Use of electron vortex beams (EVB), that is, convergent electron beams carrying an orbital angular momentum (OAM), is a novel development in the field of transmission electron microscopy. They should allow measurement of element-specific magnetic properties of thin crystals using electron magnetic circular dichroism (EMCD)-a phenomenon similar to the x-ray magnetic circular dichroism. Recently, it has been shown computationally that EVBs can detect magnetic signal in a scanning mode only at atomic resolution. In this follow-up work, we explore in detail the elastic and inelastic scattering properties of EVBs on crystals, as a function of beam diameter, initial OAM, acceleration voltage, and beam displacement from an atomic column. We suggest that for a 10-nm layer of bcc iron oriented along (001) zone axis, an optimal configuration for a detection of EMCD is an EVB with OAM of 1 and a full width at half maximum diffraction-limited beam diameter of 1.6Å, acceleration voltage 200 kV, and an annular detector with inner and outer radii of 7 and 44 mrad, respectively.
Abstract. The latest outburst of AG Peg has lasted for 150 years, which makes it the slowest nova eruption ever recorded. During the time of IUE observations (1978−1995) line profiles and intensity ratios of the N V and C IV doublet components changed remarkably, and we discuss plausible reasons. One of them is radiative pumping of Fe II which is investigated by studying the fluorescence lines from pumped levels. Three Fe II channels are pumped by C IV and one by N V. The pumping rates of those Fe II channels as derived by the modeling agree well with the strengths of the Fe II fluorescence lines seen in the spectra. We model the C IV and N V resonance doublets in IUE spectra recorded between 1978 and 1995 in order to derive optical depths, expansion velocities, and the emissivities of the red giant wind, the white dwarf wind and their collision region. The derived expansion velocities are ∼60 km s −1 for the red giant wind and ∼700 km s −1 for the white dwarf wind. We also suggest a fast outflow from the system at ∼150 km s −1 . The expansion velocity is slightly higher for N V than for C IV. Emission from the collision region strongly affects the profile of the N V and C IV resonance doublets indicating its existence.
Code generation in a compiler is commonly divided into several phases: instruction selection, scheduling, register allocation, spill code generation, and, in the case of clustered architectures, cluster assignment. These phases are interdependent; for instance, a decision in the instruction selection phase affects how an operation can be scheduled We examine the effect of this separation of phases on the quality of the generated code. To study this we have formulated optimal methods for code generation with integer linear programming; first for acyclic code and then we extend this method to modulo scheduling of loops. In our experiments we compare optimal modulo scheduling, where all phases are integrated, to modulo scheduling, where instruction selection and cluster assignment are done in a separate phase. The results show that, for an architecture with two clusters, the integrated method finds a better solution than the nonintegrated method for 27% of the instances.
Abstract. We present a semi-empirical equation for prediction of the strengths of those N III lines that are generated by the Bowen mechanism and observed in spectra of symbiotic stars. The equation assumes that the Bowen mechanism is the only source populating the 3d state in N III, and comparisons with observations of the 3s-3p and 3p-3d transitions serve as a test of this assumption. In an ongoing study of symbiotic stars the equation has been applied to two symbiotic novae, RR Tel and AG Peg, by comparing the predicted N III line strengths to observed line intensities. It is clear that besides the Bowen mechanism there is another process, most likely radiative recombination, that contributes to the N III 3d population in AG Peg and is the main population process of this state in RR Tel. It is also clear that a second, not previously considered, N III channel, 2p 2 P 1/2 -3d 2 D 3/2 at 374.198 Å, is pumped by O III in both RR Tel and AG Peg.
Tantalum spectra, emitted from a hollow cathode, have been recorded with a Fourier transform spectrometer in the region 2000–5000 Å. The observed hyperfine patterns of Ta II have been analyzed, yielding center-of-gravity wavelengths for 199 lines and hyperfine constants for 38 even and 97 odd levels of Ta II. Improved energy level values have been derived. The laboratory data have been used for a study of the tantalum abundance of the CP star χ Lupi.
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