Stellar Population Analysis of Galaxies based on Genetic Algorithms     

Abdel-Fattah Attia  H. A. Ismail  I. M. Selim  A. M. Osman  I. A. Isaa  M. A. Marie  A. A. Shaker 《中国天文和天体物理学报》2005,5(4):347-355

We present a new method for determining the age and relative contribution of different stellar populations in galaxies based on the genetic algorithm. We apply this method to the barred spiral galaxy NGC 3384, using CCD images in U, B, V, R and I bands. This analysis indicates that the galaxy NGC 3384 is mainly inhabited by old stellar population (age >109yr). Some problems were encountered when numerical simulations are used for determining the contribution of different stellar populations in the integrated color of a galaxy. The results show that the proposed genetic algorithm can search efficiently through the very large space of the possible ages.  相似文献   

White dwarf mass distribution in the SDSS     

S. O. Kepler  S. J. Kleinman  A. Nitta  D. Koester  B. G. Castanheira  O. Giovannini  A. F. M. Costa  L. Althaus 《Monthly notices of the Royal Astronomical Society》2007,375(4):1315-1324

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Models of rapidly rotating neutron stars: remnants of accretion-induced collapse     

Yuk Tung Liu  Lee Lindblom 《Monthly notices of the Royal Astronomical Society》2001,324(4):1063-1073

Equilibrium models of differentially rotating nascent neutron stars are constructed, which represent the result of the accretion-induced collapse of rapidly rotating white dwarfs. The models are built in a two-step procedure: (1) a rapidly rotating pre-collapse white dwarf model is constructed; (2) a stationary axisymmetric neutron star having the same total mass and angular momentum distribution as the white dwarf is constructed. The resulting collapsed objects consist of a high-density central core of size roughly 20 km, surrounded by a massive accretion torus extending over 1000 km from the rotation axis. The ratio of the rotational kinetic energy to the gravitational potential energy of these neutron stars ranges from 0.13 to 0.26, suggesting that some of these objects may have a non-axisymmetric dynamical instability that could emit a significant amount of gravitational radiation.  相似文献   

Mid-infrared polarisation and inferred magnetic field direction toward YSOs with outflow     

Christopher M. Wright 《Astrophysics and Space Science》2007,311(1-3):47-55

The position angle of mid-infrared polarisation can be directly related to the magnetic field direction projected onto the plane of the sky. Such observations, from both ground and space-based platforms, have been used to investigate the relation between the magnetic field and other “symmetry axes” associated with embedded young stars, such as the interstellar magnetic field, bipolar outflow and disk/toroid axes. Interpretation of the results in terms of hydromagnetic driving mechanisms of bipolar outflow is discussed.  相似文献   

Regular and chaotic motion in softened gravitational systems     

Amr A.El-Zant† 《Monthly notices of the Royal Astronomical Society》2002,331(1):23-39

The stability of the dynamical trajectories of softened spherical gravitational systems is examined, both in the case of the full N -body problem and that of trajectories moving in the gravitational field of non-interacting background particles. In the latter case, for   N 10 000  , some trajectories, even if unstable, had exceedingly long diffusion times, which correlated with the characteristic e-folding time-scale of the instability. For trajectories of   N ≈100 000  systems this time-scale could be arbitrarily large – and thus appear to correspond to regular orbits. For centrally concentrated systems, low angular momentum trajectories were found to be systematically more unstable. This phenomenon is analogous to the well-known case of trajectories in generic centrally concentrated non-spherical smooth systems, where eccentric trajectories are found to be chaotic. The exponentiation times also correlate with the conservation of the angular momenta along the trajectories. For N up to a few hundred, the instability time-scales of N -body systems and their variation with particle number are similar to those of the most chaotic trajectories in inhomogeneous non-interacting systems. For larger N (up to a few thousand) the values of the these time-scales were found to saturate, increasing significantly more slowly with N . We attribute this to collective effects in the fully self-gravitating problem, which are apparent in the time variations of the time-dependent Liapunov exponents. The results presented here go some way towards resolving the long-standing apparent paradoxes concerning the local instability of trajectories. This now appears to be a manifestation of mechanisms driving evolution in gravitational systems and their interactions – and may thus be a useful diagnostic of such processes.  相似文献   

Tidal capture of stars by intermediate-mass black holes     

H. Baumgardt  C. Hopman  S. Portegies Zwart  J. Makino 《Monthly notices of the Royal Astronomical Society》2006,372(1):467-478

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Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories     

P. A. James  M. Salaris  J. I. Davies  S. Phillipps  S. Cassisi 《Monthly notices of the Royal Astronomical Society》2006,367(1):339-348

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Investigation of the ELAIS field by Vega photometry: absolute magnitude-dependent Galactic model parameters     

S. Bilir  S. Karaali  G. Gilmore 《Monthly notices of the Royal Astronomical Society》2006,366(4):1295-1309

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Formation and evolution of compact binaries in globular clusters – I. Binaries with white dwarfs     

N. Ivanova  C. O. Heinke †  F. A. Rasio  R. E. Taam  K. Belczynski ‡  J. Fregeau 《Monthly notices of the Royal Astronomical Society》2006,372(3):1043-1059

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The origin of the initial mass function and its dependence on the mean Jeans mass in molecular clouds     

Matthew R. Bate  Ian A. Bonnell 《Monthly notices of the Royal Astronomical Society》2005,356(4):1201-1221

We investigate the dependence of stellar properties on the mean thermal Jeans mass in molecular clouds. We compare the results from the two largest hydrodynamical simulations of star formation to resolve the fragmentation process down to the opacity limit, the first of which was reported by Bate, Bonnell & Bromm. The initial conditions of the two calculations are identical except for the radii of the clouds, which are chosen so that the mean densities and mean thermal Jeans masses of the clouds differ by factors of 9 and 3, respectively.
We find that the denser cloud, with the lower mean thermal Jeans mass, produces a higher proportion of brown dwarfs and has a lower characteristic (median) mass of the stars and brown dwarfs. This dependence of the initial mass function (IMF) on the density of the cloud may explain the observation that the Taurus star-forming region appears to be deficient in brown dwarfs when compared with the Orion Trapezium cluster. The new calculation also produces wide binaries (separations >20 au), one of which is a wide binary brown dwarf system.
Based on the hydrodynamical calculations, we develop a simple accretion/ejection model for the origin of the IMF. In the model, all stars and brown dwarfs begin with the same mass (set by the opacity limit for fragmentation) and grow in mass until their accretion is terminated stochastically by their ejection from the cloud through dynamically interactions. The model predicts that the main variation of the IMF in different star-forming environments should be in the location of the peak (due to variations in the mean thermal Jeans mass of the cloud) and in the substellar regime. However, the slope of the IMF at high masses may depend on the dispersion in the accretion rates of protostars.  相似文献