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The collapse of a massive stellar core may lead to the production of a black hole surrounded by a torus of material. Such a system is a potential source for the so-called long gamma-ray bursts (GRBs). A torus will form around the black hole if the infalling material contains sufficient angular momentum. This however requires that the core of the massive star rotates extremely rapidly prior to coll
The Milky Way is expected to host an accreted disc of stars and dark matter. This forms as massive greater than or similar to 1 : 10 mergers are preferentially dragged towards the disc plane by dynamical friction and then tidally shredded. The accreted disc likely contributes only a tiny fraction of the MilkyWay's thin and thick stellar disc. However, it is interesting because (i) its associated '
The present paper investigates and discusses the ability of the Hartmann test with a 4-aperture differential image motion monitor (DIMM) to measure the atmospheric primary aberrations which, in turn, can be used for the calculation of the atmospheric coherence time. Through performing numerical simulations, we show that the 4-aperture DIMM is able to measure the defocus and astigmatism terms corre
We present detailed chemical element abundance ratios of 17 elements with eight <= Z <= 60 in three metal-poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine with extant data from the literature to assess the predictions of a novel suite of galaxy chemical evolution models. The spectroscopic data were obtained with the Keck/High-Resolution Echelle Spectrograph instrument and rev
According to our current cosmological model, galaxies like the Milky Way are expected to experience many mergers over their lifetimes. The most massive of the merging galaxies will be dragged towards the disc plane, depositing stars and dark matter into an accreted disc structure. In this work, we utilize the chemodynamical template developed in Ruchti et al. to hunt for accreted stars. We apply t
Using two epochs of HST/WFPC2 images of the metal-rich globular clusterNGC 6528 we derive the proper motions of the stars and use them toseparate the stars belonging to NGC 6528 from those of the Galacticbulge. The stellar sequences in the resulting colour-magnitude diagramfor the cluster are significantly better determined than in previouslypublished data. From comparison of the colour-magnitude
Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H2O, CH4, etc.) with accuracy, it is imperative to un
The formation of beryllium in the early Galaxy is discussed. It is shown that if spallation occurs predominantly in regions rich in heavy elements, i.e., close to supernovae, the linear relation recently obtained by Gilmore et al. (1992) between beryllium and oxygen abundances for Population II stars may be reproduced. Estimates of relevant timescales for mixing show that the decisive factor, the
In this paper we investigate the performance of a Fourier based algorithm for fast subpixel shift determination of two mutually shifted images subjected to noise. The algorithm will be used for Shack-Hartmann based adaptive optics correction of images of an extended object subjected to dynamical atmospheric fluctuations. The performance of the algorithm is investigated both analytically and by Mon
Using high-resolution spectra of nearby F and G dwarf stars, we have investigated the detailed abundance and age structure of the Hercules stream. We find that the stars in the stream have a wide range of stellar ages, metallicities, and element abundances. By comparing to existing samples of stars in the solar neighborhood with kinematics typical of the Galactic thin and thick disks, we find that
We present oxygen abundances for 72 F and G dwarf stars in the solarneighbourhood. Using the kinematics of the stars we divide them into twosub-samples with space velocities that are typical for the thick andthin disks, respectively. The metallicities of the stars range from[Fe/H] ≈ -0.9 to +0.4 and we use the derived oxygen abundances of thestars to: (1) perform a differential study of the oxygen
We discuss resonance-enhanced two-photon ionization (RETPI) and presentschemes of successive RETPI of the elements C, N, and O in nebulae.RETPI is activated by intrinsic radiation stored in the form of trappedspectral lines of HI, HeI, and HeII in the optically thick nebula. Therate of this two-step photoionization is comparable with or exceeds thelow recombination rate of the photoions formed in
Spectral lines observed in the Sr-filament of eta Car in the wavelengthregion 2480-10140{AA}. The lines are sorted by wavelength in table 2 andsorted according to the periodic table in table 3. Both tables areavailable in postscript format as the file tables23.ps where the sub andsuperscripts for the atomic transitions are kept.(2 data files).
Context. Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface composition, especially in the giants where most elements can be studied. Aims. We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mi
After the discovery of space masers based on OH radicals (Weaver et al.cite{Wea65}) and H2O (Cheung et al. cite{Che69}) such microwavelasers have been found to work in more than 100 molecular species(Elitzur cite{Eli92}; Townes cite{Tow97}), as well as in highlyexcited H atoms (Strelnitski et al. cite{Str96}). In the IR region (10mum), the effect of stimulated emission of radiation in t
About a hundred dust impacts per day were detected during the first week in December 1995 by Galileo during its approach to Jupiter. These impacts were caused by submicrometer-sized particles that were just above the detection limit. After the closest approach to Io on 7 December, impacts of these small particles ceased. This effect is expected for dust grains emitted from Io that exit the field o
Three-dimensional numerical simulations with CO5, a newradiation hydrodynamics code, result in a dynamic, thermally bifurcatedmodel of the non-magnetic chromosphere of the quiet Sun. The 3D modelincludes the middle and low chromosphere, the photosphere, and the topof the convection zone, where acoustic waves are excited by convectivemotions. While the waves propagate upwards, they steep
Analyses of their internal stellar kinematics imply that the dwarf spheroidal (dSph) companion galaxies to the Milky Way are among the most dark matter-dominated systems known. Should there be significant dark matter in the form of faint stars in these systems, the stellar luminosity function must be very different from that of a similar metallicity globular cluster, for which there is no evidence
It is shown how first-order performance estimation of high-orderadaptive optics (AO) systems may be efficiently implemented in a hybridnumerical simulation by the use of 1) sparse matrix techniques forwavefront reconstruction, 2) undersampled pupil-plane turbulence-inducedaberrations, and 3) analytical models that compensate - in the limit ofinfinite exposure time - for the errors introduced by un
