| 1. |
- Bertin, G., et al.
(författare)
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Global spiral modes in stellar disks containing gas
- 1988
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 195, s. 105-113
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Tidskriftsartikel (refereegranskat)abstract
- The authors present a linear study of the role of gas on global spiral modes in stellar disks using a simple two-fluid model. The local destabilizing role of a cold component has been known for many years and calculated by various authors. The authors confirm the effectiveness of a small amount of gas in fueling spiral instabilities with a response which is primarily at short wavelengths by constructing a synthetic diagram which summarizes the local stability analysis. Then they proceed to consider global spiral modes in two-component systems with applications to regimes of astrophysical interest that are expected to be associated with normal spiral structure. Moderate growth spiral modes are found that are very close in structure to those of one-component systems, but for substantially higher values of the stability parameter Q. A simple model of self-regulation is also presented.
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| 2. |
- Marcaide, J. M., et al.
(författare)
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The quasars 1038+528 A and B
- 1985
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Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 142:1, s. 71-84
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Tidskriftsartikel (refereegranskat)abstract
- The results of VLBI observations of the quasars 1038 + 528 A and B at 2.8, 3.6, 13, and 18 cm at various times between November 1979 and March 1981 are reported. The observations and data calibration are described, as are the mapping and astrometric techniques applied in the study. Both quasars are found to have 'core-jet' morphologies. The core of the A quasar dominates its morphology at centrimetric wavelengths with the brightness temperature of its 400 pc long jet being about 1/100 that of the core. By contrast, the 'jet' in the B quasar is very short (about 70 pc); the tail of this jet has the steepest spectral index found to date in extragalactic compact sources, indicating that high electron losses are responsible for the shortness of the jet. No evidence for appreciable morphological change in the B quasar was found over the time span of the study, whereas a new feature may be emerging from the A quasar core at superluminal speed.
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| 3. |
- Pettersson, Bertil
(författare)
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T Tauri stars and dust clouds in a region of the Gum Nebula
- 1987
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 171, s. 101-115
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Tidskriftsartikel (refereegranskat)abstract
- Photometric, spectroscopic and infrared observations are presented of 16 H-alpha emission-line stars in the Gum nebula. Nine of the stars were found to be T Tauri stars. They are located in a part of the Gum nebula rich in dark clouds near the cometary globule complex CG 30-CG 31. The globules and the associated dark clouds DC 252.9-1.6 and DC 253.1-1.7 have been mapped in the 6-cm formaldehyde line. It is concluded that they, together with the T Tauri stars and other dark clouds near the globules, define a region of star formation in the Gum nebula with preferentially low mass stars.
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| 4. |
- Stirpe, G. M., et al.
(författare)
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Broad emission line variability in the Seyfert 1 galaxies NGC 5548 and NGC 3783
- 1988
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 200:1-2, s. 9-1616
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Tidskriftsartikel (refereegranskat)abstract
- High quality spectra are presented for the Seyfert 1 galaxies NGC 5548 and NGC 3783. Halpha and Hbeta were observed at different epochs, with time intervals between 4 and 13 months. The spectra have been scaled to each other by normalizing the flux of the narrow lines. The difference spectra show that the broad emission lines have varied not only in flux, but also in shape. The authors compare their data with those published by Peterson (1987) and Peterson et al. (1987) and conclude that the supermassive binary scenario proposed by the latter is not unique. The observed variations can be interpreted in an accretion disk scenario. The whole data-set also provides evidence against models based on outflowing motion. The He II lambda4686 line discussed together with N III lambda4640 Bowen emission. A limit to the results is given by the long time intervals separating the observations, which do not allow the authors to set as strong constraints on the BLR as high quality, frequent monitoring of Seyfert 1 spectra would
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| 5. |
- Stirpe, G. M., et al.
(författare)
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Emission line variation in the Seyfert galaxy Fairall 9 and the presence of broad [O III] emission
- 1989
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 211:2, s. 310-314314
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Tidskriftsartikel (refereegranskat)abstract
- The authors present two high quality spectra of the Hbeta region in the Seyfert 1 galaxy Fairall 9, separated by a period of 4 years. The variation of the broad lines is discussed, and in particular that of the Fe II lines of multiplet 42. The difference spectrum shows clearly Fe II (42) lambda 5018 and lambda 5169 with equal intensity. There is a strong, broad (4000 km s -1) wing present under the [O III] lambda 5007 line, which varied only by a small amount. The authors show that this variation is entirely due to the variation of Fe II lambda 5018 and that the remainder of the [O III]-wing has not varied. On this basis the authors argue that the wing is not caused by the Fe II (42) lambda 5018 line, nor by any other Fe II lines, but that they are observing high velocity [O III]-emitting material. From the lack of variation in this component a minimum distance to the source can be set of 1.3 pc. Other features of the difference spectrum are also discussed
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| 6. |
- Sundelius, B., et al.
(författare)
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Tidal spiral arms in two-component galaxies - Density waves and swing amplification
- 1987
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 174:1-2, s. 67-77
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Tidskriftsartikel (refereegranskat)abstract
- The tidal spiral arms in galaxies are studied using the two-dimensional polar coordinate N-body program of Miller (1976, 1978) and a two-component disk galaxy perturbed by a point-mass model. The density wave theory, which explains spiral arms in the presence of differential rotation, and the components of the model and computer program are discussed. The spiral arms in the cold (spiral arm population) and hot (old stars) components and their velocity dispersions are examined, and consideration is given to density waves and swing amplification. The data reveal that a grand design spiral pattern can develop in the gaseous component of a disk galaxy as a result of tidal triggering from a companion, and the spiral pattern and kinematics of the particles correlate with predictions from the density wave theory.
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| 7. |
- Tang, G., et al.
(författare)
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Radio source structure from geodetic VLBI observations : evolution of the quasar 3C 345 at 8 GHz
- 1989
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Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 216:1-2, s. 31-38
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Tidskriftsartikel (refereegranskat)abstract
- Eight VLBI maps of 3C 345 were obtained at 8.3 GHz between 1980 and 1986. During this period, the two jet components C4 and C5 were shown to emerge from the core in succession and to move superluminally toward the west with angular velocities of 0.17 + or - 0.5 and 0.11 + or - 0.02 mas/yr, respectively. An emission detected between D and C5 after 1985.46 suggests that a new component emerged from the core, moving at an angular velocity of 0.2 mas/yr. The C3 component retained a velocity of about 0.44 mas/yr, and its proper motion was found to accelerate in the y-direction and decelerate in the x-direction.
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| 8. |
- Tang, Guoquiang, et al.
(författare)
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Radio source structure from geodetic VLBI observations : 8 GHz multi-epoch maps of the quasar 4C 39.25
- 1987
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Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 185:1-2, s. 87-93
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Tidskriftsartikel (refereegranskat)abstract
- Techniques for estimating and/or eliminating the effects of structure variations in the extragalactic radio sources used as fiducial points in 8.3-GHz geodetic VLBI observations are described and demonstrated. Particular attention is given to the data base statistics, the calibration of visibility data, the production of hybrid maps with AIPS, and the implementation of these steps in a software package. A set of X-band maps of 4C 39.25, obtained with the receivers of the Mark III dual-frequency VLBI network at six epochs during 1980-1985, is presented. Such maps are shown to be of value (1) for maintaining submarcsec accuracy in the extragalactic reference frame and (2) for qualitative astronomical monitoring of the sources. In 4C 39.25, for example, apparent superluminal motions, complex internal kinematics, and a double western component are observed.
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| 9. |
- Thomasson, Magnus, 1961, et al.
(författare)
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Formation of leading spiral arms in retrograde galaxy encounters
- 1989
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 211:1, s. 25-36
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Tidskriftsartikel (refereegranskat)abstract
- The formation of spiral structures in retrograde galaxy encounters was studied theoretically and with N-body simulations. A one-armed leading spiral dominates in a disk if the tidal perturbation from the retrograde companion is large enough, and the disk is surrounded by a massive halo. The leading arm is made up of particles in slightly elongated orbits, the turning points of which outline the arm. The orbits precess in such a way that the arm structure survives while it rotates in the opposite sense to the disk rotation. From the literature it is found that very few spirals in a sample of galaxies with a large companion have leading spiral arms. A possible reason for this is that very few spiral galaxies have a halo with larger mass than the disk mass.
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| 10. |
- van Groningen, Ernst, et al.
(författare)
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A search for electron-scattered wings in Halpha in Seyfert-1 galaxies
- 1989
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 211:2, s. 318-323323
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Tidskriftsartikel (refereegranskat)abstract
- Spectra were obtained of a dozen bright Seyfert-1 galaxies to search for the presence of ultra-broad wings in Halpha, caused by electron scattering in the hypothetical hot intercloud medium. The spectra, which covered the wavelength range from 5300 Aring to 8750 Aring, were carefully calibrated to obtain the accurate continuum shape. In no case were ultra-broad wings of the type expected from electron scattering found, although significantly broader wings are detected than in previous data. Synthesized spectra were used to estimate the sensitivity of the survey for electron scattered wings. The results can be converted into a lower limit on the temperature of 7times10 7 K, and an upper limit of 0.15 on the electron scattering optical depth
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| 11. |
- van Groningen, E., et al.
(författare)
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Broad emission line profiles in Seyfert-1 galaxies : [O III]-wings from a transition zone
- 1989
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 211:2, s. 293-309309
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Tidskriftsartikel (refereegranskat)abstract
- The authors demonstrate that broad wings are present under the strong [O III] lambda 5007 line in 10 out of 12 observed Seyfert-1 galaxies. The question is addressed whether the wing is due to broad [O III] emission or to some other ion. The alternatives considered are Fe II (42) lambda 5018, He I lambda 5016 and Si II lambdalambda 5041, 5056 or even Hbeta. The authors have searched for a counterpart of the wing in other forbidden and permitted lines. The observations yield a crude estimate of the density in the wing-emitting region. The authors derive a lower limit to the region's size. Because the [O III] emitting gas holds an intermediate position between the broad line region (BRL) and the narrow line region (NLR), both in density and distance, the authors denominate it the transition line region (TLR). They surmise that the outflowing part of the BLR provides the material of the TLR, before slowing down to NLR velocities
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| 12. |
- van Groningen, E.
(författare)
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Broad emission line profiles in Seyfert-1 galaxies. I. Evidence for a disk and a wind in Mkn 335
- 1987
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 186:1-2, s. 103-113113
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Tidskriftsartikel (refereegranskat)abstract
- High resolution spectroscopic observations of the Seyfert-1 galaxy Mkn 335 are presented. This galaxy is exceptional in the sense that its broad emission lines display a strong blue asymmetry. It is shown that the Halpha/Hbeta intensity ratio changes drastically with velocity shift. It is also found that the Hdelta/Hbeta ratio varies only by a small amount from line core to the wings, and is close to the theoretical case B recombination value. It is proposed that these Balmer line ratios are produced by a medium with very high densities and a temperature of about 10 4 K. These observations are interpreted by a model with two dynamically and spatially separate components: a disk which contains the very high density material and emits ~80% of the Balmer lines in a symmetric profile and an outflowing component of much lower density which emits the blue wing and most of Ly alpha and the higher excitation lines (such as C IV lambda1549)
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| 13. |
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