Theoretical and experimental lifetime and oscillator strength determination in singly ionized neodymium (Nd ii)     

H. L. Xu  S. Svanberg  R. D. Cowan  P.-H. Lefèbvre  P. Quinet  E. Biémont 《Monthly notices of the Royal Astronomical Society》2003,346(2):433-440

Radiative lifetime measurements were performed with time-resolved laser-induced fluorescence techniques for 24 levels of Nd  ii in the energy range 20 500–32 500 cm⁻¹. For 17 levels, no previous experimental data exist. These results have allowed the testing of new theoretical calculations with the relativistic Hartree–Fock method taking configuration interactions and core-polarization effects into account, and a satisfying agreement has been found for this complex ion. A new set of calculated oscillator strengths, accurate within a few per cent for the strongest transitions, is presented for 107 lines of astrophysical interest appearing in the wavelength range 358.0–1100.0 nm. These results will be useful to evaluate abundance values of neodymium in chemically peculiar stars in relation with cosmochronology.  相似文献   

Probing the core and envelope structure of DBV white dwarfs     

T. S. Metcalfe  M. H. Montgomery  S. D. Kawaler 《Monthly notices of the Royal Astronomical Society》2003,344(4):L88-L92

  相似文献   

X-ray and ultraviolet observations of the dwarf nova VW Hyi in quiescence     

Dirk Pandel  France A. Córdova  Steve B. Howell 《Monthly notices of the Royal Astronomical Society》2003,346(4):1231-1241

We present an analysis of X-ray and ultraviolet (UV) data of the dwarf nova VW Hyi that were obtained with XMM–Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles on to the white dwarf. The plasma has a continuous temperature distribution that is well described by a power law or a cooling flow model with a maximum temperature of 6–8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of  8 × 10³⁰ erg s^-1  , which is only 20 per cent of the disc luminosity. The rate of accretion on to the white dwarf is  5 × 10⁻¹² M_⊙ yr⁻¹  , about half of the rate in the disc. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km s⁻¹, which is close to the rotation velocity of the white dwarf but is significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux, which is likely to be due to the rotation of the boundary layer. The X-ray and the UV flux show strong variability on a time-scale of ∼1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ∼100 s. The correlation indicates that the variable UV flux is emitted near the transition region between the disc and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ∼100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disc is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.  相似文献   

Tidal dwarfs in the M81 group: the second generation?     

L. Makarova  E. Grebel  I. Karachentsev  A. Dolphin  V. Karachentseva  M. Sharina  D. Geisler  P. Guhathakurta  P. Hodge  A. Sarajedini  P. Seitzer 《Astrophysics and Space Science》2003,285(1):107-111

We present a quantitative star formation history derivation of the four suspected tidal dwarf galaxies in the M 81 group: Holmberg IX, BK3N,Arp-loop (A0952+69) and Garland using HST/WFPC2 images of these galaxies. We construct a library of synthetic Colour-Magnitude Diagrams(CMDs) based on theoretical isochrones and data-derived determinations of photometric errors. These synthetic CMDs were combined linearly andχ²-compared to observed photometry. All the galaxies show continuous star formation between about 20 and 200 Myr ago with star formation rates between 7.5⋅10^-3 M_⊙/yr and 7.67⋅10^-4 M_⊙/yr. The metallicity of the detected stars is spanning rather a wide range, being lower than solar abundance. We suppose, that all the galaxies were formed out of material from metal-poor outer part of the giant spiral galaxy M81after tidal interaction about 200 Myr ago. However, this suggestion requires significantly more deep color-magnitude diagrams to be sure with the scenario of the galaxy evolution. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The radio nebula surrounding the Vela pulsar     

R. Dodson †  D. Lewis  D. McConnell  A. A. Deshpande 《Monthly notices of the Royal Astronomical Society》2003,343(1):116-124

  相似文献   

Simulation of stellar instabilities with vastly different time-scales using domain decomposition     

M. Grott  S. Chernigovski  W. Glatzel 《Monthly notices of the Royal Astronomical Society》2003,344(4):1119-1130

  相似文献   

A Type IV Radio Burst Associated with Type III Burst,Pulsation and Spike Emissions     

Wang  M.  Xie  R.X.  Duan  C.C.  Yan  Y.H.  Sych  R.A.  Altyntsev  A.T. 《Solar physics》2003,212(2):407-424

A type IV radio burst accompanied by several normal- and reverse-drifting type III bursts, multiple long-term quasi-periodic pulsations and spikes was observed with the radio spectrometers (1.0–2.0 and 2.6–3.8 GHz) at National Astronomical Observatories of China (NAOC) on 23 September 1998. In combination with the images of Siberian Solar Radio Telescope (SSRT) of Russia, the complex and multiple magnetic structures inferred from the radio bursts reveal the existence of both large-scale and small-scale magnetic structures. This event suggests that the geometries of coronal magnetic fields contain multiple discrete electron acceleration/injection sites at different heights, and extended open and closed magnetic field lines. It can be shown that the energetic electrons gain access to open, diverging and closed field lines thus producing different types of radio bursts. From the characteristics of position, polarization, dispersion and displacement of the sources, the model of the type IV event is supported, which involves synchrotron emission from the electrons confined by the rapid scattering through the interaction of hydromagnetic wave with particles.  相似文献   

Optical colors of 56 near-Earth objects: trends with size and orbit     

C.L Dandy  A Fitzsimmons 《Icarus》2003,163(2):363-373

We present the results of BVRIZ photometry of 56 near-Earth objects (NEOs) obtained with the 1-m Jacobus Kapteyn telescope on La Palma during 2000 and 2001. Our sample includes many NEOs with particularly deep 1-μm pyroxene/olivine absorption bands, similar to Q-type asteroids. We also classify three NEOs with particularly blue colors. No D-type asteroids were found, placing an upper limit of ∼2% on the fraction of the NEO population originating in the outer main belt or the Trojan clouds. The ratio of dark to bright objects in our sample was found to be 0.40, significantly higher than current theoretical predictions. As well as classifying the NEOs, we have investigated color trends with size and orbit. We see a general trend for larger silicate objects to have shallower absorption bands but find no significant difference in the distribution of taxonomic classes at small and large sizes. Our data clearly show that different taxonomic classes tend to occupy different regions of (a, e) space. By comparing our data with current model predictions for NEO dynamical evolution we see that Q-, R-, and V-type NEOs tend to have orbits associated with “fast track” delivery from the main belt, whereas S-type NEOs tend to have orbits associated with “slow track” delivery. This outcome would be expected if space weathering occurs on time scales of >10⁶ years.  相似文献   

Damped Lyman alpha systems and galaxy formation models – II. High ions and Lyman-limit systems     

Ariyeh H. Maller  Jason X. Prochaska  Rachel S. Somerville  Joel R. Primack 《Monthly notices of the Royal Astronomical Society》2003,343(1):268-278

  相似文献   

Book Review: The Iron Oxides von Rochelle M. Cornell und Udo Schwertmann     

S. Peiffer 《洁净——土壤、空气、水》2003,31(6):527-527

  相似文献