762 research outputs found
Characterizing The Chemistry Of The Milky Way Stellar Halo: Detailed Chemical Analysis Of A Metal-Poor Stellar Stream
No full textWe present the results of a detailed abundance analysis of one of the confirmed building blocks of the Milky Way stellar halo, a kinematically coherent metal-poor stellar stream. We have obtained high-resolution and high signal-to-noise spectra of 12 probable stream members using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the 2dCoude spectrograph on the Smith Telescope at McDonald Observatory. We have derived abundances or upper limits for 51 species of 46 elements in each of these stars. The stream members show a range of metallicity (-3.4 < [Fe/H] < -1.5) but are otherwise chemically homogeneous, with the same star-to-star dispersion in [X/Fe] as the rest of the halo. This implies that, in principle, a significant fraction of the Milky Way stellar halo could have formed from accreted systems like the stream. The stream stars show minimal evolution in the a or Fe-group elements over the range of metallicity. This stream is enriched with material produced by the main and weak components of the rapid neutron-capture process and shows no evidence for enrichment by the slow neutron-capture process.NSF AST 06-07708, AST 09-08978Astronom
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The End Of Nucleosynthesis: Production Of Lead And Thorium In The Early Galaxy
No full textWe examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] < -1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from Hubble Space Telescope/Space Telescope Imaging Spectrograph, Keck/High Resolution Echelle Spectrograph, and Very Large Telescope/UV-Visual Echelle Spectrograph spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the third r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical "waiting-point" approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 <= log n(n) <= 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the "main" r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect-at most-only the elements beyond the third r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance "losses" by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.Deutsche Forschungsgemeinschaft KR 806/13-1Helmholtz Gemeinschaft VH-VI-061U S National Science Foundation AST 07-07447, AST 06-07708Astronom
New Abundance Determinations of Cadmium, Lutetium, and Osmium in the R-Process Enriched Star BD+17 3248
No full textWe report the detection of Cd I (Z = 48), Lu II (Z = 71), and Os II (Z = 76) in the metal-poor star BD + 17 3248. These abundances are derived from an ultraviolet spectrum obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. This is the first detection of these neutron-capture species in a metal-poor star enriched by the r process. We supplement these measurements with new abundances of Moi, Rui, and Rh i derived from an optical spectrum obtained with the High Resolution Echelle Spectrograph on Keck. Combined with previous abundance derivations, 32 neutron-capture elements have been detected in BD + 17 3248, the most complete neutron-capture abundance pattern in any metal-poor star to date. The light neutron-capture elements (38 <= Z <= 48) show a more pronounced even-odd effect than expected from current solar system r-process abundance predictions. The age for BD + 17 3248 derived from the Th ii/Os II chronometer is in better agreement with the age derived from other chronometers than the age derived from Th ii/Os i. NewHf II abundance derivations from transitions in the ultraviolet are lower than those derived from transitions in the optical, and the lower Hf abundance is in better agreement with the scaled solar system r-process distribution.NASA NAS 5-26555W. M. Keck FoundationU.S. National Science Foundation AST 09-08978, AST 09-07732, AST 07-07447Astronom
The ubiquity of the rapid neutron-capture process
No full textTo better characterize the abundance patterns produced by the r-process, we have derived new abundances or upper limits for the heavy elements zinc (Zn, Z = 30), yttrium (Y, Z = 39), lanthanum (La, Z = 57), europium (Eu, Z = 63), and lead (Pb, Z =82). Our sample of 161 metal-poor stars includes newmeasurements from 88 high-resolution and high signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 m Smith Telescope at the McDonald Observatory, and other abundances are adopted from the literature. We use models of the s-process in asymptotic giant branch stars to characterize the high Pb/Eu ratios produced in the s-process at low metallicity, and our new observations then allow us to identify a sample of stars with no detectable s-process material. In these stars, we find no significant increase in the Pb/Eu ratios with increasing metallicity. This suggests that s-process material was not widely dispersed until the overall Galactic metallicity grew considerably, perhaps even as high as [Fe/H] = -1.4, in contrast with earlier studies that suggested a much lower mean metallicity. We identify a dispersion of at least 0.5 dex in [La/Eu] in metal-poor stars with [Eu/Fe] < +0.6 attributable to the r-process, suggesting that there is no unique "pure" r-process elemental ratio among pairs of rare earth elements. We confirm earlier detections of an anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in the r-process (e.g., CS 22892-052) and those with deficiencies of the heavy elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using simulations of high-entropy neutrino winds of core-collapse supernovae that include chargedparticle and neutron-capture components of r-process nucleosynthesis. The heavy element abundance patterns in most metal-poor stars do not resemble that of CS 22892-052, but the presence of heavy elements such as Ba in nearly all metal-poor stars without s-process enrichment suggests that the r-process is a common phenomenon
Europium, Samarium, And Neodymium Isotopic Fractions In Metal-Poor Stars
No full textWe have derived isotopic fractions of europium, samarium, and neodymium in two metal-poor giants with differing neutron-capture nucleosynthetic histories. These isotopic fractions were measured from new high-resolution (R similar to 120; 000), high signal-to-noise ratio (S/N similar to 160-1000) spectra obtained with the 2d-coude spectrograph of McDonald Observatory's 2.7m Smith telescope. Synthetic spectra were generated using recent high-precision laboratory measurements of hyperfine and isotopic subcomponents of several transitions of these elements and matched quantitatively to the observed spectra. We interpret our isotopic fractions by the nucleosynthesis predictions of the stellar model, which reproduces s-process nucleosynthesis from the physical conditions expected in low-mass, thermally pulsing stars on the AGB, and the classical method, which approximates s-process nucleosynthesis by a steady neutron flux impinging on Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with an r-process origin by the classical method and is consistent with either an r- or an s-process origin by the stellar model. Our Sm isotopic fraction in HD 175305 suggests a predominantly r- process origin, and our Sm isotopic fraction in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions, while consistent with either r-process or s-process origins, have very little ability to distinguish between any physical values for the isotopic fraction in either star. This study for the first time extends the n-capture origin of multiple rare earths in metal-poor stars from elemental abundances to the isotopic level, strengthening the r-process interpretation for HD 175305 and the s-process interpretation for HD 196944.NSF AST 03-07279, AST 05-06324, AST 03-07495, AST 06-07708Astronom
New Stellar Parameters, Metallicities, and Elemental Abundance Ratios for 311 Metal-poor Stars
No full textWe present equivalent widths, improved model atmosphere parameters, and revised abundances for 14 species of 11 elements derived from high-resolution optical spectroscopy of 311 metal-poor stars. All of these stars had their parameters previously published by Roederer et al. We use color– T _eff relationships calibrated for Gaia and 2MASS photometry to calculate improved effective temperatures ( T _eff ). We calculate log of surface gravity (log g ) values using measurements derived from Gaia parallaxes and other fundamental stellar properties. We perform a standard LTE abundance analysis using MARCS model atmospheres and the MOOG line analysis software to rederive microturbulence velocity parameters, metallicities, and abundances based on O i , Na i , Mg i , Si i , K i , Ca i , Ti i , Ti ii , Cr i , Cr ii , Fe i , Fe ii , Ni i , and Zn i lines using previously measured equivalent widths. On average, the new T _eff values are 310 K warmer, the new log g values are higher by 0.64 dex, and the new [Fe/H] values are higher by 0.26 dex. We apply NLTE corrections to the abundances derived from O i , Na i , Mg i , Si i , K i , Fe i , and Fe ii lines. Our sample contains 6 stars with [Fe/H] < −3.5, 28 stars with [Fe/H] < −3.0, and 113 stars with [Fe/H] < −2.5. Our revised abundances for these 311 stars are now in better agreement with those derived by previous studies of smaller samples of metal-poor stars in the Milky Way
The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) project. I. The lithium-, s-, and r-enhanced metal-poor giant HKII 17435-00532
No full textWe present the first detailed abundance analysis of the metal-poor giant HKII 17435-00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo ( CASH). A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R similar to 15,000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor (Fe/H = -2.2) star has an unusually high lithium abundance [log epsilon(Li) +2.1], mild carbon (C/Fe = +0.7) and sodium (]Na/Fe] = +0.6) enhancement, as well as enhancement of both s-process ([Ba/Fe] = +0.8) and r-process ([Eu/Fe] = +0.5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435-00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current nondetection of radial velocity variations (over similar to 180 days), it is possible that HKII 17435 - 00532 is in a long-period or highly inclined binary system, similar to other stars with similar n-capture enrichment patterns
Lancashire, IBM-U of T Computing Centre, Letters, Meeting Minutes, Reports, 1982-1985
No full textA collection of letters, meeting minutes, and reports associated with the creation of the IBM-U of T of Toronto cooperative created in the 1980s; <?xml version=\"1.0\" encoding=\"UTF-8\"?> <!-- This XML metadata document supplements the metadata that we can enter into ERA archive records. This provides more detailed metadata that we hope will eventually be part of the system. --> <DHHistArchMeta> <title>Lancashire, IBM-U of T Computing Centre, Letters, Meeting Minutes, Reports, 1982-1985</title> <creatorTitle></creatorTitle> <creatorOriginal> <author>Lancashire, Ian</author> <author>Nowlan, D. H.</author> <author>Heyworth, Allan</author> <author>Focklerekh</author> </creatorOriginal> <digitalRecordCreator>Alicia Hibbert</digitalRecordCreator> <recordCreator>Victoria Smith</recordCreator> <subject>University of Toronto Computing Centre, IBM-U of T Cooperative, Humanities</subject> <description>A collection of letters, meeting minutes, and reports associated with the creation of the IBM-U of T of Toronto cooperative created in the 1980s</description> <physicalDescription>50 pages</physicalDescription> <originals>The original document from Ian Lancashire’s personal collection</originals> <notes></notes> <publicationInformation>Not published</publicationInformation> <archivedVersionHistory></archivedVersionHistory> <dateOrigination>1982-1985-xx-xx</dateOrigination> <dateCollection></dateCollection> <dateDigitization>2009-xx-xx</dateDigitization> <documentType>Letters, meeting minutes, report</documentType> <electronicFormat>PDF, 50 pages</electronicFormat> <provenance>University of Toronto</provenance> <language>English</language> <relatedMaterials>Lancashire, 1990, Humanities Computing - CCH Toronto-IBM</relatedMaterials> <coverage>1982-1985</coverage> <identifier></identifier> <rights></rights> <access>Level 4 - Embargoed</access> <use>Level 4 - Embargoed</use> <preferredCitation></preferredCitation> </DHHistArchMeta>
HEAVY-ELEMENT DISPERSION IN THE METAL-POOR GLOBULAR CLUSTER M92
No full textDispersion among the light elements is common in globular clusters (GCs), while dispersion among heavier elements is less common. We present detection of r-process dispersion relative to Fe in 19 red giants of the metal-poor GC M92. Using spectra obtained with the Hydra multi-object spectrograph on the WIYN Telescope at Kitt Peak National Observatory, we derive differential abundances for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are homogeneous at a level of 0.07-0.16 dex. The heavy-elements La, Eu, and Ho exhibit clear star-to-star dispersion spanning 0.5-0.8 dex. The abundances of these elements are correlated with one another, and we demonstrate that they were produced by r-process nucleosynthesis. This r-process dispersion is not correlated with the dispersion in C, N, or Na in M92, indicating that r-process inhomogeneities were present in the gas throughout star formation. The r-process dispersion is similar to that previously observed in the metal-poor GC M15, but its origin in M15 or M92 is unknown at present.National Aeronautics and Space AdministrationCarnegie Institution of WashingtonU.S. National Science Foundation AST 09-08978Astronom
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