Radiative transfer effects on Doppler measurements as sources of surface effects in sunspot seismology

We show that the use of Doppler shifts of Zeeman sensitive spectral lines to observe wavesn in sunspots is subject to measurement specific phase shifts arising from, (i) altered height range of spectral line formation and the propagating character of p mode waves in penumbrae, and (ii) Zeeman broadening and splitting. We also show that these phase shifts depend on wave frequencies, strengths and line of sight inclination of magnetic field, and the polarization state used for Doppler measurements. We discuss how these phase shifts could contribute to local helioseismic measurements of 'surface effects' in sunspot seismology.Comment: 12 pages, 4 figures, Accepted for publication in the Astrophysical Journal Letter

Domain Architectures as an Instrument to Refine Enterprise Architecture

Enterprise architecture is concerned with the fundamental organization of the operating environment of an enterprise. The enterprise architecture is used to plan and control the construction of the systems that populate the operating environment. As the scope covered can be considerable in large enterprises, introducing domain architectures to partition and detail the enterprise architecture is a plausible approach. We formulate prescriptive criteria that consistent domain architectures must meet. By integrating the creation of domain architectures into an extended strategic alignment model we develop a theory that accounts for both the creation, scope-setting and detailing. Based on the creation viewpoint we derive a multi-level classification taxonomy. The primary differentiator is that between domains that are created from business usage viewpoints and those that are created from solution construction viewpoints. Four cases of domain architectures from actual practice are described that illustrate the variety encountered. Domain classifications in all cases conform to the theoretical model. The criteria, the developed theory and the cases have both academic relevance as well as significance for practitioners

Formation of Li I lines in photospheric granulation

The possibility of significant systematic errors due to the use of 1D homogeneous atmospheres in lithium-abundance determinations of cool stars motivates a study of non-local-thermodynamic-equilibrium (NLTE) effects on Li I line formation in a 3D solar-granulation simulation snapshot. The NLTE effect on the equivalent width of the 671 nm resonance line is small in 1D models or in integrated light from the granulation model. The line-strength variations over the granulation pattern are however markedly different in NLTE compared to LTE -- observations of this may provide diagnostics to NLTE effects. The effects of horizontal photon exchange found in the granulation model are moderate and due entirely to bound-bound processes, ultraviolet overionization is unimportant.Comment: 9 pages Latex (AASTeX using aaspp4.sty) with 3 figures (PS). The former EPS figures have been replaced with safer PS due to technical problems encountered by some users. No change in content. Accepted for publication in Astrophysical Journal Letter

The Zeeman-sensitive emission lines of Mg I at 12 micron in Procyon

Emission-lines of magnesium at 12 microns have been observed in the spectrum of Procyon. We reproduce the observed, disk-averaged line flux from Procyon (as well as the observed intensity profiles from the Sun) by calculating the line formation, relaxing the assumption of Local Thermodynamic Equilibrium. We find that the lines in Procyon are formed in the photosphere in the same manner as the solar lines. We discuss our modeling of these Rydberg lines and evaluate, among other things, the importance of the ionizing flux and updated model-atom parameters. The lines are of large diagnostic value for measurements of stellar magnetic-fields through their Zeeman splitting. We have not, however, detected splitting of the MgI lines in Procyon. Using simple arguments, we believe we would have detected a magnetic field, had it been of a strength larger than approximately 800 Gauss covering more than a quarter of the surface. We discuss the prospects for future use of the Zeeman-sensitive, mid-infrared, MgI emission lines as a diagnostic tool for stellar magnetic fields.Comment: To be published in Ap

Non-thermal Mg I emission at 12 um from Procyon

We report on stellar Mg I emission at 12 um from alpha CMi (Procyon), a star slightly hotter than the Sun. Solar Mg I emission is well-known and its formation was successfully explained in detail by Carlsson et al. (1992). Here, for the first time, we compare synthetic spectra of the emission lines at 12 um with observations of a star other than the Sun. The use of these lines as stellar diagnostics has been anticipated for 10 years or more. We find that the model reproduces the observed emission in Procyon quite well. We expect that high-resolution spectrographs on 8-10 m telescopes will finally be able to exploit these new diagnostics.Comment: ApJ Letters, accepte

Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations

Calculations of spectral darkening and visibility functions for the brightness oscillations of the Sun resulting from global solar oscillations are presented. This has been done for a broad range of the visible and infrared continuum spectrum. The procedure for the calculations of these functions includes the numerical computation of depth-dependent derivatives of the opacity caused by p modes in the photosphere. A radiative-transport code was used for this purpose to get the disturbances of the opacities from temperature and density fluctuations. The visibility and darkening functions are obtained for adiabatic oscillations under the assumption that the temperature disturbances are proportional to the undisturbed temperature of the photosphere. The latter assumption is the only way to explore any opacity effects since the eigenfunctions of p-mode oscillations have not been obtained so far. This investigation reveals that opacity effects have to be taken into account because they dominate the violet and infrared part of the spectrum. Because of this dominance, the visibility functions are negative for those parts of the spectrum. Furthermore, the darkening functions show a wavelength-dependent change of sign for some wavelengths owing to these opacity effects. However, the visibility and darkening functions under the assumptions used contradict the observations of global p-mode oscillations, but it is beyond doubt that the opacity effects influence the brightness fluctuations of the Sun resulting from global oscillations

Comparison of the thin flux tube approximation with 3D MHD simulations

The structure and dynamics of small vertical photospheric magnetic flux concentrations has been often treated in the framework of an approximation based upon a low-order truncation of the Taylor expansions of all quantities in the horizontal direction, together with the assumption of instantaneous total pressure balance at the boundary to the non-magnetic external medium. Formally, such an approximation is justified if the diameter of the structure (a flux tube or a flux sheet) is small compared to all other relevant length scales (scale height, radius of curvature, wavelength, etc.). The advent of realistic 3D radiative MHD simulations opens the possibility of checking the consistency of the approximation with the properties of the flux concentrations that form in the course of a simulation. We carry out a comparative analysis between the thin flux tube/sheet models and flux concentrations formed in a 3D radiation-MHD simulation. We compare the distribution of the vertical and horizontal components of the magnetic field in a 3D MHD simulation with the field distribution in the case of the thin flux tube/sheet approximation. We also consider the total (gas plus magnetic) pressure in the MHD simulation box. Flux concentrations with super-equipartition fields are reasonably well reproduced by the second-order thin flux tube/sheet approximation. The differences between approximation and simulation are due to the asymmetry and the dynamics of the simulated structures

Direct composition profiling in III-V nanostructures by cross-sectional STM

Using cross-sectional STM we have studied the local composition in III–V nanostructures such as GaAs/InGaAs quantum wells, InGaNAs/InP quantum wells and quantum dots, and InAs/GaAs self-assembled quantum dots. We are able to determine the local composition by either simply counting the constituent atoms, measuring the local lattice constant or measuring the relaxation of the cleaved surface due to the elastic field of the buried strained nanostructures

A Godunov Method for Multidimensional Radiation Magnetohydrodynamics based on a variable Eddington tensor

We describe a numerical algorithm to integrate the equations of radiation magnetohydrodynamics in multidimensions using Godunov methods. This algorithm solves the radiation moment equations in the mixed frame, without invoking any diffusion-like approximations. The moment equations are closed using a variable Eddington tensor whose components are calculated from a formal solution of the transfer equation at a large number of angles using the method of short characteristics. We use a comprehensive test suite to verify the algorithm, including convergence tests of radiation-modified linear acoustic and magnetosonic waves, the structure of radiation modified shocks, and two-dimensional tests of photon bubble instability and the ablation of dense clouds by an intense radiation field. These tests cover a very wide range of regimes, including both optically thick and thin flows, and ratios of the radiation to gas pressure of at least 10^{-4} to 10^{4}. Across most of the parameter space, we find the method is accurate. However, the tests also reveal there are regimes where the method needs improvement, for example when both the radiation pressure and absorption opacity are very large. We suggest modifications to the algorithm that will improve accuracy in this case. We discuss the advantages of this method over those based on flux-limited diffusion. In particular, we find the method is not only substantially more accurate, but often no more expensive than the diffusion approximation for our intended applications.Comment: 42 pages, 22 figures, 2 tables, accepted by ApJ