The intrinsic shapes of galaxy groups     

V.V. Orlov  A.V. Petrova  V.G. Tarantaev 《Monthly notices of the Royal Astronomical Society》2001,325(1):133-137

The intrinsic, three-dimensional shapes of small galaxy groups, containing between three and eight members, are evaluated using three different statistics: (i) the mean sum of square sines of angles in all possible triangles formed by members of the group; (ii) the variance of square paired separations in the group; (iii) the axial ratio of a rectangle containing the group. The mean values of these parameters and their rms deviations are calculated for observed galaxy groups and simulated groups with members that are distributed randomly within prolate or oblate spheroids. Comparison of observational data and simulations shows that the observed galaxy groups have shapes consistent with the projected shapes of prolate or oblate spheroids with axial ratios of 3:1, regardless of their multiplicity, but inconsistent with the projected shapes of spherical objects.  相似文献   

Automatic Determination of Stellar Atmospheric Parameters Using Neural Networks and Instance-Based Machine Learning     

Olac Fuentes 《Experimental Astronomy》2001,12(1):21-31

In this article we show how machine learning methods can beeffectively applied to the problem of automatically predictingstellar atmospheric parameters from spectral information, a veryimportant problem in stellar astronomy. We apply feedforwardneural networks, Kohonen's self-organizing maps andlocally-weighted regression to predict the stellar atmosphericparameters effective temperature, surface gravity and metallicityfrom spectral indices. Our experimental results show that thethree methods are capable of predicting the parameters with verygood accuracy. Locally weighted regression gives slightly betterresults than the other methods using the original dataset asinput, while self-organizing maps outperform the other methods when significant amounts of noise are added. We also implemented a heterogeneous ensemble of predictors, combining the results given by the three algorithms. This ensemble yields better results than any of the three algorithms alone, using both the original and the noisy data.  相似文献   

Surface Photometry of Comet Hale-Bopp Carbon Images     

Sizonenko  Yu. V. 《Earth, Moon, and Planets》1997,78(1-3):137-142

A very long series of photographic observations of the comet Hale-Bopp has been made during January–April 1997 at the double astrograph (400/2000) of the Main Astronomical Observatory (Kyiv, Ukraine). Some of the cometary photos were obtained with two wide-band filter combinations. One of these combinations isolates C₂ emission, another — the nearby dust continuum. The images were digitized by means of AMDPH-XY machine and then calibrated following the standard procedure. After subtraction of the dust continuum the distribution of surface brightness in the C₂ emission coma of comet Hale-Bopp was studied. We found an asymmetric brightness distribution both pre- and post-perihelion. On 21.77 April 1997 a secondary brightness peak is found at the distance of 1.03 × 10⁵ km from the nucleus. It is possible that this peak is related to the extended source of the C₂ molecules. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Observational Selection of Comet Discoveries     

Kalinicheva  O. V. 《Solar System Research》2002,36(3):260-266

The distribution of perihelion distances of nearly periodic comets was studied using different assumptions about comet discovery probabilities. It was shown that the concentration of perihelia at small heliocentric distances cannot be explained solely by the influence of visibility conditions.  相似文献   

The influence of the 0-0 edge distances on the Fe2+ → Fe3+ charge transfer bands energies in amphiboles     

V. M. Khomenko  A. N. Platonov 《Physics and Chemistry of Minerals》1996,23(4-5):243-243

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Radar observations of the Geminid meteoroid stream     

G. Cevolani  G. Bortolotti  L. Foschini  C. Franceschi  G. Grassi  G. Trivellone  A. Hajduk  V. Porubčan 《Earth, Moon, and Planets》1995,68(1-3):247-255

Continuous radio-wave monitoring of the Geminid activity in December 1992 and 1993 by using a forward scatter (FS) bistatic radar over the Bologna-Lecce baseline (700 km) in Italy, reveals peculiar structural aspects of the stream in terms of signal amplitude-rate and duration-rate dependence. The observational results of the Geminid display obtained in the two consecutive years with differentiated peak levels of transmitted power, exhibit different time distributions of underdense meteors against the signal received power. Both sets of the data relative to the peak activity in December 12–14, show reflection properties of Geminids which are atypical if compared with echoes from cometary-type showers, with really high echo counts at mid-upper levels of the peak received power. An assymmetric curve of activity of the Geminids complex is evidenced, with the peak flux of smaller particles occurring earlier than that of larger ones.  相似文献   

Interactions and the Evolution of Cluster Galaxies     

R. Vílchez-Gómex  G. A. Bruzual  R. Pelló  R. Domínguez 《Astrophysics and Space Science》2001,276(2-4):757-764

We have developed model predictions for the morphological distribution of cluster galaxies as a function of the cluster-centric distance and the local galaxy density, using a semi-analytical code. This code allows us to obtain magnitudes and colours for cluster galaxies at different redshifts, and thus to study in detail the evolution of the colour–magnitude relation of specific distant clusters. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

A comparison of theoreticalCIV emission line strengths with active region observations obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)     

F. P. Keenan  R. J. Thomas  W. M. Neupert  E. S. Conlon  V. M. Burke 《Solar physics》1993,144(1):69-74

Theoretical line ratios involving 2s ² S - 3p ² P, 2p ² P - 3s ² S, and 2p ² S - 3d ² D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed.  相似文献   

Accretion disk spectra of super-luminal jet sources and ultra-luminous compact X-ray sources in nearby spiral galaxies     

Ken Ebisawa  Aya Kubota  Tsunefumi Mizuno  Piotr Życki 《Astrophysics and Space Science》2001,276(1):11-14

The Ultra-luminous Compact X-ray Sources (ULXs)in nearby spiral galaxies and the Galactic super-luminaljet sources sharethe common spectral characteristic that they haveextremely high disk temperatures which cannot be explainedin the framework of the standard accretion disk modelin the Schwarzschild metric. We have calculated an extreme Kerr disk model to examine if the Kerr disk model can instead explain the observed `too hot' accretion disk spectra.We found that the Kerr disk spectrum becomes significantly hardercompared to the Schwarzschild disk only when the disk is highlyinclined.For super-luminal jet sources, which are known to beinclined systems, the Kerr disk model may thuswork if we choose proper values for the black hole angular momentum. For the ULXs, however, the Kerr disk interpretation will be problematic,as is is highly unlikely that their accretion disks are preferentiallyinclined.  相似文献   

The mass of the white dwarf in the recurrent nova U Scorpii     

T.D. Thoroughgood  V.S. Dhillon  S.P. Littlefair  T.R. Marsh  D.A. Smith 《Monthly notices of the Royal Astronomical Society》2001,327(4):1323-1333

We present spectroscopy of the eclipsing recurrent nova U Sco. The radial velocity semi-amplitude of the primary star was found to be     from the motion of the wings of the He  ii λ 4686-Å emission line. By detecting weak absorption features from the secondary star, we find its radial velocity semi-amplitude to be     . From these parameters, we obtain a mass of     for the white dwarf primary star and a mass of     for the secondary star. The radius of the secondary is calculated to be     , confirming that it is evolved. The inclination of the system is calculated to be     , consistent with the deep eclipse seen in the light-curves. The helium emission lines are double-peaked, with the blueshifted regions of the disc being eclipsed prior to the redshifted regions, clearly indicating the presence of an accretion disc. The high mass of the white dwarf is consistent with the thermonuclear runaway model of recurrent nova outbursts, and confirms that U Sco is the best Type Ia supernova progenitor currently known. We predict that U Sco is likely to explode within ∼700 000 yr.  相似文献