| 51. |
- Larsson, Josefin, et al.
(författare)
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A new constraint for gamma-ray burst progenitor mass
- 2007
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 376:3, s. 1285-1290
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Tidskriftsartikel (refereegranskat)abstract
- Recent comparative observations of long-duration gamma-ray bursts (LGRBs) and core collapse supernova (cc SN) host galaxies demonstrate that these two, highly energetic transient events are distributed very differently upon their hosts. LGRBs are much more concentrated on their host galaxy light than cc SN. Here we explore the suggestion that this differing distribution reflects different progenitor masses for LGRBs and cc SN. Using a simple model we show that, assuming cc SN arise from stars with main-sequence masses > 8 M-circle dot, GRBs are likely to arise from stars with initial masses > 20 M-circle dot. This difference can naturally be explained by the requirement that stars which create a LGRB must also create a black hole.
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| 52. |
- Levan, Andrew J., et al.
(författare)
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Neutron star binaries and long-duration gamma-ray bursts
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 372:3, s. 1351-1356
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Tidskriftsartikel (refereegranskat)abstract
- Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes (BHs) of stripped massive stars. Those with sufficient rotation form a centrifugally supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50 per cent of the observed systems. As these systems created a neutron star and not a BH, they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity.
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| 53. |
- Mackey, A D, et al.
(författare)
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Black holes and core expansion in massive star clusters
- 2008
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 386:1, s. 65-95
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Forskningsöversikt (refereegranskat)abstract
- In this study we present the results from realistic N-body modelling of massive star clusters in the Magellanic Clouds. We have computed eight simulations with N similar to 10(5) particles; six of these were evolved for at least a Hubble time. The aim of this modelling is to examine in detail the possibility of large-scale core expansion in massive star clusters, and search for a viable dynamical origin for the radius-age trend observed for such objects in the Magellanic Clouds. We identify two physical processes which can lead to significant and prolonged cluster core expansion - mass-loss due to rapid stellar evolution in a primordially mass-segregated cluster, and heating due to a retained population of stellar mass black holes, formed in the supernova explosions of the most massive cluster stars. These two processes operate over different time-scales and during different periods of a cluster's life. The former occurs only at early times and cannot drive core expansion for longer than a few hundred Myr, while the latter typically does not begin until several hundred Myr have passed, but can result in core expansion lasting for many Gyr. We investigate the behaviour of each of these expansion mechanisms under different circumstances - in clusters with varying degrees of primordial mass segregation, and in clusters with varying black hole retention fractions. In combination, the two processes can lead to a wide variety of evolutionary paths on the radius-age plane, which fully cover the observed cluster distribution and hence define a dynamical origin for the radius-age trend in the Magellanic Clouds. We discuss in some detail the implications of core expansion for various aspects of globular cluster research, as well as the possibility of observationally inferring the presence of a significant population of stellar mass black holes in a cluster.
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| 54. |
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| 55. |
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| 56. |
- Malcheva, G, et al.
(författare)
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Radiative lifetimes and transition probabilities of astrophysical interest in ZrII
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 367:2, s. 754-762
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Tidskriftsartikel (refereegranskat)abstract
- Radiative lifetimes of 16 odd levels belonging to the 4d(2)5p configuration of Zr II have been measured using a time-resolved laser-induced fluorescence technique with a single-step excitation process either from the ground state or from metastable levels belonging to the 4d(2)5s and 4d(3) configurations. For 12 levels, there were no previous results available. The new experimental results and the lifetime values available in the literature have allowed to test a theoretical relativistic Hartree-Fock (HFR) model including core-polarization effects and to deduce transition probabilities for 242 transitions of astrophysical interest in the range 187.8-535.0 nm.
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| 57. |
- Malcheva, G., et al.
(författare)
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Radiative parameters of Nb i excited states
- 2011
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 412:3, s. 1823-1827
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Tidskriftsartikel (refereegranskat)abstract
- Radiative lifetimes of 17 excited levels of Nb i, in the energy range 27 400-47 700 cm-1 (5p y 6D degrees(9/2), 5p x 6D degrees(7/2), 5p w 4G degrees(7/2,9/2,11/2), 5s5p v 4D degrees(1/2,3/2,5/2,7/2), 5s6p n 4D degrees(1/2,3/2,5/2,7/2), 5s6p o 4F degrees(3/2,5/2,7/2,9/2)), have been measured. For 15 of these levels, the lifetimes are obtained for the first time. The lifetimes were measured using the time-resolved laser-induced fluorescence technique and the experiments are complemented by a theoretical investigation using a relativistic Hartree-Fock method including core polarization. By combining the experimental lifetimes and the calculated branching fractions, we obtain transition probabilities for the individual de-excitation channels from the investigated levels.
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| 58. |
- Malmberg, Daniel, et al.
(författare)
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Close encounters in young stellar clusters: implications for planetary systems in the solar neighbourhood
- 2007
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 378:3, s. 1207-1216
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Tidskriftsartikel (refereegranskat)abstract
- The stars that populate the solar neighbourhood were formed in stellar clusters. Through N-body simulations of these clusters, we measure the rate of close encounters between stars. By monitoring the interaction histories of each star, we investigate the singleton fraction in the solar neighbourhood. A singleton is a star which formed as a single star, has never experienced any close encounters with other stars or binaries, or undergone an exchange encounter with a binary. We find that, of the stars which formed as single stars, a significant fraction is not singletons once the clusters have dispersed. If some of these stars had planetary systems, with properties similar to those of the Solar System, the planets' orbits may have been perturbed by the effects of close encounters with other stars or the effects of a companion star within a binary. Such perturbations can lead to strong planet-planet interactions which eject several planets, leaving the remaining planets on eccentric orbits. Some of the single stars exchange into binaries. Most of these binaries are broken up via subsequent interactions within the cluster, but some remain intact beyond the lifetime of the cluster. The properties of these binaries are similar to those of the observed binary systems containing extrasolar planets. Thus, dynamical processes in young stellar clusters will alter significantly any population of Solar System-like planetary systems. In addition, beginning with a population of planetary systems exactly resembling the Solar System around single stars, dynamical encounters in young stellar clusters may produce at least some of the extrasolar planetary systems observed in the solar neighbourhood.
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| 59. |
- Malmberg, Daniel, et al.
(författare)
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The effects of fly-bys on planetary systems
- 2011
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 411:2, s. 859-877
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Tidskriftsartikel (refereegranskat)abstract
- Most of the observed extrasolar planets are found on tight and often eccentric orbits. The high eccentricities are not easily explained by planet-formation models, which predict that planets should be on rather circular orbits. Here we explore whether fly-bys involving planetary systems with properties similar to those of the gas giants in the Solar system can produce planets with properties similar to the observed planets. Using numerical simulations, we show that fly-bys can cause the immediate ejection of planets, and sometimes also lead to the capture of one or more planets by the intruder. More common, however, is that fly-bys only perturb the orbits of planets, sometimes leaving the system in an unstable state. Over time-scales of a few million to several hundred million years after the fly-by, this perturbation can trigger planet-planet scatterings, leading to the ejection of one or more planets. For example, in the case of the four gas giants of the Solar system, the fraction of systems from which at least one planet is ejected more than doubles in 108 yr after the fly-by. The remaining planets are often left on more eccentric orbits, similar to the eccentricities of the observed extrasolar planets. We combine our results of how fly-bys affect Solar-system-like planetary systems, with the rate at which encounters in young stellar clusters occur. For example, we measure the effects of fly-bys on the four gas giants in the Solar system. We find, that for such systems, between 5 and 15 per cent suffer ejections of planets in 108 yr after fly-bys in typical open clusters. Thus, encounters in young stellar clusters can significantly alter the properties of any planets orbiting stars in clusters. As a large fraction of stars which populate the solar neighbourhood form in stellar clusters, encounters can significantly affect the properties of the observed extrasolar planets.
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| 60. |
- McMillan, P.~J., et al.
(författare)
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Disassembling the Galaxy with angle-action coordinates
- 2008
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 390, s. 429-437
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Tidskriftsartikel (refereegranskat)
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