| 1. |
- Carlqvist, Per
(författare)
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A new mechanism for the crab fingers
- 2004
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Ingår i: Astrophysics and Space Science. - 0004-640X .- 1572-946X. ; 289:1-2, s. 47-53
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Tidskriftsartikel (refereegranskat)abstract
- Images of the outer filamentary parts of the Crab Nebula, earlier obtained with the Hubble Space Telescope WFPC2, are reconsidered. On inspection of the images it is found that most of the mainly radially oriented fingers possess an internal structure, usually in the form of two or more filaments. For some of the fingers there are clear signs that the filaments are twisted around each other. A mechanism for the fingers taking the internal structure into account is proposed. The mechanism is based on the reshaping of magnetized filaments under the influence of inertia and magnetic forces. When interacting with an expanding shell, driven by the pressure of the synchrotron nebula, part of such a filament may develop into a double helix pointing radially inwards. For this transformation to take place it is required that the magnetic field within the filament is sufficiently twisted and that the filament contains a mass condensation. The finger is identified with the double helix. Similar structures of double helices are found to be present also in other environments.
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| 2. |
- Carlqvist, Per, et al.
(författare)
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Formation of Twisted Elephant Trunks in the Rosette Nebula
- 2002
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Ingår i: Astrophysics and Space Science. - 0004-640X .- 1572-946X. ; 280:4, s. 405-412
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Tidskriftsartikel (refereegranskat)abstract
- New observations show that dark elephant trunks in the Rosette nebula are often built up by thin filaments. In several of the trunks the filaments seem to form a twisted pattern. This pattern is hard to reconcile with current theory. We propose a new model for the formation of twisted elephant trunks in which electromagnetic forces play an important role. The model considers the behaviour of a twisted magnetic filament in a molecular cloud, where a cluster of hot stars has been recently born. As a result of stellar winds, and radiation pressure, electromagnetic forces, and inertia forces part of the filament can develop into a double helix pointing towards the stars. The double helix represents the twisted elephant trunk. A simple analogy experiment visualizes and supports the trunk model.
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| 3. |
- Wang, De Yu, et al.
(författare)
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On the envelope soliton below the H+ gyrofrequency in low auroral region
- 2002
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Ingår i: Astrophysics and Space Science. - 0004-640X .- 1572-946X. ; 282:2, s. 421-432
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Tidskriftsartikel (refereegranskat)abstract
- Two envelope soliton events below the H+ gyrofrequency with localized density depletionwere discovered in low auroral region (∼ 1760 km) by Freja satellite. These events werecorrelated in time with the observations of the ratio of oxygen ion density to hydrogen ion densitysharp increase and the electrons energization. These envelope solitons have a characteristic frequencyat ∼ 180–190 Hz, which are obviously different from the electron-ion lower hybrid wave frequencyand the helium ion gyrofrequency in low auroral plasma, but it is close to the resonance frequency ofhydrogen ion-oxygen ion hybrid wave. A modulational instability model of an ion-ion hybrid wavehas been discussed here. It is found that the envelope soliton below the H+ gyrofrequency in lowauroral region may be generated by this modulational instability on condition that the local oxygenion density is larger than the local hydrogen ion density.
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| 4. |
- Fabricius, C, et al.
(författare)
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The spectro point spread function for GAIA
- 2002
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Ingår i: Astrophysics and Space Science. - 0004-640X. ; 280:1-2, s. 51-56
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the point spread function for the Spectro instrument based on realistic effects of the time delayed integration, transverse image motion and pixelization, but assuming rather small wave front errors. Stars brighter than 10 mag, which will saturate the CCD, may be observed photometrically using the image wings, but the central wavelengths will be slightly shifted towards the red.
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| 5. |
- Holmberg, Johan, et al.
(författare)
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Local galactic evolution
- 2003
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Ingår i: Astrophysics and Space Science. - 0004-640X. ; 284:2, s. 685-688
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of the Solar neighbourhood is followed using a unique,magnitude complete and kinematically unbiased sample of 14,000 F, G, andK dwarfs. Metallicity, age, space motion and galactic orbits have beendetermined for all stars. The result is a detailed view of the complexevolution of the local Milky Way, which must be matched by any model forthe chemical and dynamical evolution of the Galactic disk. E.g., suchmodels must explain the shape and large scatter in the age-metallityrelation as well as the overall metallicity distribution; the evolutionof stellar kinematics with age; the distribution of stars in velocityspace; and the contributions from the thick and thin disks to all theserelations.
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| 6. |
- Myasnikov, AV, et al.
(författare)
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Gasdynamics of detached shells around carbon stars with variable mass loss
- 2000
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Ingår i: ASTROPHYSICS AND SPACE SCIENCE. - : KLUWER ACADEMIC PUBL. - 0004-640X. ; 274:1-2, s. 231-241
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Tidskriftsartikel (refereegranskat)abstract
- Gasdynamic features of detached shells around carbon stars with variable mass loss rate are investigated in detail numerically. It is shown that a shell is unstable and also, 2D perturbations are less developed that 3D ones. The structure of perturbed flo
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| 7. |
- Nordström, Birgitta
(författare)
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Chemical enrichment in the early galaxy
- 2003
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Ingår i: Astrophysics and Space Science. - 0004-640X. ; 284:2, s. 779-782
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Tidskriftsartikel (refereegranskat)abstract
- New, precise abundance data for a large number of elements in a growingsample of extremely metal-poor stars are accumulating from the new 8-mtelescopes. Combined with theoretical models, these results advance ourunderstanding of the first generations of stars, whose nucleosynthesisproducts are fossilised in the oldest stars we see today and thus giveclues to the earliest phases of evolution in the Galaxy. In particular,the heaviest elements give us insight into the different neutron capturemechanisms and the stellar sites where such elements could be produced.They also afford an independent way to determine the age of the Galaxy,by radioactive chronology.
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| 8. |
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| 9. |
- Reimers, C, et al.
(författare)
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Shaping of planetary nebulae by dust driven AGB winds
- 2000
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Ingår i: ASTROPHYSICS AND SPACE SCIENCE. - : KLUWER ACADEMIC PUBL. - 0004-640X. ; 272:1-3, s. 205-212
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Tidskriftsartikel (refereegranskat)abstract
- We have investigated the interacting winds model (IWM) in which the shapes of elliptical Planetary Nebulae (PNe) can be explained by the asymmetric mass loss produced by a rotating AGB star. The mass loss mechanism is based on a dust driven wind calculate
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