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151.
Daniel Malmberg Francesca De Angeli Melvyn B. Davies Ross P. Church Dougal Mackey Mark I. Wilkinson 《Monthly notices of the Royal Astronomical Society》2007,378(3):1207-1216
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. 相似文献
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Paul A. Russell Trevor J. Ponman Alastair J. R. Sanderson 《Monthly notices of the Royal Astronomical Society》2007,378(4):1217-1228
We present an X-ray analysis of the radio-quiet cool-core galaxy group NGC 4325 ( z = 0.026) based on Chandra and ROSAT observations. The Chandra data were analysed using xspec deprojection, 2D spectral mapping and forward-fitting with parametric models. Additionally, a Markov Chain Monte Carlo method was used to perform a joint Bayesian analysis of the Chandra and ROSAT data. The results of the various analysis methods are compared, particularly those obtained by forward-fitting and deprojection. The spectral mapping reveals the presence of cool gas displaced up to 10 kpc from the group centre. The Chandra X-ray surface brightness shows the group core to be highly disturbed, and indicates the presence of two small X-ray cavities within 15 kpc of the group core. The xspec deprojection analysis shows that the group has a particularly steep entropy profile, suggesting that an active galactic nucleus (AGN) outburst may be about to occur. With the evidence of prior AGN activity, but with no radio emission currently observed, we suggest that the group in a pre-outburst state, with the cavities and displaced gas providing evidence of a previous, weak AGN outburst. 相似文献
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S. L. Casewell P. D. Dobbie S. T. Hodgkin E. Moraux R. F. Jameson N. C. Hambly J. Irwin N. Lodieu 《Monthly notices of the Royal Astronomical Society》2007,378(3):1131-1140
We present the results of a deep optical–near-infrared (optical–NIR) multi-epoch survey covering 2.5 deg2 of the Pleiades open star cluster to search for new very low-mass brown dwarf members. A significant (∼5 yr) epoch difference exists between the optical (CFH12k I -, Z -band) and NIR (UKIRT WFCAM J -band) observations. We construct I , I − Z and Z , Z − J colour–magnitude diagrams to select candidate cluster members. Proper motions are computed for all candidate members and compared to the background field objects to further refine the sample. We recover all known cluster members within the area of our survey. In addition, we have discovered nine new candidate brown dwarf cluster members. The seven faintest candidates have red Z − J colours and show blue NIR colours. These are consistent with being L- and T-type Pleiads. Theoretical models predict their masses to be around 11 M Jup . 相似文献
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In many magnetized, dilute astrophysical plasmas, thermal conduction occurs almost exclusively parallel to magnetic field
lines. In this case, the usual stability criterion for convective stability, the Schwarzschild criterion, which depends on
entropy gradients, is modified. In the magnetized long mean free path regime, instability occurs for small wavenumbers when
(∂ P/∂z) (∂ ln T/∂ z) > 0, which we refer to as the Balbus criterion. We refer to the convective-type instability that results
as the magnetothermal instability (MTI). We use the equations of MHD with anisotropic electron heat conduction to numerically
simulate the linear growth and nonlinear saturation of the MTI in plane-parallel atmospheres that are unstable according to
the Balbus criterion. The linear growth rates measured from the simulations are in excellent agreement with the weak field
dispersion relation. The addition of isotropic conduction, e.g. radiation, or strong magnetic fields can damp the growth of
the MTI and affect the nonlinear regime. The instability saturates when the atmosphere becomes isothermal as the source of
free energy is exhausted. By maintaining a fixed temperature difference between the top and bottom boundaries of the simulation
domain, sustained convective turbulence can be driven. MTI-stable layers introduced by isotropic conduction are used to prevent
the formation of unresolved, thermal boundary layers. We find that the largest component of the time-averaged heat flux is
due to advective motions as opposed to the actual thermal conduction itself. Finally, we explore the implications of this
instability for a variety of astrophysical systems, such as neutron stars, the hot intracluster medium of galaxy clusters,
and the structure of radiatively inefficient accretion flows.
J. M. Stone: Program in Applied and Computational Mathematics, Princeton University, Princeton, NJ 08544 相似文献
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