Effects of resolution and helium abundance in A star surface convection simulations

We present results from 2D radiation-hydrodynamical simulations of fully compressible convection for the surface layers of A-type stars with the ANTARES code. Spectroscopic indicators for photospheric convective velocity fields show a maximum of velocities near Teff ~8000 K. In that range the largest values are measured for the subgroup of Am stars. Thus far, no prognostic model, neither theoretical nor numerical, is able to exactly reproduce the line profiles of sharp line A and Am stars in that temperature range. In general, the helium abundance of A stars is not known from observations. Hence, we have considered two extreme cases for our simulations: a solar helium abundance as an upper limit and zero helium abundance as a lower limit. The simulation for the helium free case is found to differ from the case with solar helium abundance by larger velocities, larger flow structures, and by a sign reversal of the flux of kinetic energy inside the hydrogen ionisation zone. Both simulations show extended shock fronts emerging from the optical surface, as well as mixing far below the region of partial ionisation of hydrogen, and vertical oscillations emerging after initial perturbations have been damped. We discuss problems related to the rapid radiative cooling at the surface of A-type stars such as resolution and efficient relaxation. The present work is considered as a step towards a systematic study of convection in A- to F-type stars, encouraged by the new data becoming available for these objects from both asteroseismological missions and from high resolution spectroscopy.Comment: submitted to CoAst, preprint version with 26 pages (29 pages in CoAst layout), 8 figures, 1 tabl

Structure of the solar photosphere studied from the radiation hydrodynamics code ANTARES

The ANTARES radiation hydrodynamics code is capable of simulating the solar granulation in detail unequaled by direct observation. We introduce a state-of-the-art numerical tool to the solar physics community and demonstrate its applicability to model the solar granulation. The code is based on the weighted essentially non-oscillatory finite volume method and by its implementation of local mesh refinement is also capable of simulating turbulent fluids. While the ANTARES code already provides promising insights into small-scale dynamical processes occurring in the quiet-Sun photosphere, it will soon be capable of modeling the latter in the scope of radiation magnetohydrodynamics. In this first preliminary study we focus on the vertical photospheric stratification by examining a 3-D model photosphere with an evolution time much larger than the dynamical timescales of the solar granulation and of particular large horizontal extent corresponding to $25\!" \!\! \times \, 25\!"$ on the solar surface to smooth out horizontal spatial inhomogeneities separately for up- and downflows. The highly resolved Cartesian grid thereby covers $\sim 4~\mathrm{Mm}$ of the upper convection zone and the adjacent photosphere. Correlation analysis, both local and two-point, provides a suitable means to probe the photospheric structure and thereby to identify several layers of characteristic dynamics: The thermal convection zone is found to reach some ten kilometers above the solar surface, while convectively overshooting gas penetrates even higher into the low photosphere. An $\approx 145\,\mathrm{km}$ wide transition layer separates the convective from the oscillatory layers in the higher photosphere.Comment: Accepted for publication in Astrophysics and Space Science; 18 pages, 12 figures, 2 tables; typos correcte

Turbulent convection: comparing the moment equations to numerical simulations

The non-local hydrodynamic moment equations for compressible convection are compared to numerical simulations. Convective and radiative flux typically deviate less than 20% from the 3D simulations, while mean thermodynamic quantities are accurate to at least 2% for the cases we have investigated. The moment equations are solved in minutes rather than days on standard workstations. We conclude that this convection model has the potential to considerably improve the modelling of convection zones in stellar envelopes and cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures), Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ

Fractal Dimension Analysis of Solar Granulation- Boxcounting dimension

The fractal dimension of high resolution Hinode solar granulation observations and numerical simulations is studied and the results are compared. These observations are not influenced by atmospheric seeing conditions and therefore allow a more realistic estimate of the fractal dimension than in previous works. Though arriving at similar results for observations and simulation data, non integer fractal dimension $<2$, some differences in the numerical values occur, and these are discussed

The size distribution of magnetic bright points derived from Hinode/SOT observations

Context. Magnetic Bright Points (MBPs) are small-scale magnetic features in the solar photosphere. They may be a possible source of coronal heating by rapid footpoint motions that cause magnetohydrodynamical waves. The number and size distribution are of vital importance in estimating the small scale-magnetic-field energy. Aims. The size distribution of MBPs is derived for G-band images acquired by the Hinode/SOT instrument. Methods. For identification purposes, a new automated segmentation and identification algorithm was developed. Results. For a sampling of 0.108 arcsec/pixel, we derived a mean diameter of (218 +- 48) km for the MBPs. For the full resolved data set with a sampling of 0.054 arcsec/pixel, the size distribution shifted to a mean diameter of (166 +- 31) km. The determined diameters are consistent with earlier published values. The shift is most probably due to the different spatial sampling. Conclusions. We conclude that the smallest magnetic elements in the solar photosphere cannot yet be resolved by G-band observations. The influence of discretisation effects (sampling) has also not yet been investigated sufficiently.Comment: Astronomy and Astrophysics, Volume 498, Issue 1, 2009, pp.289-29

High-resolution models of solar granulation: the 2D case

Using grid refinement, we have simulated solar granulation in 2D. The refined region measures 1.97*2.58 Mm (vertical*horizontal). Grid spacing there is 1.82*2.84 km. The downflows exhibit strong Kelvin-Helmholtz instabilities. Below the photosphere, acoustic pulses are generated. They proceed laterally (in some cases distances of at least the size of our refined domain) and may be enhanced when transversing downflows) as well as upwards where, in the photosphere they contribute significantly to 'turbulence' (velocity gradients, etc.) The acoustic pulses are ubiquitous in that at any time several of them are seen in our high-resolution domain. Their possible contributions to p-mode excitation or heating of the chromosphere needs to be investigated

Vortices in simulations of solar surface convection

We report on the occurrence of small-scale vortices in simulations of the convective solar surface. Using an eigenanalysis of the velocity gradient tensor, we find the subset of high vorticity regions in which the plasma is swirling. The swirling regions form an unsteady, tangled network of filaments in the turbulent downflow lanes. Near-surface vertical vortices are underdense and cause a local depression of the optical surface. They are potentially observable as bright points in the dark intergranular lanes. Vortex features typically exist for a few minutes, during which they are moved and twisted by the motion of the ambient plasma. The bigger vortices found in the simulations are possibly, but not necessarily, related to observations of granular-scale spiraling pathlines in "cork animations" or feature tracking.Comment: 11 pages, 13 figures, accepted for publication in A&A, complementary movies at http://www.mps.mpg.de/homes/moll/strudel/papermovies

Dynamics of isolated magnetic bright points derived from Hinode/SOT G-band observations

Small-scale magnetic fields in the solar photosphere can be identified in high-resolution magnetograms or in the G-band as magnetic bright points (MBPs). Rapid motions of these fields can cause magneto-hydrodynamical waves and can also lead to nanoflares by magnetic field braiding and twisting. The MBP velocity distribution is a crucial parameter for estimating the amplitudes of those waves and the amount of energy they can contribute to coronal heating. The velocity and lifetime distributions of MBPs are derived from solar G-band images of a quiet sun region acquired by the Hinode/SOT instrument with different temporal and spatial sampling rates. We developed an automatic segmentation, identification and tracking algorithm to analyse G-Band image sequences to obtain the lifetime and velocity distributions of MBPs. The influence of temporal/spatial sampling rates on these distributions is studied and used to correct the obtained lifetimes and velocity distributions for these digitalisation effects. After the correction of algorithm effects, we obtained a mean MBP lifetime of (2.50 +- 0.05) min and mean MBP velocities, depending on smoothing processes, in the range of (1 - 2) km/s. Corrected for temporal sampling effects, we obtained for the effective velocity distribution a Rayleigh function with a coefficient of (1.62 +- 0.05) km/s. The x- and y- components of the velocity distributions are Gaussians. The lifetime distribution can be fitted by an exponential function.Comment: Astronomy and Astrophysics (in press

Re II and Other Exotic Spectra in HD 65949

Powerful astronomical spectra reveal an urgent need for additional work on atomic lines, levels, and oscillator strengths. The star HD 65949 provides some excellent examples of species rarely identified in stellar spectra. For example, the Re II spectrum is well developed, with 17 lines between 3731 and 4904 [A], attributed wholly or partially to Re II. Classifications and oscillator strengths are lacking for a number of these lines. The spectrum of Os II is well identified. Of 14 lines attributed wholly or partially to Os II, only one has an entry in the VALD database. We find strong evidence that Te II is present. There are NO Te II lines in the VALD database. Ru II is clearly present, but oscillator strengths for lines in the visual are lacking. There is excellent to marginal evidence for a number of less commonly identified species, including Kr II, Nb II, Sb II, Xe II, Pr III, Ho III, Au II, and Pt II (probably Pt-198), to be present in the spectrum of HD 65949. The line Hg II at 3984 [A] is of outstanding strength, and all three lines of Multiplet 1 of Hg I are present, even though the surface temperature of HD 65949 is relatively high. Finally, we present the case of an unidentified, 24 [mA], line at 3859.63 [A], which could be the same feature seen in magnetic CP stars. It is typically blended with a putative U II line used in cosmochronology.Comment: ASOS9 Poster (Lund, Sweden, August 2007), to be published in Journal of Physics: Conference Series (JPCS), 6 pages 1 figur