| 1. |
- Aidas, Kestutis, et al.
(författare)
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The Dalton quantum chemistry program system
- 2014
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Ingår i: WIREs Computational Molecular Science. - : Wiley. - 1759-0876 .- 1759-0884. ; 4:3, s. 269-284
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Tidskriftsartikel (refereegranskat)abstract
- Dalton is a powerful general-purpose program system for the study of molecular electronic structure at the Hartree-Fock, Kohn-Sham, multiconfigurational self-consistent-field, MOller-Plesset, configuration-interaction, and coupled-cluster levels of theory. Apart from the total energy, a wide variety of molecular properties may be calculated using these electronic-structure models. Molecular gradients and Hessians are available for geometry optimizations, molecular dynamics, and vibrational studies, whereas magnetic resonance and optical activity can be studied in a gauge-origin-invariant manner. Frequency-dependent molecular properties can be calculated using linear, quadratic, and cubic response theory. A large number of singlet and triplet perturbation operators are available for the study of one-, two-, and three-photon processes. Environmental effects may be included using various dielectric-medium and quantum-mechanics/molecular-mechanics models. Large molecules may be studied using linear-scaling and massively parallel algorithms. Dalton is distributed at no cost from for a number of UNIX platforms.
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| 2. |
- Davies, Melvyn B, et al.
(författare)
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Blue straggler production in globular clusters
- 2004
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 349:1, s. 129-134
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Tidskriftsartikel (refereegranskat)abstract
- Recent Hubble Space Telescope observations of a large sample of globular clusters reveal that every cluster contains between 40 and 400 blue stragglers.The population does not correlate with either stellar collision rate (as would be expected if all blue stragglers were formed via collisions) or total mass (as would be expected if all blue stragglers were formed via the unhindered evolution of a subset of the stellar population). In this paper, we support the idea that blue stragglers are made through both channels. The number produced via collisions tends to increase with cluster mass. In this paper we show how the current population produced from primordial binaries decreases with increasing cluster mass;exchange encounters with third, single stars in the most massivec lusters tend to reduce the fraction of binaries containing a primary close to the current turn-off mass. Rather, their primaries tend to be somewhat more massive (~1-3 Msolar) and have evolved off the main sequence, filling their Roche lobes in the past, often converting their secondaries into blue stragglers (but more than 1 Gyr or so ago and thus they are no longer visible as blue stragglers). We show that this decline in the primordial blue straggler population is likely to be offset by the increase in the number of blue stragglers produced via collisions. The predicted total blue straggler population is therefore relatively independent of cluster mass, thus matching the observed population. This result does not depend on any particular assumed blue straggler lifetime.
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| 3. |
- 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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| 4. |
- Malmberg, Daniel, et al.
(författare)
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The Singleton Fraction of Stars Formed in Stellar Clusters
- 2008
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Ingår i: Extreme Solar Systems (Astronomical Society of the Pacific Conference Series). - 9781583816660 - 9781583816677 ; 398, s. 75-76
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Konferensbidrag (refereegranskat)abstract
- Most stars form in some sort of stellar cluster or association. In such clusters the number density of stars can be very high. Thus, single stars in such clusters may undergo frequent close encounters with other stars and exchange encounters with binary systems. The perturbation caused by the other star in a close encounter or by the companion star in a binary can have significant effects on the evolution of any planetary system around the initially single star. If the planetary system which formed around the single star was originally solar-system-like, planet-planet interactions, induced by the perturbation from other stars, may change it significantly and leave it more like some of the planetary systems which are observed around other stars. Only if the host star of an initially solar-system-like planetary system never undergoes any encounters with other stars and is never exchanged into a binary may the planetary system remain solar-system like. We define such a star to be a SINGLETON.
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| 5. |
- Piotto, Giampaolo, et al.
(författare)
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Relative Frequencies of Blue Stragglers in Galactic Globular Clusters: Constraints for the Formation Mechanisms
- 2004
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 604:2, s. 109-112
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the main properties of the Galactic globular cluster (GC)blue straggler stars (BSSs), as inferred from our new catalog containingnearly 3000 BSSs. The catalog has been extracted from thephotometrically homogeneous V versus (B-V) color-magnitude diagrams(CMDs) of 56 GCs, based on Wide Field Planetary Camera 2 images of theircentral cores. In our analysis, we used consistent relative distancesbased on the same photometry and calibration. The number of BSSs hasbeen normalized to obtain relative frequencies (FBSS) andspecific densities (NS) using different stellar populationsextracted from the CMD. The cluster FBSS is significantlysmaller than the relative frequency of field BSSs. We find a significantanticorrelation between the BSS relative frequency in a cluster and itstotal absolute luminosity (mass). There is no statistically significanttrend between the BSS frequency and the expected collision rate. Thevalue of FBSS does not depend on other cluster parameters,apart from a mild dependence on the central density. Post-core-collapseclusters act like normal clusters as far as the BSS frequency isconcerned. We also show that the BSS luminosity function for the mostluminous clusters is significantly different, with a brighter peak andextending to brighter luminosities than in the less luminous clusters.These results imply that the efficiency of BSS production mechanisms andtheir relative importance vary with the cluster mass.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
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