| 1. |
- Biemont, E, et al.
(author)
-
Lifetimes and oscillator strengths for ultraviolet transitions in neutral sulfur
- 1998
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 502:2, s. 1010-1014
-
Journal article (peer-reviewed)abstract
- A consistent set of f-values for vacuum ultraviolet lines of S I is deduced from a combination of laser lifetime measurements and theoretical branching ratios. The new results are compared with results deduced from absorption by interstellar gas toward zeta Ophiuchi, and good agreement is observed for most transitions.
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| 2. |
- Brage, Tomas, et al.
(author)
-
Hyperfine induced transitions as diagnostics of isotopic composition and densities of low-density plasmas
- 1998
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 500:1, s. 507-521
-
Journal article (peer-reviewed)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.
(author)
-
Intercombination and allowed transitions in O IV
- 1996
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 464:2, s. 1030-1043
-
Journal article (peer-reviewed)abstract
- We report on large-scale ab initio multiconfiguration Hartree-Fock calculations for important multiplets including the UV 0.01 multiplet, 2s(2)2p(2)P(J-)2s2p(24)P(J'), in O IV. The resulting transition probabilities should be accurate to +/- 5%-10% for intersystem lines and 1% or better for permitted lines. We present new calculations of line emission coefficients using these transition probabilities and collisional rate coefficients published by Zhang, Graziani, & Pradhan in 1994. We readdress the use of these lines as diagnostics of electron densities, paying particular attention to uncertainties in density determinations. We find (1) the absolute uncertainties in derived densities are difficult to assess, but are at least +/-40%, controlled by uncertainties in collision strengths; (2) our new calculations bring observed and computed line ratios into better agreement (earlier papers yielding systematically different densities make the agreement worse); (3) there is additional evidence for strong blends in the 1404.8 emission feature, as argued by other authors, and/or evidence for inaccurate laboratory wavelengths; and (4) the computed branching ratios are in good agreement with observed ratios to within observational uncertainties of +/-7%. We determine electron densities in a variety of solar features from HRTS data from the first and second flights of this instrument. The derived electron densities vary remarkably little between quiet Sun network and active regions, but resonance line intensities vary dramatically, and we discuss reasons for this. Finally, we discuss how new high-quality data from the SUMER instrument to be flown on Solar and Heliosphereic Observatory (SOHO) could be used to address outstanding problems concerning blends and heating of the solar transition region.
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| 4. |
- Brage, Tomas, et al.
(author)
-
Theoretical oscillator strengths and hyperfine structure in HgII
- 1999
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 513:1, s. 524-534
-
Journal article (peer-reviewed)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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| 5. |
- Brage, Tomas, et al.
(author)
-
Theoretical oscillator strengths for Sr II and Y III, with application to abundances in the HgMn-type star chi Lupi
- 1998
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 496:2, s. 1051-1057
-
Journal article (peer-reviewed)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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| 6. |
- Crenshaw, D. M., et al.
(author)
-
Multiwavelength observations of short-timescale variability in NGC 4151. I. Ultraviolet observations
- 1996
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 470:1, s. 322-335335
-
Journal article (peer-reviewed)abstract
- Presents the results of an intensive ultraviolet monitoring campaign on the Seyfert 1 galaxy NGC 4151, as part of an effort to study its short-timescale variability over a broad range in wavelength. The nucleus of NGC 4151 was observed continuously with the International Ultraviolet Explorer for 9.3 days, yielding a pair of LWP and SWP spectra every ~70 minutes, and during 4 hr periods for 4 days prior to and 5 days after the continuous-monitoring period. The sampling frequency of the observations is an order of magnitude higher than that of any previous UV monitoring campaign on a Seyfert galaxy. The continuum fluxes in bands from 1275 to 2688 Aring went through four significant and well-defined ldquoeventsrdquo of duration 2-3 days during the continuous-monitoring period. The authors find that the amplitudes of the continuum variations decrease with increasing wavelength, which extends a general trend for this and other Seyfert galaxies to smaller timescales (i.e., a few days). The continuum variations in all the UV bands are simultaneous to within an accuracy of ~0.15 days, providing a strict constraint on continuum models. The emission-line light curves show only one major event during the continuous monitoring (a slow rise followed by a shallow dip) and do not correlate well with continuum light curves over the short duration of the campaign, because the timescale for continuum variations is apparently smaller than the response times of the emission lines
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| 7. |
- Edelson, R. A., et al.
(author)
-
Multiwavelength observations of short-timescale variability in NGC 4151. IV. Analysis of multiwavelength continuum variability
- 1996
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 470:1, s. 364-377377
-
Journal article (peer-reviewed)abstract
- For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)
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| 8. |
- Elmegreen, B.G., et al.
(author)
-
Dust Spirals and Acoustic Noise in the Nucleus of the Galaxy NGC 2207
- 1998
-
In: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 503:2, s. L119-L122
-
Journal article (peer-reviewed)abstract
- Observations with the Hubble Space Telescope reveal an irregular network of dust spiral arms in the nuclear region of the interacting disk galaxy NGC 2207. The spirals extend from ~50 to ~300 pc in galactocentric radius, with a projected width of ~20 pc. Radiative transfer calculations determine the gas properties of the spirals and the inner disk and imply a factor of ~4 local gas compression in the spirals. The gas is not strongly self-gravitating, nor is there a nuclear bar, so the spirals could not have formed by the usual mechanisms applied to main galaxy disks. Instead, they may result from acoustic instabilities that amplify at small galactic radii. Such instabilities may promote gas accretion into the nucleus.
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| 9. |
- Godefroid, M, et al.
(author)
-
Accurate Multiconfiguration Hartree-fock Calculations of Oscillator-strengths In Light-atoms - the Boron (b-ii) Line At 1362 Angstrom
- 1995
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 450:1, s. 473-476
-
Journal article (peer-reviewed)abstract
- A new method to deal with nonorthogonalities in transition calculations has been implemented and tested. Large-scale transition calculations using the multiconfiguration Hartree-Fock (MCHF) method have been performed for the resonance line of B II at 1362 Angstrom. In these calculations the orbital sets of the initial and final-state wavefunctions have not been assumed to be equal but have been optimized independently. The calculated weighted multiplet oscillator strength gf= 0.999 has an estimated uncertainty of less than 0.5%. Together with the previously calculated isotope shift and hyperfine structure (Jonsson, Johansson, and Froese Fischer 1994), all the important atomic parameters needed for a detailed modeling of the B II resonance line in high-resolution astrophysical spectra are now available with high accuracy.
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| 10. |
- Henderson, M, et al.
(author)
-
Lifetime measurements for ground term transitions in TaII, WII, and ReII
- 1999
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 520:2, s. 805-810
-
Journal article (peer-reviewed)abstract
- Lifetime measurements have been made for the radiation emitted in the ground term transitions of the nominal form 5s(2)5p(6)5d(x)6s-5s(2)5p(6)5d(x)6p in Ta II (x = 3), W II (x = 4), and Re II (x = 5). Supporting theoretical calculations have also been performed to characterize configuration interaction and to estimate the degree of decay branching of the upper levels. The lifetime measurements have been combined with these theoretical considerations and with branching fractions deduced from the emission measurements of Corliss & Bozman to specify the oscillator strengths of these transitions. The results are discussed in the context of spectroscopic data obtained from orbiting telescopes and applied to Hubble Space Telescope spectra to estimate the tungsten abundance in Sirius.
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