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.Comment: Accepted for A&

A simulation of solar convection at supergranulation scale

We present here numerical simulations of surface solar convection which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of 315$\times315\times$82, which is used to investigate the dynamics of scales larger than granulation. No structure resembling supergranulation is present; possibly higher Reynolds numbers (i.e. higher numerical resolution), or magnetic fields, or greater depth are necessary. The results also show interesting aspects of granular dynamics which are briefly presented, like extensive p-mode ridges in the k-$\omega$ diagram and a ringlike distribution of horizontal vorticity around granules. At large scales, the horizontal velocity is much larger than the vertical velocity and the vertical motion is dominated by p-mode oscillations.Comment: Contribution to the proceedings of the workshop entitled "THEMIS and the new frontiers of solar atmosphere dynamics" (March 2001), 6 pages, to appear in Nuovo Cimento

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.Comment: Accepted for publication in ApJ (scheduled for Dec 10, 2000 issue). 17 pages, 27 figures, some with reduced resolution -- high resolution versions available at http://www.astro.ku.dk/~aake/astro-ph/0008048

Phase-Dependent Properties of Extrasolar Planet Atmospheres

Recently the Spitzer Space Telescope observed the transiting extrasolar planets, TrES-1 and HD209458b. These observations have provided the first estimates of the day side thermal flux from two extrasolar planets orbiting Sun-like stars. In this paper, synthetic spectra from atmospheric models are compared to these observations. The day-night temperature difference is explored and phase-dependent flux densities are predicted for both planets. For HD209458b and TrES-1, models with significant day-to-night energy redistribution are required to reproduce the observations. However, the observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical Journa

Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2

We report on the observation of nanoscale density fluctuations in 2 μm thick amorphous SiO₂ layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5 × 10¹² ions/cm², where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track “annihilation” process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.The authors acknowledge the Australian Research Council and the Australian Synchrotron Research Program for financial support and thank the staff at the ANU Heavy Ion facility for their continued technical assistance. O.P., F.D., and K.N. acknowledge financial support from the Academy of Finland under its Centre of Excellence program as well as the OPNA project, and grants of computer capacity from CSC

A Solution to the Protostellar Accretion Problem

Accretion rates of order 10^-8 M_\odot/yr are observed in young protostars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for protostars of smaller mass, approximately proportional to the second power of the stellar mass, \dot{M}_accr\propto M^2. The traditional view is that the observed accretion is the consequence of the angular momentum transport in isolated protostellar disks, controlled by disk turbulence or self--gravity. However, these processes are not well understood and the observed protostellar accretion, a fundamental aspect of star formation, remains an unsolved problem. In this letter we propose the protostellar accretion rate is controlled by accretion from the large scale gas distribution in the parent cloud, not by the isolated disk evolution. Describing this process as Bondi--Hoyle accretion, we obtain accretion rates comparable to the observed ones. We also reproduce the observed dependence of the accretion rate on the protostellar mass. These results are based on realistic values of the ambient gas density and velocity, as inferred from numerical simulations of star formation in self--gravitating turbulent clouds.Comment: 4 pages, 2 figures, ApJ Letters, in pres

Origin of atomic clusters during ion sputtering

Previous studies have shown that the size distributions of small clusters ( n<=40 n = number of atoms/cluster) generated by sputtering obey an inverse power law with an exponent between -8 and -4. Here we report electron microscopy studies of the size distributions of larger clusters ( n>=500) sputtered by high-energy ion impacts. These new measurements also yield an inverse power law, but one with an exponent of -2 and one independent of sputtering yield, indicating that the large clusters are produced when shock waves, generated by subsurface displacement cascades, ablate the surface

NEAR-SURFACE EFFECTS IN MODELLING OSCILLATIONS OF ETA BOO

Following the report of solar-like oscillations in the G0 V star eta Boo (Kjeldsen et al. 1995, AJ 109, 1313), a first attempt to model the observed frequencies was made by Christensen-Dalsgaard et al. (1995, ApJ Letters, in press). This attempt succeeded in reproducing the observed frequency separations, although there remained a difference of about 10 microHz between observed and computed frequencies. In those models, the near-surface region of the star was treated rather crudely. Here we consider more sophisticated models that include non-local mixing-length theory, turbulent pressure and nonadiabatic oscillations.Comment: uuencoded and compressed Postscript (2 pages, including figure); To appear in Proceedings of IAU Colloquium 155, "Astrophysical Applications of Stellar Pulsation", Cape Town, South Afric

Probing 5f-state configurations in URu2Si2 with U L3-edge resonant x-ray emission spectroscopy

Resonant x-ray emission spectroscopy (RXES) was employed at the U L3 absorption edge and the La1 emission line to explore the 5f occupancy, nf, and the degree of 5f orbital delocalization in the hidden order compound URu2Si2. By comparing to suitable reference materials such as UF4, UCd11, and alpha-U, we conclude that the 5f orbital in URu2Si2 is at least partially delocalized with nf = 2.87 +/- 0.08, and does not change with temperature down to 10 K within the estimated error. These results place further constraints on theoretical explanations of the hidden order, especially those requiring a localized f2 ground state.Comment: 11 pages,7 figure

A simulation of solar convection of supergranulation scale

We present here numerical simulations of surface solar convection which cover a box of 30×30×3.2 Mm3 with a resolutionof 315×315×82, which is used to investigate the dynamics of scales larger than granulation. No structure resembling supergranulation is present; possibly higher Reynolds numbers (i.e. higher numerical resolution), or magnetic fields, or greater depth are necessary. The results also show interesting aspects of granular dynamics which are briefly presented, like extensive p-mode ridges inthe k-ω diagram and a ringlike distribution of horizontal vorticity around granules. At large scales, the horizontal velocity is much larger than the vertical velocity and the vertical motion is dominated by p-mode oscillations