Orbits

We have determined Li abundances in 55 dwarfs and subgiants that are metal-poor (-3.6 < [Fe/H] < -0.7) and have extreme orbital kinematics. Our purpose is to examine the Li abundance in the Li plateau stars and its decrease in low-temperature, low-mass stars. For the stars in our sample we have determined chemical profiles given in 2002 by Stephens & Boesgaard. The Li observations are primarily from the echelle spectrograph on the 10 m Keck I telescope, with HIRES covering 4700-6800 A with a spectral resolution of ~48,000. The spectra have high signal-to-noise ratios, from 70 to 700 pixel-1, with a median of 140. The Li I resonance doublet was detected in 42 of the 55 stars. Temperatures were found spectroscopically by Stephens & Boesgaard. Abundances or upper limits were determined for all stars, with typical errors of 0.06 dex. Corrections for the deviations from nonlocal thermodynamical equilibrium for Li in the stellar atmospheres have been made, which range from -0.04 to +0.11 dex. Our 14 dwarf and turnoff stars on the Li plateau with temperatures greater than 5700 K and [Fe/H] < -1.5 give A(Li) = log N(Li)/N(H) + 12.00 of 2.215 ± 0.110, consistent with earlier results. We find a dependence of the Li abundance on metallicity as measured by [Fe/H] and the Fe-peak elements Cr and Ni, with a slope of ~0.18. We have examined the possible trends of A(Li) with the chemical abundances of other elements and find similar dependences of A(Li) with the α-elements Mg, Ca, and Ti. These slopes are slightly steeper at ~0.20, resulting from an excess in [α/Fe] with decreasing [Fe/H]. For the n-capture, rare-earth element Ba, we find a relation between A(Li) and [Ba/H] that has a shallower slope of ~0.13; over a range of 2.6 dex in [Ba/H], the Li abundance spans only a factor of 2. We have also examined the possible trends of A(Li) with the characteristics of the orbits of our halo stars. We find no trends in A(Li) with kinematic or dynamic properties. For the stars with temperatures below the Li plateau, there are several interesting results. The group of metal-poor stars possess, on average, more Li at a given temperature than metal-rich stars. When we divide the cool stars into smaller subsets with similar metallicities, we find trends of A(Li) with temperature for the different metallicity groups. The decrease in A(Li) sets in at hotter temperatures for the higher metallicity stars than for the lower metallicity stars. The increased Li depletion in cooler stars could be a result of the increased action of convection, since cooler stars have deeper convection zones. This would also make it easier for additional mixing mechanisms, such as those induced by rotation, to have a greater effect in cooler stars. Since the model depth of the convection zone is almost independent of metallicity at a given effective temperature, the apparent metallicity dependence of the Li depletion in our data may be pointing to subtle but poorly understood mixing effects in low-mass halo dwarfs. Predictions for Li depletion from standard and nonstandard models seem to underestimate the degree of depletion inferred from the observations of the cool stars.

Chemical Composition in the Globular Cluster M71 from Keck HIRES Spectra of Turnoff Stars

We have made observations with the Keck I telescope and HIRES at a resolution of ~45,000 of five nearly identical stars at the turnoff of the metal-rich globular cluster M71. We derive stellar parameters and abundances of several elements. Our mean Fe abundance, [Fe/H] = -0.80 ± 0.02, is in excellent agreement with previous cluster determinations from both giants and near-turnoff stars. There is no clear evidence for any star-to-star abundance differences or correlations in our sample. Abundance ratios of the Fe peak elements (Cr, Ni) are similar to Fe. The turnoff stars in M71 have remarkably consistent enhancements of 0.2-0.3 dex in [Si/Fe], [Ca/Fe], and [Ti/Fe], like the red giants. Our [Mg/Fe] ratio is somewhat lower than that suggested by other studies. We compare our mean abundances for the five M71 stars with field stars of similar metallicity [Fe/H]: eight with halo kinematics and 17 with disk kinematics. The abundances of the α-fusion products (Mg, Si, Ca, Ti) agree with both samples but seem a closer match to the disk stars. The Mg abundance in M71 is at the lower edge of the disk and halo samples. The neutron-capture elements, Y and Ba, are enhanced relative to solar in the M71 turnoff stars. Our ratio [Ba/Fe] is similar to that of the halo field stars but a factor of 2 above that for the disk field stars. The important [Ba/Y] ratio is significantly lower than M71 giant values; the precluster material may have been exposed to a higher neutron flux than the disk stars or self-enrichment has occurred subsequent to cluster star formation. The Na content of the M71 turnoff stars is remarkably similar to that in the disk field stars but more than a factor of 2 higher than the halo field star sample. We find [Na/Fe] = +0.14 ± 0.04 with a spread less than half of that found in the red giants in M71. Excluding Mg, the lack of intracluster α-element variations (turnoff vis-a-vis giants) suggests that the polluting material needed to explain the abundance patterns in M71 did not arise from explosive nucleosynthesis but in a more traditional s-process environment such as AGB stars. The determination of light s-peak abundances should reveal whether this pollution occurred before or after cluster formation.

Chemical abundances in globular cluster turn-off stars from Keck/HIRES observations

Spectroscopic observations at high spectral resolution of unevolved stars in globular clusters have only been possible since the 1993 advent of the Keck-I 10-meter telescope and its high-resolution spectrometer. Our program of deriving chemical abundances of lithium and several other elements in some unevolved, but identical, stars in three globular clusters is described. For M13 we have found a spread in Li abundances of a factor of five in four very similar stars. For six stars in M92 the range is a factor of three and for five in M71 the range is a factor of two. The stars with the highest abundances of Li show values that are a factor of two above the field halo star Li plateau. The abundances of several other species - Na, Mg, Si, Ca, Ti, Cr, Ni, Fe, Y, and Ba - show no such star-to-star variations. These abundances are compared with those of halo field stars which were derived from Keck HIRES spectra.

New Keck observations of lithium in very metal-poor stars

Lithium abundances have been determined in more than 100 metal-poor halo stars both in the field and in clusters. From these data we find trends of Li with both temperature and metallicity and a real dispersion in Li abundances in the Spite Li plateau. We attribute this dispersion primarily to Li depletion (presumably due to extra mixing induced by stellar rotation) and to Galactic chemical evolution. We derive a primordial Li of 2.44

Accretion in the Galactic Halo

\,{\pm}\,

Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

0.18 for A(Li)