arXiv subjects

R. F. Stein

Publications and source records attributed to R. F. Stein.

29 records · Page 2Linked to original sources

arXiv2001

Magnetohydrodynamic turbulence in warped accretion discs

Warped, precessing accretion discs appear in a range of astrophysical systems, for instance the X-ray binary Her X-1 and in the active nucleus of NGC4258. In a warped accretion disc there are horizontal pressure gradients that drive an epicyclic motion. We have studied the interaction of this epicyclic motion with the magnetohydrodynamic turbulence in numerical simulations. We find that the turbulent stress acting on the epicyclic motion is comparable in size to the stress that drives the accretion, however an important ingredient in the damping of the epicyclic motion is its parametric decay into inertial waves.

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arXiv2000

Solar Oscillations and Convection: II. Excitation of Radial Oscillations

Solar p-mode oscillations are excited by the work of stochastic, non-adiabatic, pressure fluctuations on the compressive modes. We evaluate the expression for the radial mode excitation rate derived by Nordlund and Stein (Paper I) using numerical simulations of near surface solar convection. We first apply this expression to the three radial modes of the simulation and obtain good agreement between the predicted excitation rate and the actual mode damping rates as determined from their energies and the widths of their resolved spectral profiles. We then apply this expression for the mode excitation rate to the solar modes and obtain excellent agreement with the low l damping rates determined from GOLF data. Excitation occurs close to the surface, mainly in the intergranular lanes and near the boundaries of granules (where turbulence and radiative cooling are large). The non-adiabatic pressure fluctuations near the surface are produced by small instantaneous local imbalances between the divergence of the radiative and convective fluxes near the solar surface. Below the surface, the non-adiabatic pressure fluctuations are produced primarily by turbulent pressure fluctuations (Reynolds stresses). The frequency dependence of the mode excitation is due to effects of the mode structure and the pressure fluctuation spectrum. Excitation is small at low frequencies due to mode properties -- the mode compression decreases and the mode mass increases at low frequency. Excitation is small at high frequencies due to the pressure fluctuation spectrum -- pressure fluctuations become small at high frequencies because they are due to convection which is a long time scale phenomena compared to the dominant p-mode periods.

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arXiv2000

Solar Oscillations and Convection: I. Formalism for Radial Oscillations

We present a formalism for investigating the interaction between pmode oscillations and convection by analyzing realistic, three-dimensional simulations of the near-surface layers of the solar convection zone. By choosing suitable definitions for fluctuations and averages, we obtain a separation that retains exact equations. The equations for the horizontal averages contain one part that corresponds directly to the wave equations for a 1-D medium, plus additional terms that arise from the averaging and correspond to the turbulent pressure gradient in the momentum equation and the divergence of the convective and kinetic energy fluxes in the internal energy equation. These terms cannot be evaluated in closed form, but they may be measured in numerical simulations. The additional terms may cause the mode frequencies to shift, relative to what would be obtained if only the terms corresponding to a 1-D medium were retained---most straightforwardly by changing the mean stratification, and more subtly by changing the effective compressibility of the medium. In the presence of time dependent convection, the additional terms also have a stochastic time dependence, that acts as a source of random excitation of the coherent modes. In the present paper, we derive an expression for the excitation power and test it by applying it to a numerical experiment of sufficient duration for the excited modes to be spectrally resolved.

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arXiv2000

The effects of numerical resolution on hydrodynamical surface convection simulations and spectral line formation

The computationally demanding nature of radiative-hydrodynamical simulations of stellar surface convection warrants an investigation of the sensitivity of the convective structure and spectral synthesis to the numerical resolution and dimension of the simulations, which is presented here. With too coarse a resolution the predicted spectral lines tend to be too narrow, reflecting insufficient Doppler broadening from the convective motions, while at the currently highest affordable resolution the line shapes have converged essentially perfectly to the observed profiles. Similar conclusions are drawn from the line asymmetries and shifts. In terms of abundances, weak FeI and FeII lines show a very small dependence (~0.02 dex) while for intermediate strong lines with significant non-thermal broadening the sensitivity increases (~0.10 dex). Problems arise when using 2D convection simulations to describe an inherent 3D phenomenon, which translates to inaccurate atmospheric velocity fields and temperature and pressure structures. In 2D the theoretical line profiles tend to be too shallow and broad compared with the 3D calculations and observations, in particular for intermediate strong lines. In terms of abundances, the 2D results are systematically about 0.1 dex lower than for the 3D case for FeI lines. Furthermore, the predicted line asymmetries and shifts are much inferior in 2D. Given these shortcomings and computing time considerations it is better to use 3D simulations of even modest resolution than high-resolution 2D simulations.

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arXiv2000

Line formation in solar granulation: I. Fe line shapes, shifts and asymmetries

Realistic ab-initio 3D, radiative-hydrodynamical convection simulations of the solar granulation have been applied to FeI and FeII line formation. In contrast to classical analyses based on hydrostatic 1D model atmospheres the procedure contains no adjustable free parameters but the treatment of the numerical viscosity in the construction of the 3D, time-dependent, inhomogeneous model atmosphere and the elemental abundance in the 3D spectral synthesis. However, the numerical viscosity is introduced purely for numerical stability purposes and is determined from standard hydrodynamical test cases with no adjustments allowed to improve the agreement with the observational constraints from the solar granulation. The non-thermal line broadening is mainly provided by the Doppler shifts arising from the convective flows in the solar photosphere and the solar oscillations. The almost perfect agreement between the predicted temporally and spatially averaged line profiles for weak Fe lines with the observed profiles and the absence of trends in derived abundances with line strengths, seem to imply that the micro- and macroturbulence concepts are obsolete in these 3D analyses. Furthermore, the theoretical line asymmetries and shifts show a very satisfactory agreement with observations with an accuracy of typically 50-100 m/s on an absolute velocity scale. The remaining minor discrepancies point to how the convection simulations can be refined further.

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arXiv2000

Line formation in solar granulation: II. The photospheric Fe abundance

The solar photospheric Fe abundance has been determined using realistic ab initio 3D, time-dependent, hydrodynamical model atmospheres. The study is based on the excellent agreement between the predicted and observed line profiles directly rather than equivalent width, since the intrinsic Doppler broadening from the convective motions and oscillations provide the necessary non-thermal broadening. Thus, three of the four hotly debated parameters (equivalent widths, microturbulence and damping enhancement factors) in the center of the recent solar Fe abundance dispute regarding FeI lines no longer enter the analysis, leaving the transition probabilities as the main uncertainty. Both FeI (using the samples of lines of both the Oxford and Kiel studies) and FeII lines have been investigated, which give consistent results: log FeI = 7.44 +- 0.05 and log FeII = 7.45 +- 0.10. Also the wings of strong FeI lines return consistent abundances, log FeII = 7.42 +- 0.03, but due to the uncertainties inherent in analyses of strong lines we give this determination lower weight than the results from weak and intermediate strong lines. In view of the recent slight downward revision of the meteoritic Fe abundance log Fe = 7.46 +- 0.01, the agreement between the meteoritic and photospheric values is very good, thus appearingly settling the debate over the photospheric Fe abundance from FeI lines.

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arXiv1999

Simulations of Oscillation Modes of the Solar Convection Zone

We use the three-dimensional hydrodynamic code of Stein and Nordlund to realistically simulate the upper layers of the solar convection zone in order to study physical characteristics of solar oscillations. Our first result is that the properties of oscillation modes in the simulation closely match the observed properties. Recent observations from SOHO/MDI and GONG have confirmed the asymmetry of solar oscillation line profiles, initially discovered by Duvall et al. In this paper we compare the line profiles in the power spectra of the Doppler velocity and continuum intensity oscillations from the SOHO/MDI observations with the simulation. We also compare the phase differences between the velocity and intensity data. We have found that the simulated line profiles are asymmetric and have the same asymmetry reversal between velocity and intensity as observed. The phase difference between the velocity and intensity signals is negative at low frequencies and jumps in the vicinity of modes as is also observed. Thus, our numerical model reproduces the basic observed properties of solar oscillations, and allows us to study the physical properties which are not observed.

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arXiv1999

3D hydrodynamical model atmospheres of metal-poor stars: Evidence for a low primordial Li abundance

Realistic 3-dimensional (3D), radiative hydrodynamical surface convection simulations of the metal-poor halo stars HD 140283 and HD 84937 have been performed. Due to the dominance of adiabatic cooling over radiative heating very low atmospheric temperatures are encountered. The lack of spectral lines in these metal-poor stars thus causes much steeper temperature gradients than in classical 1D hydrostatic model atmospheres where the temperature of the optically thin layers is determined by radiative equilibrium. The modified atmospheric structures cause changes in the emergent stellar spectra. In particular, the primordial Li abundances may have been overestimated by 0.2-0.35 dex with 1D model atmospheres. However, we caution that our result assumes local thermodynamic equilibrium (LTE), while the steep temperature gradients may be prone to e.g. over-ionization.

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arXiv1999

Convective contributions to the frequencies of solar oscillations

Differences between observed and theoretical eigenfrequencies of the Sun have characteristics which identify them as arising predominantly from properties of the oscillations in the vicinity of the solar surface: in the super-adiabatic, convective boundary layer and above. These frequency differences may therefore provide useful information about the structure of these regions, precisely where the theory of solar structure is most uncertain. In the present work we use numerical simulations of the outer part of the Sun to quantify the influence of turbulent convection on solar oscillation frequencies. Separating the influence into effects on the mean model and effects on the physics of the modes, we find that the main model effects are due to the turbulent pressure that provides additional support against gravity, and thermal differences between average 3-D models and 1-D models. Surfaces of constant pressure in the visible photosphere are elevated by about 150 km, relative to a standard envelope model. As a result, the turning points of high-frequency modes are raised, while those of the low-frequency modes remain essentially unaffected. The corresponding gradual lowering of the mode frequencies accounts for most of the frequency difference between observations and standard solar models. Additional effects are expected to come primarily from changes in the physics of the modes, in particular from the modulation of the turbulent pressure by the oscillations.

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arXiv1998

Test of Convective Frequency Effects with SOI/MDI High-Degree Data

Advances in hydrodynamical simulations have provided new insights into the effects of convection on the frequencies of solar oscillations. As more accurate observations become available, this may lead to an improved understanding of the dynamics of convection and the interaction between convection and pulsation (\cite{Rosenthal+99}). Recent high-resolution observations from the SOI/MDI instrument on the SOHO spacecraft have provided the so-far most-detailed observations of high-degree modes of solar oscillations, which are particularly sensitive to the near-surface properties of the Sun. Here we present preliminary results of a comparison between these observations and frequencies computed for models based on realistic simulations of near-surface convection. Such comparisons may be expected to help in identifying the causes of the remaining differences between the observed frequencies and those of solar models.

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arXiv1998

Dynamics of Magnetic Flux Elements in the Solar Photosphere

The interaction of magnetic fields and convection is investigated in the context of the coronal heating problem. We study the motions of photospheric magnetic elements using filtergrams obtained at the Swedish Vacuum Solar Telescope at La Palma. We use potential-field modeling to extrapolate the magnetic and velocity fields to larger height. We find that the velocity in the chromosphere can be locally enhanced at the separatrix surfaces between neighboring flux tubes. The predicted velocities are several km/s, significantly larger than those of the photospheric flux tubes, which may have important implications for coronal heating. s

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