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 Physics , 2015, DOI: 10.1051/0004-6361/201526373 Abstract: Non-local thermodynamic equilibrium (NLTE) effects in diagnostically important solar Fe I lines are important due to the strong sensitivity of Fe I to ionizing UV radiation, which may lead to a considerable under-population of the Fe I levels in the solar atmosphere and, therefore, to a sizeable weakening of Fe I lines. Such NLTE effects may be intensified or weakened by horizontal radiative transfer (RT) in a three-dimensionally (3-D) structured atmosphere. We analyze the influence of horizontal RT on commonly used Fe I lines in a snapshot of a 3-D radiation magneto-hydrodynamic (MHD) simulation of a plage region. NLTE- and horizontal RT effects occur with considerable strength (up to 50% in line depth or equivalent width) in the analyzed snapshot. As they may have either sign and both signs occur with approximately the same frequency and strength, the net effects are small when considering spatially averaged quantities. The situation in the plage atmosphere turns out to be rather complex. Horizontal transfer leads to line-weakening relative to 1-D NLTE transfer near the boundaries of kG magnetic elements. Around the centers of these elements, however, we find an often significant line-strengthening. This behavior is in contrast to that expected from previous 3-D RT computations in idealized flux-tube models, which display only a line weakening. The origin of this unexpected behavior lies in the fact that magnetic elements are surrounded by dense and relatively cool down-flowing gas, which forms the walls of the magnetic elements. The continuum in these dense walls is often formed in colder gas than in the central part of the magnetic elements. Consequently, the central parts of the magnetic element experience a sub-average UV-irradiation leading to the observed 3-D NLTE line strengthening.
 Physics , 2015, DOI: 10.1093/mnrasl/slv122 Abstract: We revisit the Galactic chemical evolution of oxygen, addressing the systematic errors inherent in classical determinations of the oxygen abundance that arise from the use of one dimensional hydrostatic (1D) model atmospheres and from the assumption of local thermodynamic equilibrium (LTE). We perform detailed 3D non-LTE radiative transfer calculations for atomic oxygen lines across a grid of 3D hydrodynamic stag- ger model atmospheres for dwarfs and subgiants. We apply our grid of predicted line strengths of the [OI] 630 nm and OI 777 nm lines using accurate stellar parameters from the literature. We infer a steep decay in [O/Fe] for [Fe/H] $\gtrsim$ -1.0, a plateau [O/Fe] $\approx$ 0.5 down to [Fe/H] $\approx$ -2.5 and an increasing trend for [Fe/H] $\lesssim$ -2.5. Our 3D non-LTE calculations yield overall concordant results from the two oxygen abundance diagnostics.
 Physics , 2015, DOI: 10.1093/mnras/stv2608 Abstract: The OI 777 nm lines are among the most commonly used diagnostics for the oxygen abundances in the atmospheres of FGK-type stars. However, they form in conditions that are far from local thermodynamic equilibrium (LTE). We explore the departures from LTE of atomic oxygen, and their impact on OI lines, across the Stagger-grid of three-dimensional hydrodynamic model atmospheres. For the OI 777 nm triplet we find significant departures from LTE. These departures are larger in stars with larger effective temperatures, smaller surface gravities, and larger oxygen abundances. We present grids of predicted 3D non-LTE based equivalent widths for the OI616nm, [OI] 630 nm, [OI] 636 nm, and OI 777 nm lines, as well as abundance corrections to 1D LTE based results.
 Physics , 2014, Abstract: Colliding wind binaries (CWBs) are unique laboratories for X-ray astrophysics. The massive stars in these systems possess powerful stellar winds with speeds up to $\sim$3000 km s$^{-1}$, and their collision leads to hot plasma (up to $\sim10^8$K) that emit thermal X-rays (up to $\sim$10 keV). Many X-ray telescopes have observed CWBs, including Suzaku, and our work aims to model these X-ray observations. We use 3D smoothed particle hydrodynamics (SPH) to model the wind-wind interaction, and then perform 3D radiative transfer to compute the emergent X-ray flux, which is folded through X-ray telescopes' response functions to compare directly with observations. In these proceedings, we present our models of Suzaku observations of the multi-year-period, highly eccentric systems $\eta$ Carinae and WR 140. The models reproduce the observations well away from periastron passage, but only $\eta$ Carinae's X-ray spectrum is reproduced at periastron; the WR 140 model produces too much flux during this more complicated phase.
 Physics , 2009, DOI: 10.1051/0004-6361/200911741 Abstract: We construct a 3D radiative-hydrodynamic model atmosphere of parameters Teff = 4820 K, log g = 4.5, and solar chemical composition. The theoretical line profiles computed with this model are asymmetric, with their bisectors having a characteristic C-shape and their core wavelengths shifted with respect to their laboratory values. The line bisectors span from about 10 to 250 m/s, depending on line strength, with the stronger features showing larger span. The corresponding core wavelength shifts range from about -200 m/s for the weak Fe I lines to almost +100 m/s in the strong Fe I features. Based on observational results for the Sun, we argue that there should be no core wavelength shift for Fe I lines of EW > 100 mA. The cores of the strongest lines show contributions from the uncertain top layers of the model, where non-LTE effects and the presence of the chromosphere, which are important in real stars, are not accounted for. The comparison of model predictions to observed Fe I line bisectors and core wavelength shifts for a reference star, HIP86400, shows excellent agreement, with the exception of the core wavelength shifts of the strongest features, for which we suspect inaccurate theoretical values. Since this limitation does not affect the predicted line equivalent widths significantly, we consider our 3D model validated for photospheric abundance work.
 Physics , 2002, Abstract: We simulate convection near the solar surface, where the continuum optical depth is of order unity. Hence, to determine the radiative heating and cooling in the energy conservation equation, we must solve the radiative transfer equation (instead of using the diffusion or optically thin cooling approximations). A method efficient enough to calculate the radiation for thousands of time steps is needed. We assume LTE and a non-gray opacity grouped into 4 bins according to strength. We perform a formal solution of the Feautrier equation along a vertical and four straight, slanted, rays (at four azimuthal angles which are rotated 15 deg. every time step). We present details of our method. We also give some results: comparing simulated and observed line profiles for the Sun, showing the importance of 3D transfer for the structure of the mean atmosphere and the eigenfrequencies of p-modes, illustrating Stokes profiles for micropores, and analyzing the effect of radiation on p-mode asymmetries.
 Physics , 2010, DOI: 10.1017/S1743921310004126 Abstract: In this review we discuss possible systematic errors inherent in classical 1D LTE abundance analyses of late-type stars for the light elements (here: H, He, Li, Be and B). The advent of realistic 3D hydrodynamical model atmospheres and the availability of non-LTE line formation codes place the stellar analyses on a much firmer footing and indeed drastically modify the astrophysical interpretations in many cases, especially at low metallicities. For the Teff-sensitive hydrogen lines both stellar granulation and non-LTE are likely important but the combination of the two has not yet been fully explored. A fortuitous near-cancellation of significant but opposite 3D and non-LTE effects leaves the derived 7Li abundances largely unaffected but new atomic collisional data should be taken into account. We also discuss the impact on 3D non-LTE line formation on the estimated lithium isotopic abundances in halo stars in light of recent claims that convective line asymmetries can mimic the presence of 6Li. While Be only have relatively minor non-LTE abundance corrections, B is sensitive even if the latest calculations imply smaller non-LTE effects than previously thought.
 Physics , 1999, Abstract: The high resolution and sensitivity provided by proposed Atacama Large Millimeter Array will reveal new small scale structures in many sources, e.g. star forming regions. Such inhomogeneities may not have been considered in the analysis of past observations but they will be essential to the understanding of future data. Radiative transfer methods are needed to interpret the observations and the presence of complicated source structures and small scale inhomogeneities requires 3D modelling. We will describe studies of molecular line emission we have made using models of inhomogeneous molecular clouds. These are based on MHD models and include e.g. thermal balance calculations for molecular clouds.
 Physics , 1995, Abstract: We study the formation of O I and OH spectral lines in three-dimensional hydrodynamic models of the solar photosphere. The line source function of the O I 777 nm triplet is allowed to depart from local thermodynamic equilibrium (LTE), within the two-level-atom approximation. Comparison with results from 1D models show that the 3D models alleviate, but do not remove, the discrepancy between the oxygen abundances reported from non-LTE work on the 777 nm triplet and from the [O I] 630 nm and OH lines. Results for the latter two could imply that the solar oxygen abundance is below 8.8 (lg(H) = 12). If this is confirmed, the discrepancy between theory and observation for the 777 nm triplet lines might fall within the range of errors in equivalent width measurements and f-values. The line source function of the 777 nm triplet in the 1.5D approximation is shown to differ insignificantly from the full 3D non-LTE result.
 Physics , 2012, DOI: 10.1051/0004-6361/201219477 Abstract: In network and active region plages, the magnetic field is concentrated into structures often described as flux tubes (FTs) and sheets (FSs). 3-D radiative transfer (RT) is important for energy transport in these concentrations. It is also expected to be important for diagnostic purposes but has rarely been applied for that purpose. Using true 3-D, non-LTE (NLTE) RT in FT/FS models, we compute Fe line profiles commonly used to diagnose the Sun's magnetic field by comparing the results with those obtained from LTE/1-D (1.5-D) NLTE calculations. Employing a multilevel iron atom, we study the influence of basic parameters such as Wilson depression, wall thickness, radius/width, thermal stratification or magnetic field strength on all Stokes $I$ parameters in the thin-tube approximation. The use of different levels of approximations of RT may lead to considerable differences in profile shapes, intensity contrasts, equivalent widths, and the determination of magnetic field strengths. In particular, LTE, which often provides a good approach in planar 1-D atmospheres, is a poor approximation in our flux sheet model for some of the most important diagnostic Fe I lines (524.7nm, 525.0nm, 630.1nm, and 630.2nm). The observed effects depend on parameters such as the height of line formation, field strength, and internal temperature stratification. Differences between the profile shapes may lead to errors in the determination of magnetic fields on the order of 10 to 20%, while errors in the determined temperature can reach 300-400K. The empirical FT models NET and PLA turn out to minimize the effects of 3D RT, so that results obtained with these models by applying LTE may also remain valid for 3-D NLTE calculations. Finally, horizontal RT is found to only insignificantly smear out structures such as the optically thick walls of FTs and FSs, allowing features as narrow as 10km to remain visible.
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