| 1. |
- Clavel, J., et al.
(författare)
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Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. I. An 8 month campaign of monitoring NGC 5548 with IUE
- 1991
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 366:1, s. 64-8181
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Tidskriftsartikel (refereegranskat)abstract
- The authors present emission-line and ultraviolet continuum observations of a type I Seyfert galaxy in which the time resolution is adequate for describing the character of variability. Using the IUE satellite, the nucleus of NGC 5548 was observed every 4 days for a period of 8 months. Its mean properties-continuum shape, line ratios-are not unusual for type I Seyfert galaxies, but it was found to be strongly variable. The ultraviolet continuum flux and broad emission line fluxes varied significantly, going through three large maxima and three deep minima. The great majority of all variations were well resolved in time. The data lend qualitative support to the view that photoionization by the nuclear continuum is responsible for driving the emission lines
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| 2. |
- Feltzing, Sofia, et al.
(författare)
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The production of beryllium in the early Galaxy
- 1994
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 423:1, s. 68-72
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Tidskriftsartikel (refereegranskat)abstract
- The formation of beryllium in the early Galaxy is discussed. It is shown that if spallation occurs predominantly in regions rich in heavy elements, i.e., close to supernovae, the linear relation recently obtained by Gilmore et al. (1992) between beryllium and oxygen abundances for Population II stars may be reproduced. Estimates of relevant timescales for mixing show that the decisive factor, the ratio of the timescale of cosmic-ray diffusion out of the locally enriched regions to the decay time of cosmic rays in the Galaxy as a whole, admits this scenario. Although energetically possible, it requires, however, very high local cosmic-ray fluxes (greater than or equal to 105/sq cm/s), which predicts gamma-ray fluxes above those observed from supernova remnants and the Galaxy. We therefore consider other models, such as those given by Duncan et al. (1992) or Prantzos et al. (1993), more probable.
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| 3. |
- Johansson, S. G, et al.
(författare)
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Comparison of New Experimental and Astrophysical F-values For Some Ru-ii Lines, Observed In Hst Spectra of Chi-lupi
- 1994
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 421:2, s. 809-815
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Tidskriftsartikel (refereegranskat)abstract
- We report on experimental absolute oscillator strengths for 18 UV lines of Ru II, obtained by combining laser-induced fluorescence measurements of radiative lifetimes and branching fractions from line intensities in a calibrated Fourier-transform spectrum. HST/GHRS observations of the spectrum of the sharp-lined B star chi Lupi contain six of these lines, for which ''astrophysical'' relative f-values have been determined. The agreement is within 0.10 dex for a Ru abundance of log N(Ru)/N(H) = -7.90, which is 2.3 dex above the solar abundance.
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| 4. |
- Jönsson, Per, et al.
(författare)
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Accurate Calculation of the Isotope Shift and Hyperfine-structure In the Boron (b-ii) Line At 1362 Angstrom
- 1994
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 429:1
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Tidskriftsartikel (refereegranskat)abstract
- An extensive multiconfiguration Hartree-Fock (MCHF) calculation of the isotope shift and hyperfine structure in the resonance line of B II at 1362 angstrom has been performed. The calculated B-11-B-10 isotope shift is 13.3 mangstrom with an estimated uncertainty of 1%. The calculated interaction constants A and B give a hyperfine structure splitting of less than 0.45 mangstrom for both isotopes. The size of the isotope shift in the B II line is similar to the limit of detection in high-resolution observations with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope when side A becomes operational after the service mission.
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| 5. |
- Thomasson, Magnus, 1961, et al.
(författare)
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A computer-generated galaxy model with long-lived two-armed spiral structure
- 1990
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 356, s. L9-L13
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Tidskriftsartikel (refereegranskat)abstract
- A long-lived two-armed spiral has been generated in an N-body computer simulation of a galaxy with a static bulge and halo and an active disk composed of 60,000 particles. The spiral lasts for about three pattern revolutions without severe distortion and persists for at least two more revolutions with distortions and bifurcations resulting from an increasingly clumpy ISM. This suggests that two-armed grand design spirals in nonbarred noninteracting galaxies can be long-lived if star formation and other heat sources not present in the simulation maintain a steady interstellar medium.
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| 6. |
- Wahlgren, Glenn, et al.
(författare)
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The pursuit of heavy elements in the HgMn-type star Chi Lupi: Observations with the GHRS in the COSTAR ERA
- 1994
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 435:1, s. 67-70
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Tidskriftsartikel (refereegranskat)abstract
- Observations of the ultra-sharp-lined HgMn star chi Lupi made with the Goddard High Resolution Spectrograph after the Hubble Space Telescope First Servicing Mission display the effect of COSTAR corrective optics upon the spectral resolution of several observing modes. The resolution is restored to levels expected in the absence of the telescope spherical aberration for all observations made through the GHRS Large Science Aperture (LSA). The Small Science Aperture (SSA) spectra are nearly identical in resolution to the pre-COSTAR values while benefiting from higher throughput. The line spread function for the LSA relative to the SSA shows little wavelength dependence and reflects a Gaussian profile approximately 20%-30% broader than for SSA spectra. Absorption features containing components of As II and Bi II represent upper limit enhancements of 2.9 and 2.1 dex for As II and Bi II, respectively, relative to meteoritic abundance determinations.
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