678 research outputs found
First optical images of circumstellar dust surrounding the debris disk candidate HD 32297
Near-infrared imaging with the Hubble Space Telescope recently revealed a
circumstellar dust disk around the A star HD 32297. Dust scattered light is
detected as far as 400 AU radius and the linear morphology is consistent with a
disk ~10 degrees away from an edge-on orientation. Here we present the first
optical images that show the dust scattered light morphology from 560 to 1680
AU radius. The position angle of the putative disk midplane diverges by 31
degrees and the color of dust scattering is most likely blue. We associate HD
32297 with a wall of interstellar gas and the enigmatic region south of the
Taurus molecular cloud. We propose that the extreme asymmetries and blue disk
color originate from a collision with a clump of interstellar material as HD
32297 moves southward, and discuss evidence consistent with an age of 30 Myr or
younger.Comment: 5 pages; Accepted for publication in ApJ Letter
Photoionization microscopy on magnesium atom and comparison with hydrogenic theory
International audienc
Sexuality-based stigma and access to care: intersecting perspectives between healthcare providers and men who have sex with men in HIV care centres in Senegal
Percolating magmas in three dimensions
The classical models of volcanic eruptions assume that they originate as a consequence of critical stresses or critical strain rates being exceeded in the magma followed by catastrophic fragmentation. In a recent paper (Gaonac'h et al., 2003) we proposed an additional mechanism based on the properties of complex networks of overlapping bubbles; that extreme multibubble coalescence could lead to catastrophic changes in the magma rheology at a critical vesicularity. This is possible because at a critical vesicularity <i>P<sub>c</sub></i> (the percolation threshold), even in the absence of external stresses the magma fragments. By considering 2-D percolation with the (observed) extreme power law bubble distributions, we showed numerically that <i>P<sub>2c</sub></i> had the apparently realistic value &asymp;0.7. <br><br> The properties of percolating systems are, however, significantly different in 2-D and 3-D. In this paper, we discuss various new features relevant to 3-D percolation and compare the model predictions with empirical data on explosive volcanism. The most important points are a) bubbles and magma have different 3-D critical percolation points; we show numerically that with power law bubble distributions that the important magma percolation threshold <i>P<sub>3c,m</sub></i> has the high value &asymp;0.97&plusmn;0.01, b) a generic result of 3-D percolation is that the resulting primary fragments will have power law distributions with exponent <i>B<sub>3f</sub></i>&asymp;1.186&plusmn;0.002, near the empirical value (for pumice) &asymp;1.1&plusmn;0.1; c) we review the relevant percolation literature and point out that the elastic properties may have lower &ndash; possibly more realistic &ndash; critical vesicularities relevant to magmas; d) we explore the implications of long range correlations (power law bubble distributions) and discuss this in combination with bubble anisotropy; e) we propose a new kind of intermediate "elliptical" dimensional percolation involving differentially elongated bubbles and show that it can lead to somewhat lower critical thresholds. <br><br> These percolation mechanisms for catastrophically weakening magma would presumably operate in conjunction with the classical critical stress and critical strain mechanisms. We conclude that percolation theory provides an attractive theoretical framework for understanding highly vesicular magma
Coupled-Channel Theory of Photoionization Microscopy
We develop a quantum-mechanical coupled-channel theory to simulate spatial distributions of electron current densities, produced in photoionization for nonhydrogenic atoms in the presence of a uniform external electric field. The coupled Schrodinger equations are numerically solved using the renormalized Numerov method. The expression for the outgoing wave function for photoelectrons ejected from the nonhydrogenic atomic source is derived. The theory is applied to investigations of photoionization for ground-state Li atoms. The distributions of electron current densities are computed and compared to the corresponding experimental images. Excellent agreement is obtained. It is, furthermore, found that the presence of the nonhydrogenic residual ion significantly changes the differential cross sections and/or electron current densities with respect to the hydrogenic case. Finally, the implications of the presence of the atomic core for quantum resonance tunneling are also analyzed
Observations of GRB 060526 Optical Afterglow with Russian-Turkish 1.5-m Telescope
We present the results of the photometric multicolor observations of GRB
060526 optical afterglow obtained with Russian-Turkish 1.5-m Telescope (RTT150,
Mt. Bakirlitepe, Turkey). The detailed measurements of afterglow light curve,
starting from about 5 hours after the GRB and during 5 consecutive nights were
done. In addition, upper limits on the fast variability of the afterglow during
the first night of observations were obtained and the history of afterglow
color variations was measured in detail. In the time interval from 6 to 16
hours after the burst, there is a gradual flux decay, which can be described
approximately as a power law with an index of -1.14+-0.02. After that the
variability on the time scale \delta t < t is observed and the afterglow
started to decay faster. The color of the afterglow, V-R=~0.5, is approximately
the same during all our observations. The variability is detected on time
scales up to \delta t/t =~ 0.0055 at \Delta F_\nu/F_\nu =~ 0.3, which violates
some constraints on the variability of the observed emission from
ultrarelativistic jet obtained by Ioka et al. (2005). We suggest to explain
this variability by the fact that the motion of the emitting shell is no longer
ultrarelativistic at this time.Comment: 6 pages, 7 figures, Astronomy Letters, 2007, 33, 797, The on-line
data tables and the original text in Russian can be found at
http://hea.iki.rssi.ru/grb/060526/indexeng.htm
aromatic hydrocarbons. pseudomonas strain growing on polycyclic Biosurfactant production by a soil
Ionization of strontium atoms by ultrashort pulses of a Ti-Sapphire laser
We present experimental photoelectron spectra obtained by ionizing two-valence electron Sr atoms with 4825 fs Ti- Sapphire laser pulses and we show preliminary evidence that electron-electron correlation effects dominate for laser intensities <0.4
71013 W-cm-2, while a single electron picture prevails when the laser intensity is increased further
Electrical detection of spin accumulation in a p-type GaAs quantum well
We report on experiments in which a spin-polarized current is injected from a
ferromagnetic electrode into a quantum well through an AlAs
barrier. The resulting spin polarization in the GaAs well is detected by
measuring how the current, tunneling to a second ferromagnetic
electrode, depends on the orientation of its magnetization. Our results can be
accounted for the non-relaxed spin splitting of the chemical potential, that is
spin accumulation, in the well. We discuss the conditions on the hole
spin relaxation time in GaAs that are required to obtain the large effects we
observe.Comment: 4 pages - 2 figues; one added note; some numbers corrected on page
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