| 1. |
- Brage, Tomas, et al.
(författare)
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Atomic calculations inspired by the GHRS - gf-values, hyperfine structure constants, and isotope shifts for heavy elements
- 1998
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Ingår i: The Scientific Impact of the Goddard High Resolution Spectrograph (Astronomical Society of the Pacific Conference Series). - 1886733635 ; 143, s. 378-381
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Konferensbidrag (refereegranskat)abstract
- GHRS observations of chemically peculiar stars have initiated a new project for calculating atomic properties of heavy elements (Z approximate to 80). The method used is based on large-scale, fully relativistic MCDF, and the data involve accurate gf-values, hyperfine structure parameters, and relative isotope shifts.
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| 2. |
- Brage, Tomas, et al.
(författare)
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Hyperfine induced transitions as diagnostics of isotopic composition and densities of low-density plasmas
- 1998
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 500:1, s. 507-521
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Tidskriftsartikel (refereegranskat)abstract
- The J = 0 --> J' = 0 radiative transitions, usually viewed as allowed through two-photon decay, may also be induced by the hyperfine (HPF) interaction in atoms or ions having a nonzero nuclear spin. We compute new and review existing decay rates for the nsnp P-3(J)0 --> nS(2 1)S(J'= 0) transitions in ions of the Be (n = 2) and Mg (n = 3) isoelectronic sequences. The HPF induced decay rates for the J = 0 --> J' = 0 transitions are many orders of magnitude larger than those for the competing two-photon processes, and when present are typically 1 or 2 orders of magnitude smaller than the decay rates of the magnetic quadrupole (J = 2 --> J' = 0) transitions for these ions. Several HPF induced transitions are potentially of astrophysical interest in ions of C, N, Na, Mg, Al, Si, K, Cr, Fe, and Ni. We highlight those cases that may be of particular diagnostic value for determining isotopic abundance ratios and/or electron densities from UV or EUV emission-line data. We present our atomic data in the form of scaling laws so that, given the isotopic nuclear spin and magnetic moment, a simple expression yields estimates for HPF induced decay rates. We examine some UV and EUV solar and nebular data in light of these new results and suggest possible applications for future study. We could not find evidence for the existence of HPF induced lines in the spectra we examined, but we demonstrate that existing data have come close to providing interesting upper limits. For the planetary nebula SMC N2, we derive an upper limit of 0.1 for C-13/C-12 from Goddard High-Resolution Spectrograph data obtained by Clegg. It is likely that more stringent limits could be obtained using newer data with higher sensitivities in a variety of objects.
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| 3. |
- Brage, Tomas, et al.
(författare)
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Theoretical oscillator strengths and hyperfine structure in HgII
- 1999
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 513:1, s. 524-534
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Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical model ion for Hg II, including oscillator strengths and hyperfine structure constants for a large number of transitions and levels. Different computational models have been used, depending on the observability of the lines and their importance in spectral synthesis of stellar atmospheres. For the resonance lines, we use highly systematic, accurate, and fully relativistic multiconfiguration Dirac-Fock methods. These methods are well suited to an accurate treatment of both the relativistic problem and the strong correlation effects in this ion. The predicted gf-values are probably accurate to within a few percent. A larger number of transitions are treated with a more flexible, but less accurate, version of the method. This is based on the idea of crosswise optimization to represent a number of states in the same calculation. The results are used in stellar atmosphere models, assuming local thermodynamic equilibrium (LTE), where a line-by-line investigation is important. A larger set of levels are treated with semiempirical methods, for use in large scale non-LTE calculations. The lower accuracy of these are well-suited to a more statistical treatment of the structure of the ion. We discuss the importance of a correct treatment of core-valence correlation and relativistic effects for predicting accurate oscillator strengths. These tend to reduce their values by as much as a factor of 2 for the resonance line.
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| 4. |
- Brage, Tomas, et al.
(författare)
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Theoretical oscillator strengths for Sr II and Y III, with application to abundances in the HgMn-type star chi Lupi
- 1998
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 496:2, s. 1051-1057
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Tidskriftsartikel (refereegranskat)abstract
- Oscillator strengths for selected transitions of Sr II and Y III have been determined using ab initio multiconfiguration Hartree-Fock techniques. The importance of including an accurate treatment of the core-valence correlation is emphasized. The results are used to determine the abundances of Sr and Y in the chemically peculiar star chi Lupi from HST/GHRS Echelle spectra. Overabundances of 2.09 dex for Sr and 2.8 dex for Y relative to the solar abundance are derived, and an ionization imbalance of +1.1 dex is evident from abundance determinations using Y III and Y II.
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| 5. |
- Brandt, JC, et al.
(författare)
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A Goddard High Resolution Spectrograph atlas of echelle observations of the HgMn star chi Lupi
- 1999
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Ingår i: The Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 117:3, s. 1505-1548
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Tidskriftsartikel (refereegranskat)abstract
- Observations of the ultra-sharp-lined, chemically peculiar star chi Lupi taken by the Goddard High Resolution Spectrograph in echelle mode are presented. Thirty-six intervals of the spectral region between 1249 and 2688 Angstrom are covered with resolving powers in the range 75,000-93,000. Line identifications are provided, and the observed spectra are compared with synthetic spectra calculated using the SYNTHE program and associated line lists with changes to the line lists. The significance of these spectra for the chi Lupi Pathfinder Project and the closely related atomic physics effort is discussed in a companion paper.
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| 6. |
- Johansson, Sveneric, et al.
(författare)
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Hyperfine structure and isotope shift in Tl II with astrophysical applications
- 1996
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 462:2, s. 943-947
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Tidskriftsartikel (refereegranskat)abstract
- Fourier transform (FT) spectra of Tl II with resolved hyperfine structure (hfs) have been analyzed in the region 1790-3400 Angstrom. Measured isotope shifts (IS) for two transitions, including the S-1-P-3 intercombination line to the ground state at 1908.6 Angstrom are reported. Accurate Ritz wavelengths for isotope and hyperfine components of the S-1-P-1 resonance line at 1321.6 Angstrom are given. The accuracy is better than 0.3 m Angstrom for measured wavelengths and better than 0.4 m Angstrom for the Ritz wavelengths. Theoretical multi-configuration Dirac-Fock (MCDF) calculations, including core polarization, show that the LS-allowed 1321.6 Angstrom transition is a factor of 30 stronger than the intercombination line at 1908.6 Angstrom. The application of the new Tl II data to astrophysical spectra obtained with the Hubble Space Telescope is discussed.
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| 7. |
- Leckrone, DS, et al.
(författare)
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Abundance and isotopic anomalies of thallium in the atmosphere of the HgMn star chi Lupi
- 1996
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 462:2, s. 937-942
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Tidskriftsartikel (refereegranskat)abstract
- We announce the detection of two fully resolved hyperfine components of the intercombination line of Tl II (Z = 81) at 1908.6 Angstrom in the ultraviolet spectrum of the sharp-lined, nonmagnetic, chemically peculiar star chi Lupi (BB.5pHgMn + A2 Vm). The wavelengths and profiles of these absorption lines strongly suggest that they arise from the heaviest isotope, Tl-205. The echelle observations were obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph (HST/GHRS). Spectrum synthesis fits to the line profiles yield a logarithmic overabundance of thallium, relative to the solar system abundance, of +3.8 dex. We believe this to be the first detection of thallium in a star other than the Sun. It is also the first time hyperfine structure has been resolved in the satellite ultraviolet spectrum of an astrophysical source. The large overabundance and the presence of only the heaviest isotope of the element continue the pattern observed previously in chi Lup for platinum (Z = 78), gold (Z = 79), and mercury (Z = 80).
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| 8. |
- Leckrone, DS, et al.
(författare)
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GHRS spectroscopy of chemically peculiar stars: The chi Lupi Pathfinder Project
- 1998
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Ingår i: The Scientific Impact of the Goddard High Resolution Spectrograph (Astronomical Society of the Pacific Conference Series). - 1886733635 ; 143, s. 135-153
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Konferensbidrag (refereegranskat)abstract
- The HST has provided visual images of the universe of unprecedented clarity and sensitivity. It is less widely known that the HST/GHRS has provided equally breathtaking views of a different kind Of "universe" - a "spectroscopic universe". Nowhere is this better seen than in the extraordinarily complex and beautiful ultraviolet spectrum of the ultra-sharp-lined, non-magnetic, chemically peculiar (CP) star chi Lupi (B9.5pHgMn + A2 Vm). We have begun a systematic exploration and quantitative analysis of this spectrum with the echelle mode of the GHRS. We call this the "The chi Lupi Pathfinder Project", because it is analogous to the exploration and mapping of a previously uncharted land by a diverse team of specialists - in this case astrophysicists and atomic physicists working synergistically. Our objective is to characterize, comprehensively and accurately, the full range of abundance and isotopic anomalies found in the atmosphere of at least one CP star to test and constrain the most widely accepted theoretical scenario for the production of abundance anomalies - radiatively-driven diffusion and gravitational settling. These physical mechanisms are important in several other astrophysical contexts. The surface layers of the CP stars provide a laboratory to investigate the conditions which foster them and the competing mechanisms which retard them. The GHRS echelle data set for chi Lupi covers 345 Angstrom, with S/N approximate to 50-100. We have quadrupled the number of elements for which abundances have been measured in this star, spanning most of the periodic table. These include several elements which have rarely, if ever, been seen before in a star other than the Sun and several examples of isotopically anomalous abundances. We have found systematic trends in these abundance data, which qualitatively support the mechanism of radiatively-driven diffusion and gravitational settling. However, our state-of-the-art theoretical, non-LTE diffusion models have not as yet been successful in sustaining the observed magnitudes of the largest abundance anomalies nor in reproducing anomalous isotope blends.
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| 9. |
- Leckrone, DS, et al.
(författare)
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Stellar chemical abundances with the GHRS
- 1996
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Ingår i: Physica Scripta. - 0031-8949. ; T65, s. 110-114
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Tidskriftsartikel (refereegranskat)abstract
- The accurate quantitative analysis of high resolution ultraviolet spectra of ultra-sharp-lined early-type, chemically peculiar stars, obtained with the Goddard High Resolution spectrograph on the Hubble Space Telescope, has been made possible by the synergism between state-of-the-art astro-physics and state-of-the-art atomic spectroscopy. We illustrate this with several examples, including the discovery of spin-forbidden ''parasite'' transitions of Fe II, the first observations of hyperfine components and isotopic shifts of stellar lines of thallium, and measurements of the photospheric abundances of a large number of normally trace elements.
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| 10. |
- Leckrone, DS, et al.
(författare)
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Very high resolution ultraviolet spectroscopy of a chemically peculiar star: Results of the chi Lupi Pathfinder project
- 1999
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Ingår i: The Astronomical Journal. - : American Astronomical Society. - 1538-3881 .- 0004-6256. ; 117:3, s. 1454-1470
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Tidskriftsartikel (refereegranskat)abstract
- We summarize here the results of a major eight-year investigation of the extraordinarily detailed UV spectrum of the sharp-lined, nonmagnetic, main-sequence, chemically peculiar star chi Lupi (B9.5p HgMn + A2 Vm). The UV observations are composed of 345 Angstrom of the spectrum acquired with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope at an average resolution of 0.023 Angstrom. The complete set of echelle spectrograms is presented as an atlas in a companion paper. These data were supplemented by optical-wavelength spectra obtained at the Angle-Australian Telescope. Quantitatively accurate analysis and theoretical interpretation of these data required major improvements in the accuracy and completeness of available atomic data-wavelengths, transition probabilities, hyperfine structure, and isotope shifts-for the lowest ionization states of many elements. A large, international group of theoretical and experimental atomic physicists has collaborated in this investigation, and their results are summarized or referenced in this paper. In turn, the GHRS observations of chi Lupi have become a useful source of data for atomic spectroscopy, displaying many transitions that are difficult to observe in a laboratory setting. Measured abundances or upper limits are presented for 72 ions of 51 chemical elements, spanning the periodic table. We have confirmed and refined previously identified isotopic abundance anomalies in mercury and platinum and have discovered similar isotopic anomalies in thallium and, tentatively, in lead. Large discrepancies among the LTE abundances derived, using a chemically homogeneous model atmosphere, from two or three ionization states of the same element are found to be common. In some cases these are due to departures from LTE in the ionization equilibria, but the largest such discrepancies probably result from chemical stratification within the photosphere. We find qualitative trends in the abundances of the elements that clearly signify radiatively driven diffusion and gravitational settling as the primary mechanism producing abundance anomalies. However, detailed non-LTE diffusion calculations for mercury and thallium show that there is insufficient unsaturated radiative force within the chemically enriched atmosphere to sustain the observed huge overabundances of these elements in equilibrium with gravity. Either other hydrodynamic processes, such as slow mass motions or unexpectedly strong stellar winds must assist radiation pressure in supporting the enriched material, or the observed abundance patterns simply provide a snapshot in time of a nonequilibrium, time-variable phenomenon.
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| 11. |
- Proffitt, CR, et al.
(författare)
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Mercury in the HgMn stars chi Lupi and HR 7775
- 1999
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 512:2, s. 942-960
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Tidskriftsartikel (refereegranskat)abstract
- Observations of mercury lines in the HgMn stars chi Lupi and HR 7775 made with the Hubble Space Telescope Goddard High Resolution Spectrograph are presented and analyzed. In chi Lupi we find that all observed lines are consistent with the same isotopic mixture (essentially pure (204)Hg). Strong ionization anomalies are present, with UV Hg I lines being too weak and Hg III lines too strong for the abundance derived from lines of the majority ionization state, Hg rr. Observations of mercury in I;IR 7775 show less extreme isotope and ionization anomalies. We find that the ionization anomaly in the Hg I resonance lines can be plausibly explained as a non-LTE effect, but the same non-LTE calculations show that the Hg III ionization anomaly in chi Lupi cannot be explained in this way. Radiative force calculations show that the observed mercury abundance cannot be supported in the atmosphere by the radiative forces alone. We suggest that weak mixing brings mercury into the line-forming region from below the photosphere, while a wind of order 10(-14) M. yr(-1) supports a cloud of Hg III at very small optical depths.
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| 12. |
- Proffitt, CR, et al.
(författare)
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Selections from the GHRS atlas of chi Lupi
- 1998
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Ingår i: The Scientific Impact of the Goddard High Resolution Spectrograph (Astronomical Society of the Pacific Conference Series). - 1886733635 ; 143, s. 322-325
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Konferensbidrag (refereegranskat)abstract
- High S/N echelle data of chi Lupi have been obtained at 36 different wavelength settings. The high resolution of the observations combined with the low rotational velocity of chi Lupi (v sin i approximate to 1 km s(-1)), allow the UV spectrum of this star to be seen with unprecedented detail, and provides a unique resource for the study of both atomic physics and stellar atmospheres.
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| 13. |
- Wahlgren, Glenn, et al.
(författare)
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Very heavy elements in the HgMn star chi Lupi
- 1998
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Ingår i: The Scientific Impact of the Goddard High Resolution Spectrograph (Astronomical Society of the Pacific Conference Series). - 1886733635 ; 143, s. 330-333
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Konferensbidrag (refereegranskat)abstract
- Abundances for the elements W, Os, and Bi are presented for the HgMn star chi Lupi, based upon GHRS echelle-mode spectra and a synthetic spectrum analysis. The tungsten abundance is consistent with a solar-system value while the osmium and bismuth abundances reflect enhancements. With these new data we are able to quantitatively define an abundance peak within the element range Z = 74 through 83.
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