| 1. |
- Cowley, CR, et al.
(författare)
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Abundances of vanadium and bromine in 3 Centauri A - Additional odd-Z anomalies
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 447:2, s. 681-684
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Tidskriftsartikel (refereegranskat)abstract
- We report abundance excesses of 1.2 and 2.6 dex, respectively, for vanadium and bromine in the hot, peculiar star 3 Cen A. Abundances for these two odd-Z elements have not been previously reported for this star. Taken with previous work, they strengthen the case of the origin of the abundance peculiarities by diffusion.
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| 2. |
- Eriksson, Mattias, et al.
(författare)
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Bowen excitation of NIII lines in symbiotic stars
- 2005
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 434:2, s. 397-404
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Tidskriftsartikel (refereegranskat)abstract
- We present a semi-empirical equation for prediction of the strengths of those N III lines that are generated by the Bowen mechanism and observed in spectra of symbiotic stars. The equation assumes that the Bowen mechanism is the only source populating the 3d state in N III, and comparisons with observations of the 3s-3p and 3p-3d transitions serve as a test of this assumption. In an ongoing study of symbiotic stars the equation has been applied to two symbiotic novae, RR Tel and AG Peg, by comparing the predicted N III line strengths to observed line intensities. It is clear that besides the Bowen mechanism there is another process, most likely radiative recombination, that contributes to the N III 3d population in AG Peg and is the main population process of this state in RR Tel. It is also clear that a second, not previously considered, N III channel, 2p P-2(1/2)-3d D-2(3/2) at 374.198 angstrom, is pumped by O III in both RR Tel and AG Peg.
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| 3. |
- Eriksson, Mattias, et al.
(författare)
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The nature of ultraviolet spectra of AG Pegasi and other symbiotic stars: locations, origins, and excitation mechanisms of emission lines
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 451:1, s. 89-157
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Tidskriftsartikel (refereegranskat)abstract
- A detailed study of ultraviolet spectra of the symbiotic star AG Peg has been undertaken to derive the atomic excitation mechanisms and origin of formation for the lines common in symbiotic systems. More than 600 emission lines are observed in spectra from IUE, HST and FUSE of which 585 are identified. Population mechanisms and origin of formation are given for a majority of those lines. Based on the understanding of the AG Peg spectra IUE data of 19 additional symbiotic stars are investigated and differences and similarities of their spectra are discussed. Fe II fluorescence lines pumped by strong emission lines between 1000 and 2000 angstrom are observed in 13 of these systems. Some of the symbiotic systems belonging to the subclass symbiotic novae have more than 100 Fe II fluorescence lines in the ultraviolet wavelength region. Forbidden lines are detected for 13 of the stars, mostly from highly-ionized spectra such as Ar V, Ne V and Mg V. Further, [Mg VI] and [Mg VII] lines are observed in a symbiotic star (AG Dra) for the first time. Five of the symbiotic stars have broad white-dwarf wind profiles (FWHM > 400 km s(-1)) for a few lines in their spectra. The stars with no such broad lines can be divided into two similarly sized groups, one where all lines have FWHM less than 70 km s(-1) and the other where one, a few or all of the broad (FWHM > 400 km s(-1)) lines of AG Peg have an enhanced broad wing (110-140 km s(-1)).
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| 4. |
- Faria, Daniel, et al.
(författare)
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The usage of Stromgren photometry in studies of local group dwarf spheroidal galaxies - Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients
- 2007
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 465:2, s. 28-357
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Tidskriftsartikel (refereegranskat)abstract
- Aims. In this paper we demonstrate how Stromgren uvby photometry can be efficiently used to: 1. identify red giant branch stars that are members in a dwarf spheroidal galaxy; 2. derive age-independent metallicities for the same stars and quantify the associated errors. Methods. Stromgren uvby photometry in a 11 x 22 arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were identified using the surface gravity sensitive c(1) index which discriminates between red giant and dwarf stars. Thus enabling us to distinguish the (red giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy. The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions. The metallicity sensitive m(1) index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy. The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good. Results. We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy (the largest sample to date). The metallicity distribution function for the Draco dSph galaxy shows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old, and coeval population. There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars.
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| 5. |
- Gurell, Jonas, 1981-, et al.
(författare)
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Wavelengths, energy levels and HFS in Ho II
- 2009
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 79:3, s. 035306-
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Tidskriftsartikel (refereegranskat)abstract
- We recorded spectra of Ho II emitted by hollow-cathode lamps, using the ultraviolet Fourier transform spectrometer (FTS) at Lund Observatory and the 2-m FTS at the National Institute of Standards and Technology. The combined wavenumber coverage spans the interval from 8000 cm-1 to 49600 cm-1(12500 Å to 2016 Å). We measured 303 lines in these spectra and used them to determine energies and hyperfine structure constants of 100 levels. Ofthese, 41 energy levels have not been reported previously and hyperfine structure constants have not been reported previously for 83 levels. Comparisonsbetween the hyperfine structure constants and energy levels presented in this article and those previously determined from other studies are included anddiscrepancies are discussed. A theoretical study of the odd-parity levels leads to hyperfine structure parameters and to predictions for unknown excited levelsof the ground-state configuration.
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| 6. |
- Gurell, J., et al.
(författare)
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Wavelengths, energy levels and hyperfine structure constants in Ho II
- 2009
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Ingår i: Physica Scripta. - : IOP Publishing. - 0031-8949 .- 1402-4896. ; 79:3
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Tidskriftsartikel (refereegranskat)abstract
- We recorded spectra of Ho II emitted by hollow-cathode lamps, using the ultraviolet Fourier transform spectrometer (FTS) at Lund Observatory and the 2-m FTS at the National Institute of Standards and Technology. The combined wavenumber coverage spans the interval from 8000 to 49 600 cm(-1) (12 500 to 2016 angstrom). We measured 303 lines in these spectra and used them to determine energies and hyperfine structure constants of 100 levels. Of these, 41 energy levels have not been reported previously and hyperfine structure constants have not been reported previously for 83 levels. Comparisons between the hyperfine structure constants and energy levels presented in this article and those previously determined from other studies are included and discrepancies are discussed. A theoretical study of the odd-parity levels leads to hyperfine structure parameters and to predictions for unknown excited levels of the ground-state configuration.
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| 7. |
- Johansson, Sveneric, et al.
(författare)
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Commission 14: Atomic & Molecular Data
- 2007
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Ingår i: Transactions of the International Astronomical Union, Reports on Astronomy. ; 26A, s. 337-375
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Bokkapitel (populärvet., debatt m.m.)
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| 8. |
- Lundqvist, Martin, et al.
(författare)
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Improved oscillator strengths and wavelengths in Hf II, with applications to stellar elemental abundances
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 450:1, s. 115-407
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We present new and improved radiative lifetimes for eight levels in HfI and 18 levels in HfII, along with oscillator strengths and wavelengths for 195 transitions in HfII. With these data we determine the abundance of hafnium in two chemically peculiar stars: the hot-Am star HR 3383 and the HgMn star chi Lupi, and discuss the implications of the new data to the hafnium abundance for the Sun and the metal-poor galactic halo stars CS 22892-052 and CS 31082-001. Methods. The oscillator strengths are derived by combining radiative lifetimes measured with the laser induced fluorescence technique and branching fractions determined from intensity calibrated Fourier transform spectra. The hafnium abundance in the two sharp-lined peculair stars is determined by comparison of spectra obtained from instruments onboard the Hubble Space Telescope with synthetic spectra, while the abundance of hafnium in the solar photosphere and the metal-poor halo stars is discussed in terms of rescaling previous investigations using the new g f values. Results. The abundance enhancement of hafnium has been determined in HR 3383 to be +1.7 dex and that for chi Lupi A is +1.3 dex. In the course of the analysis we have also determined an abundance enhancement for molybdenum in HR 3383 to be +1.2 dex, which is similar to that known for. Lupi A. The abundances in the metal-poor halo stars CS 31082-001 and CS 22892-052 were rescaled to log epsilon(Hf) = -0.75 and -0.82 respectively, with smaller 1 sigma uncertainty. This has the effect of improving the theoretical fits of r-process nucleosynthesis to abundance data for heavy elements. The change of g f values also implies that the hafnium abundance in the solar photosphere should be reduced by up to 0.2 dex, thereby inducing a discrepancy with the meteoritic hafnium abundance.
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| 9. |
- Lundqvist, Martin, et al.
(författare)
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Isotope structure of SmII as an indicator of r- vs. s-process nucleosynthesis
- 2007
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 463:2, s. 693-702
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The isotope composition of samarium provides a previously unexploited means of determining the r- versus s-process content of e. g. metal-poor stars. The seven stable isotopes of samarium can be divided into two groups, where in one group the r- process is the dominant production mechanism, while for the second group it is the s-process that is dominant. The center-of-gravity for spectral lines produced by these two groups are slightly shifted with respect to each other, where the heavier and most abundant isotopes are mainly produced by the r- process. This relative shift makes the line position dependent upon the content of r- and s-processed samarium. The isotope composition also affects the width and the shape of the line, making it narrower and slightly more asymmetric when the composition deviates from the solar system composition. Methods. The behaviour of Sm II lines in stellar spectra is studied using synthetic spectrum techniques with line data accounting for both isotope and hyperfine structure. Isotope shifts in 11 lines of Sm II have been measured from a laboratory spectrum obtained with a Fourier transform spectrometer. Hyperfine structure constants have been taken from the literature. Results. The new atomic data are applied to the spectrum of the metal-poor halo star CS 31082-001 and suggests that this star contains predominantly r- processed material. We also discuss the uncertainties involved in this type of high precision measurement.
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| 10. |
- Lundqvist, Martin, et al.
(författare)
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s-Process elements in the M supergiant alpha Ori
- 2005
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Ingår i: Nuclear Physics, Section A. - : Elsevier BV. - 0375-9474. ; 758, s. 304-307
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Tidskriftsartikel (refereegranskat)abstract
- We present new results on a study of the weak s-process elemental abundances in the M supergiant alpha Ori (M2Iab). We have conducted the abundance analysis through comparison of synthetic with observed spectra in the two infrared spectral regions 0.7 - 1.1 mu m and 2.1 - 2.3 mu m, where the molecular and atomic line densities are less than in the optical region and where many of the relevant elements have strong tines. Our results show that there has been weak s-process production occurring during the life of the star and among the s-process elements is Sr the most, overabundant with 1.4 dex higher abundance than the sun. The abundance of Fe in alpha Ori is essentially solar. Other elements for which we have determined abundances are Rb, Y and Zr. We compare our results to nuclear evolutionary stellar model calculations and discuss their relevance.
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