Browsing by Subject "oxygen abundances"
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Item Atmospheric Composition Of Weak G Band Stars: CNO And Li Abundances(2013-03) Adamczak, Jens; Lambert, David L.; Adamczak, Jens; Lambert, David L.We determined the chemical composition of a large sample of weak G band stars-a rare class of G and K giants of intermediate mass with unusual abundances of C, N, and Li. We have observed 24 weak G band stars with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory and derived spectroscopic abundances for C, N, O, and Li, as well as for selected elements from Na-Eu. The results show that the atmospheres of weak G band stars are highly contaminated with CN-cycle products. The C underabundance is about a factor of 20 larger than for normal giants and the C-12/C-13 ratio approaches the CN-cycle equilibrium value. In addition to the striking CN-cycle signature the strong N overabundance may indicate the presence of partially ON-cycled material in the atmospheres of the weak G band stars. The exact mechanism responsible for the transport of the elements to the surface has yet to be identified but could be induced by rapid rotation of the main sequence progenitors of the stars. The unusually high Li abundances in some of the stars are an indicator for Li production by the Cameron-Fowler mechanism. A quantitative prediction of a weak G band star's Li abundance is complicated by the strong temperature sensitivity of the mechanism and its participants. In addition to the unusual abundances of CN-cycle elements and Li, we find an overabundance of Na that is in accordance with the NeNa chain running in parallel with the CN cycle. Apart from these peculiarities, the element abundances in a weak G band star's atmosphere are consistent with those of normal giants.Item Boron Abundances In B-Type Stars: A Test Of Rotational Depletion During Main-Sequence Evolution(2002-01) Venn, Kim A.; Brooks, A. M.; Lambert, David L.; Lemke, M.; Langer, N.; Lennon, D. J.; Keenan, F. P.; Lambert, David L.Boron abundances have been derived for seven main-sequence B-type stars from Hubble Space Telescope STIS spectra around the B III lambda2066 line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but is clearly depleted. In the other four stars, boron is undetectable, implying depletions of 1-2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. In addition, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near-solar iron group abundances, as expected for young stars in the solar neighborhood. We have also analyzed the halo B-type star PG 0832 + 676. We find [Fe/H] = -0.88 +/- 0.10, and the absence of the B III line gives the upper limit [B/H] < -2.5. These and other published abundances are used to infer the star's evolutionary status as a post-asymptotic giant branch star.Item Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars(2008-06) Melendez, J.; Asplund, M.; Alves-Brito, A.; Cunha, K.; Barbuy, B.; Bessell, M. S.; Chiappini, C.; Freeman, K. C.; Ramirez, I.; Smith, V. V.; Yong, D.; Ramirez, I.Context. The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. Aims. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge red giant stars. We also completed an identical study of local thin disk, thick disk and halo giants to establish the chemical differences and similarities between the various populations. Methods. High-resolution infrared spectra of 19 bulge giants and 49 comparison giants in the solar neighborhood were acquired with Gemini/Phoenix. All stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk stars ensured that systematic errors were minimized. Results. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf stars in the solar neighborhood. Conclusions. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shared, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.Item The Distribution Of The Elements In The Galactic Disk. III. A Reconsideration Of Cepheids From L=30 Degrees To 250 Degrees(2011-10) Luck, R. Earle; Lambert, David L.; Lambert, David L.This paper reports on the spectroscopic investigation of 238 Cepheids in the northern sky. Of these stars, about 150 are new to the study of the galactic abundance gradient. These new Cepheids bring the total number of Cepheids involved in abundance distribution studies to over 400. In this work, we also consider systematics between various studies and also those which result from the choice of models. We find that systematic variations exist at the 0.06 dex level both between studies and model atmospheres. In order to control the systematic effects our final gradients depend only on abundances derived herein. A simple linear fit to the Cepheid data from 398 stars yields a gradient d[Fe/H]/dR(G) = -0.062 +/- 0.002 dex kpc(-1) which is in good agreement with previously determined values. We have also re-examined the region of the >metallicity island> of Luck et al. With the doubling of the sample in that region and our internally consistent abundances, we find that there is scant evidence for a distinct island. We also find in our sample the first reported Cepheid (V1033 Cyg) with a pronounced Li feature. The Li abundance is consistent with the star being on its redward pass toward the first giant branch.Item Hd 91669B: A New Brown Dwarf Candidate From The McDonald Observatory Planet(2009-03) Wittenmyer, Robert A.; Endl, Michael; Cochran, William D.; Ramirez, Ivan; Reffert, Sabine; MacQueen, Phillip J.; Shetrone, Matthew; Wittenmyer, Robert A.; Endl, Michael; Cochran, William D.; Ramirez, Ivan; MacQueen, Phillip J.; Shetrone, Matthew D.We report the detection of a brown dwarf candidate orbiting the metal-rich K dwarf HD 91669, based on radial-velocity data from the McDonald Observatory Planet Search. HD 91669b is a substellar object in an eccentric orbit (e = 0.45) at a separation of 1.2 AU. The minimum mass of 30.6M(Jup) places this object firmly within the brown dwarf desert for inclinations i greater than or similar to 23 degrees. This is the second rare close-in brown dwarf candidate discovered by the McDonald planet search program.Item Line Lists for the A(2)Pi-X-2 Sigma(+) (Red) and B-2 Sigma(+)-X-2 Sigma(+) (Violet) Systems of Cn, (Cn)-C-13-N-14, and (Cn)-C-12-N-15, and Application To Astronomical Spectra(2014-10) Sneden, Christopher; Lucatello, Sara; Ram, Ram S.; Brooke, James S. A.; Bernath, Peter; Sneden, ChristopherNew red and violet system line lists for the CN isotopologues (CN)-C-13-N-14 and (CN)-C-12-N-15 have been generated. These new transition data are combined with those previously derived for (CN)-C-12-N-14, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright, very low-metallicity star HD 122563, and the carbon-enhanced metal-poor stars HD 196944 and HD 201626. When both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work are also generally in accord with published values.Item Metallicity And Alpha-Element Abundance Measurement In Red Giant Stars From Medium-Resolution Spectra(2008-08) Kirby, Evan N.; Guhathakurta, Puragra; Sneden, Christopher; Sneden, ChristopherWe present a technique that applies spectral synthesis to medium-resolution spectroscopy (MRS; R similar to 6000) in the red (6300 angstrom < lambda < 9100 angstrom) to measure [Fe/H] and [alpha/Fe] of individual red giant stars over a wide metallicity range. We apply our technique to 264 red giant stars in seven Galactic globular clusters and demonstrate that it reproduces the metallicities and alpha-enhancements derived from high-resolution spectroscopy (HRS). The MRS technique excludes the three Ca II triplet lines and instead relies on a plethora of weaker lines. Unlike empirical metallicity estimators, such as the equivalent width of the Ca II triplet, the synthetic method presented here is applicable over an arbitrarily wide metallicity range and is independent of assumptions about the alpha-enhancement. Estimates of cluster mean [Fe/H] from different HRS studies show typical scatter of similar to 0.1 dex but can be larger than 0.2 dex for metal-rich clusters. The scatter in HRS abundance estimates among individual stars in a given cluster is also comparable to 0.1 dex. By comparison, the scatter among MRS [Fe/H] estimates of individual stars in a given cluster is similar to 0.1 dex for most clusters but 0.17 dex for the most metal-rich cluster, M71 (<[Fe/H]> = - 0.8). A star-by-star comparison of HRS versus MRS [alpha/Fe] estimates indicates that the precision in [alpha/Fe](MRS) is 0.05 dex. The errors in [Fe/H](MRS) and [alpha/Fe](MRS) increase beyond 0.25 dex only below signal-to-noise ratios of 20 angstrom(-1), which is typical for existing MRS of the red giant stars in Leo I, one of the most distant Milky Way satellites (250 kpc).Item Very Metal-Poor Stars in the Outer Galactic Bulge Found By the APOGEE Survey(2013-04) Garcia Perez, Ana E.; Cunha, Katia; Shetrone, Matthew; Majewski, Steven R.; Johnson, Jennifer A.; Smith, Verne V.; Schiavon, Ricardo P.; Holtzman, Jon; Nidever, David; Zasowski, Gail; Allende Prieto, Carlos; Beers, Timothy C.; Bizyaev, Dmitry; Ebelke, Garrett; Eisenstein, Daniel J.; Frinchaboy, Peter M.; Girardi, Leo; Hearty, Fred R.; Malanushenko, Elena; Malanushenko, Viktor; Meszaros, Szabolcs; O'Connell, Robert W.; Oravetz, Daniel; Pan, Kaike; Robin, Annie C.; Schneider, Donald P.; Schultheis, Mathias; Skrutskie, Michael F.; Simmonsand, Audrey; Wilson, John C.; Shetrone, MatthewDespite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge Formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] <= -1.7), including two that are very metal-poor [Fe/H] similar to -2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the alpha-elements O, Mg, and Si without significant alpha-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.