| 11. |
- 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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| 12. |
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| 13. |
- van Groningen, E.
(författare)
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Accretion disks in Seyfert nuclei : broad line profiles and asymmetries
- 1983
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 126:2, s. 363-371371
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Tidskriftsartikel (refereegranskat)abstract
- Keplerian rotation around a massive object in the centre of a Seyfert galaxy is considered as the main broadening mechanism for the broad emission lines. The resultant line profiles are shown to be of power-law form in the wings and are compared with recently published high quality observations. Several possible solutions are given for the [O III]-problem raised by Shields (1978). Asymmetries in the line profiles are explained by anisotropically radiating filaments in two oppositely directed jets emanating from the centre of the disk and by radial motions in or above the disk
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| 14. |
- Alfvén, Hannes
(författare)
-
Memoirs of a dissident scientist
- 1988
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Ingår i: American Scientist. - : Sigma Xi. - 0003-0996 .- 1545-2786. ; 76:3, s. 249-251
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Tidskriftsartikel (refereegranskat)
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| 15. |
- 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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| 16. |
- 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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| 17. |
- van Groningen, E.
(författare)
-
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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| 18. |
- van Genderen, A. M., et al.
(författare)
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A discussion on new VBLUW observations of the X-ray binary Sk. 160=SMC X-1
- 1981
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 101:1, s. 101-104104
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Tidskriftsartikel (refereegranskat)abstract
- VBLUW photometry (Walraven system) of Sk. 160=SMC X-1 made in 1973, 1975, and 1977 are discussed. They are compared with those of van Paradijs (1977) made in 1976. Using all available photometric and X-ray mid-eclipse data, a period is determined by a weighted least-squares solution: P=3.89235 dplusmn0.00002 d m.e
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| 19. |
- 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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| 20. |
- 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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