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Martin Asplund - One of the best experts on this subject based on the ideXlab platform.
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A Reappraisal of the Solar Photospheric C/O Ratio
The Astrophysical Journal, 2002Co-Authors: Carlos Allende Prieto, David L. Lambert, Martin AsplundAbstract:Accurate determination of photospheric solar abundances requires detailed modeling of the solar granulation and accounting for departures from local thermodynamical equilibrium (LTE). We argue that the forbidden C I line at 8727 A is largely immune to departures from LTE, and can be realistically modeled using LTE radiative transfer in a time-dependent three-dimensional simulation of solar surface convection. We analyze the [C I] line in the solar flux spectrum to derive the abundance Log Epsilon(C)= 8.39 +/- 0.04 dex. Combining this result with our parallel analysis of the [O I] 6300 A line, we find C/O=0.50 +/- 0.07, in agreement with the ratios measured in the solar corona from gamma-ray spectroscopy and solar energetic particles.
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Line formation in solar granulation III. The photospheric Si and meteoritic Fe abundances
Astronomy and Astrophysics, 2000Co-Authors: Martin AsplundAbstract:Using realistic hydrodynamical simulations of the solar surface convection as 3D, time-dependent, inhomogeneous model atmospheres, the solar photospheric Si abundance has been determined to be Log Epsilon(Si) = 7.51 +/- 0.04. This constitutes a difference
Christopher Sneden - One of the best experts on this subject based on the ideXlab platform.
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Atomic Transition Probabilities for UV and Blue Lines of Fe II and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937
The Astrophysical Journal Supplement Series, 2019Co-Authors: E. A. Den Hartog, James E. Lawler, Christopher Sneden, John J. Cowan, A. BrukhoveskyAbstract:We report new branching fractions for 121 UV lines from the low-lying odd-parity levels of Fe II belonging to the z6Do, z6Fo, z6Po, z4Fo, z4Do and z4Po terms of the 3d6(5D)4p configuration. These lines range in wavelength from 2250 - 3280 {\AA} and originate in levels ranging in energy from 38459 - 47626 cm-1. In addition, we report branching fractions for 10 weak blue lines connecting to the z4Do term which range in wavelength from 4173 - 4584 {\AA}. The BFs are combined with radiative lifetimes from the literature to determine transition probabilities and Log(gf) values. Comparison is made to selected experimental and theoretical data from the literature. Our new data are applied to iron abundance determinations in the Sun and in metal-poor star HD 84937. For the Sun, eight blue lines yield Log {\Epsilon}(Fe) = 7.46 +/- 0.03, in agreement with standard solar abundance estimates. For HD 84937 the observable wavelength range extends to the vacuum UV ({\lambda} >= 2327 {\AA}), and from 75 lines we derive Log {\Epsilon}(Fe) = 5.26 +/- 0.01 ({\sigma} = 0.07), near to the metallicity estimates of past HD 84937 studies.
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Improved Laboratory Transition Probabilities for Neutral Chromium and Re-determination of the Chromium Abundance for the Sun and Three Stars
The Astrophysical Journal, 2007Co-Authors: Jennifer Sobeck, James E. Lawler, Christopher SnedenAbstract:Branching fraction measurements from Fourier transform spectra in conjunction with published radiative lifetimes are used to determine transition probabilities for 263 lines of neutral chromium. These laboratory values are employed to derive a new photospheric abundance for the Sun: Log $\Epsilon$(Cr I)$_{\odot}$ = 5.64$\pm$0.01 ($\sigma = 0.07$). These Cr I solar abundances do not exhibit any trends with line strength nor with excitation energy and there were no obvious indications of departures from LTE. In addition, oscillator strengths for singly-ionized chromium recently reported by the FERRUM Project are used to determine: Log $\Epsilon$(Cr II)$_{\odot}$ = 5.77$\pm$0.03 ($\sigma = 0.13$). Transition probability data are also applied to the spectra of three stars: HD 75732 (metal-rich dwarf), HD 140283 (metal-poor subgiant), and CS 22892-052 (metal-poor giant). In all of the selected stars, Cr I is found to be underabundant with respect to Cr II. The possible causes for this abundance discrepancy and apparent ionization imbalance are discussed.
David L. Lambert - One of the best experts on this subject based on the ideXlab platform.
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Carbon, nitrogen and oxygen abundances in atmospheres of the 5–11 M⊙ B-type main-sequence stars
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Leonid S. Lyubimkov, David L. Lambert, Dmitry B. Poklad, Tamara M. Rachkovskaya, S. I. RostopchinAbstract:Fundamental parameters and the carbon, nitrogen and oxygen abundances are determined for 22 B-type stars with distances up to 600 pc and slow rotation (vsini up to 66 km/s). The stars are selected according to their effective temperatures Teff and surface gravities Log g, namely: Teff is between 15300 and 24100 K and Log g is mostly greater than 3.75; therefore, stars with medium masses of 5-11 M are selected. Theory predicts for the stars with such parameters that the C, N and O abundances in their atmospheres should correspond to their initial values. Non-LTE analysis of C II, N II and O II lines is implemented. The following mean C, N and O abundances are obtained: Log \Epsilon(C) = 8.31+-0.13, Log \Epsilon(N) = 7.80+-0.12 and Log \Epsilon(O) = 8.73+-0.13. These values are in very good agreement with recent data on the C, N and O abundances for nearby B stars from other authors; it is important that different techniques are applied by us and other authors. When excluding for the stars HR 1810 and HR 2938, which can be mixed, we obtain the following mean abundances for the remaining 20 stars: Log \Epsilon(C) = 8.33+-0.11, Log \Epsilon(N) = 7.78+-0.09 and Log \Epsilon(O) = 8.72+-0.12; these values are in excellent agreement with a present-day Cosmic Abundance Standard (CAS) of Nieva & Przybilla.
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Three Li-rich K giants: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952
The Astronomical Journal, 2005Co-Authors: Bacham E. Reddy, David L. LambertAbstract:We report on spectroscopic analyses of three K giants previously suggested to be Li-rich: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952. High-resolution optical spectra and the LTE model atmospheres are used to derive the stellar parameters: ($T_{\rm eff}$, Log $g$, [Fe/H]), elemental abundances, and the isotopic ratio $^{12}$C/$^{13}$C. IRAS 13539-4153 shows an extremely high Li abundance of $\Log\Epsilon$(Li) $\approx$ 4.2, a value ten times more than the present Li abundance in the local interstellar medium. This is the third highest Li abundance yet reported for a K giant. IRAS 12327-6523 shows a Li abundances of $\Log\Epsilon$(Li)$\approx$ 1.4. IRAS 17596-3952 is a rapidly rotating ($V{\sin i}$ $\approx$ 35 km s$^{-1}$) K giant with $\Log\Epsilon$(Li) $\approx$ 2.2. Infrared photometry which shows the presence of an IR excess suggesting mass-loss. A comparison is made between these three stars and previously recognized Li-rich giants.
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A Reappraisal of the Solar Photospheric C/O Ratio
The Astrophysical Journal, 2002Co-Authors: Carlos Allende Prieto, David L. Lambert, Martin AsplundAbstract:Accurate determination of photospheric solar abundances requires detailed modeling of the solar granulation and accounting for departures from local thermodynamical equilibrium (LTE). We argue that the forbidden C I line at 8727 A is largely immune to departures from LTE, and can be realistically modeled using LTE radiative transfer in a time-dependent three-dimensional simulation of solar surface convection. We analyze the [C I] line in the solar flux spectrum to derive the abundance Log Epsilon(C)= 8.39 +/- 0.04 dex. Combining this result with our parallel analysis of the [O I] 6300 A line, we find C/O=0.50 +/- 0.07, in agreement with the ratios measured in the solar corona from gamma-ray spectroscopy and solar energetic particles.
A. Brukhovesky - One of the best experts on this subject based on the ideXlab platform.
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Atomic Transition Probabilities for UV and Blue Lines of Fe II and Abundance Determinations in the Photospheres of the Sun and Metal-poor Star HD 84937
The Astrophysical Journal Supplement Series, 2019Co-Authors: E. A. Den Hartog, James E. Lawler, Christopher Sneden, John J. Cowan, A. BrukhoveskyAbstract:We report new branching fractions for 121 UV lines from the low-lying odd-parity levels of Fe II belonging to the z6Do, z6Fo, z6Po, z4Fo, z4Do and z4Po terms of the 3d6(5D)4p configuration. These lines range in wavelength from 2250 - 3280 {\AA} and originate in levels ranging in energy from 38459 - 47626 cm-1. In addition, we report branching fractions for 10 weak blue lines connecting to the z4Do term which range in wavelength from 4173 - 4584 {\AA}. The BFs are combined with radiative lifetimes from the literature to determine transition probabilities and Log(gf) values. Comparison is made to selected experimental and theoretical data from the literature. Our new data are applied to iron abundance determinations in the Sun and in metal-poor star HD 84937. For the Sun, eight blue lines yield Log {\Epsilon}(Fe) = 7.46 +/- 0.03, in agreement with standard solar abundance estimates. For HD 84937 the observable wavelength range extends to the vacuum UV ({\lambda} >= 2327 {\AA}), and from 75 lines we derive Log {\Epsilon}(Fe) = 5.26 +/- 0.01 ({\sigma} = 0.07), near to the metallicity estimates of past HD 84937 studies.
Kim A. Venn - One of the best experts on this subject based on the ideXlab platform.
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Chemical Abundances and the Evolutionary Status of 22 Galactic A-type Supergiants
Publications of the Astronomical Society of the Pacific, 1996Co-Authors: Kim A. VennAbstract:The A-type supergiants have an interesting location on the HR-diagram for testing stellar evolution theories since various evolution scenarios describe vastly different histories for these stars. In particular, the predicted abundances of carbon and nitrogen in the stellar atmospheres differ significantly since these stars may have evolved directly from the main-sequence or may be returning from the red giant branch where they would undergo the first dredge-up of CN-cycled H-burned gas from the stellar interior. This thesis provides new elemental abundances for a large group of Galactic A-type supergiants in the 5 to 20 solar mass range, while also addressing the difficulties in determining reliable abundances in these stars. The atmospheric analysis of each star was performed using the most recent Kurucz LTE model atmospheres. Atmospheric parameters (Teff and Log g) have been determined from ionization equilibrium of weak Mg I and Mg II lines and fitting the wings of the H-gamma line profiles; the final parameters were chosen at the intersection of the loci of possible Teff-gravity values from each indicator. Calculations show that NLTE effects on the weak Mg I and Mg II spectral lines used in this analysis are small, therefore they are ideal atmospheric parameter indicators. The metal abundances, Log Epsilon (O, Mg, Si, Ca, Sc, Ti, Cr, Fe, Ni) are solar to within about ±0.2~dex, calculated assuming LTE. Overabundances of Na are found, which are discussed as a combination of possible NLTE effects and/or pollution of newly synthesized Na from a NeNa proton capture reaction that could occur in the stellar core. Otherwise, we see no evidence of slight overall metal enrichments in these young stars that might be expected due to Galactic chemical evolution. More details of the atmospheric and LTE metal abundance analyses can be found in Venn 1994 (ApJS, 99, 659). The carbon and nitrogen abundances are examined in order to study the evolutionary status of these stars. LTE abundances show that nitrogen is enriched and carbon depleted, as expected after the first dredge-up, however the quantities are much greater than predicted and there is a very strong trend in the nitrogen abundances with Teff. New NLTE line formation calculations have been carried out for nitrogen and carbon to investigate these results (details in Venn 1994 ApJ, 449, 839). For nitrogen, it was necessary to construct a new and detailed N I/II model atom (which was calibrated with respect to Vega, see Lemke & Venn 1995, A&A, in press). Application to the A-supergiants shows that departures from LTE strongly affect the nitrogen abundances, even for weak lines. NLTE corrections (=Log (X)NLTE - Log EpsilonXLTE) for the weak lines used in this analysis range from -1.0 dex in the A0 supergiants to -0.3 in the F0 supergiants. The NLTE corrections remove the strong trend with effective temperature seen in the LTE nitrogen abundances. The average NLTE abundance is Log Epsilon (N)NLTE = 8.05 ± 0.19 for the 22 A0-F0 supergiants analyzed. For carbon, the Sturenburg & Holweger (1990, A&A, 237, 125) model atom was adopted and tested extensively. Again, the NLTE corrections are significant, ranging from -0.1 in the F0 stars to -0.5 in the A3 stars. For the hotter stars, we show that the only observable C I lines, near 9100A, do not yield reliable elemental abundances. The mean NLTE carbon abundance is Log Epsilon (C)NLTE =8.14 ± 0.13 for the 14 A3-F0 supergiants. The A-supergiant NLTE abundances are compared to the mean carbon and nitrogen abundances in unevolved B-stars in the same mass range, which are not the same as solar ([C/H]B*= -0.35 and [N/H]B*= -0.21). We find Log Epsilon (N/C)A I - Log Epsilon (N/C)B* =+0.38 ± 0.26. This value is significantly less than the first dredge-up ratio (~0.65 for 10 solar mass stars and ~0.98 for 20 solar mass stars) predicted by several evolution scenarios, including the popular Geneva tracks. However, the non-zero [N/C] ratio suggests that the A-type supergiants have undergone some partial mixing of CN-cycled gas, which is not currently predicted by Galactic metallicity evolution tracks. Therefore, Galactic A-type supergiants are not evolving from the red giant branch - they have evolved directly from the main-sequence, but have undergone some mixing of CN-cycled gas not currently predicted by stellar evolution theory.
