Browsing by Subject "atomic transition-probabilities"

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Improved Co I Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937
(2015-09) Lawler, James E.; Sneden, Christopher; Cowan, John J.; Sneden, Christopher
New emission branching fraction measurements for 898 lines of the first spectrum of cobalt (Co I) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer on Kitt Peak, AZ and a high-resolution echelle spectrometer. Published radiative lifetimes from laser induced fluorescence measurements are combined with the branching fractions to determine accurate absolute atomic transition probabilities for the 898 lines. Hyperfine structure (hfs) constants for levels of neutral Co in the literature are surveyed and selected values are used to generate complete hfs component patterns for 195 transitions of Co I. These new laboratory data are applied to determine the Co abundance in the Sun and metal-poor star HD 84937, yielding log epsilon(Co) = 4.955 +/- 0.007 (sigma = 0.059) based on 82 Co I lines and log epsilon(Co) = 2.785 +/- 0.008 (sigma = 0.065) based on 66 Co I lines, respectively. A Saha or ionization balance test on the photosphere of HD 84937 is performed using 16 UV lines of Co II, and good agreement is found with the Co I result in this metal-poor ([Fe I/H] = -2.32, [Fe II/H] = -2.32) dwarf star. The resulting value of [Co/Fe]= +0.14 supports a rise of Co/Fe at low metallicity that has been suggested in other studies.
Improved Ti II Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937
(2013-10) Wood, M. P.; Lawler, James E.; Sneden, Christopher; Cowan, John J.; Sneden, Christopher
Atomic transition probability measurements for 364 lines of Ti II in the UV through near-IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer (FTS) and a new echelle spectrometer are combined with published radiative lifetimes to determine these transition probabilities. The new results are in generally good agreement with previously reported FTS measurements. Use of the new echelle spectrometer, independent radiometric calibration methods, and independent data analysis routines enables a reduction of systematic errors and overall improvement in transition probability accuracy over previous measurements. The new Ti II data are applied to high-resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ti abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. The Ti abundances derived using Ti II for these two stars match those derived using Ti I and support the relative Ti/Fe abundance ratio versus metallicity seen in previous studies.
On The Binary Helium Star DY Centauri: Chemical Composition And Evolutionary State
(2014-10) Pandey, Gajendra; Rao, N. Kameswara; Jeffery, C. Simon; Lambert, David L.; Lambert, David L.
DY Cen has shown a steady fading of its visual light by about one magnitude in the last 40 yr, suggesting a secular increase in its effective temperature. We have conducted non-local thermodynamic equilibrium (LTE) and LTE abundance analyses to determine the star's effective temperature, surface gravity, and chemical composition using high-resolution spectra obtained over two decades. The derived stellar parameters for three epochs suggest that DY Cen has evolved at a constant luminosity and has become hotter by about 5000 K in 23 yr. We show that the derived abundances remain unchanged for the three epochs. The derived abundances of the key elements, including F and Ne, are as observed for the extreme helium stars resulting from a merger of a He white dwarf with a C-O white dwarf. Thus DY Cen by chemical composition appears to also be a product of a merger of two white dwarfs. This appearance seems to be at odds with the recent suggestion that DY Cen is a single-lined spectroscopic binary.
the SDSS-III APOGEE Spectral Line List for H-Band Spectroscopy
(2015-12) Shetrone, Matthew; Bizyaev, D.; Lawler, James E.; Prieto, Carlos Allende; Johnson, J. A.; Smith, V. V.; Cunha, K.; Holtzman, J.; Perez, A. E. G.; Meszaros, S.; Sobeck, J.; Zamora, O.; Garcia-Hernandez, D. A.; Souto, D.; Chojnowski, D.; Koesterke, L.; Majewski, S.; Zasowski, G.; Shetrone, Matthew
We present the H-band spectral line lists adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). The APOGEE line lists comprise astrophysical, theoretical, and laboratory sources from the literature, as well as newly evaluated astrophysical oscillator strengths and damping parameters. We discuss the construction of the APOGEE line list, which is one of the critical inputs for the APOGEE Stellar Parameters and Chemical Abundances Pipeline, and present three different versions that have been used at various stages of the project. The methodology for the newly calculated astrophysical line lists is reviewed. The largest of these three line lists contains 134,457 molecular and atomic transitions. In addition to the format adopted to store the data, the line lists are available in MOOG, Synspec, and Turbospectrum formats. The limitations of the line lists along with guidance for its use on different spectral types are discussed. We also present a list of H-band spectral features that are either poorly represented or completely missing in our line list. This list is based on the average of a large number of spectral fit residuals for APOGEE observations spanning a wide range of stellar parameters.