1,854 research outputs found
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 303.2 Mm with a resolution of
31582, 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- 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
Channeling maps for Si ions in Si : Assessing the binary collision approximation
Simulations based on the binary collision approximation (BCA) are in principle less accurate than molecular dynamics (MD) simulations. In this work, we present a comprehensive comparison between BCA and MD for Si ions impinging on a (001)-Si surface by comparing "channeling maps", i.e., projected ranges of the ions as a function of incidence direction in a representative part of the angular space. We find quantitative differences to develop as the energy decreases below similar to 100 eV, but find qualitative agreement down to similar to 10 eV. Moreover, the quality of the BCA channeling maps depends on the implementation of the BCA, which is explained in terms of double-hits and missed collisions.Peer reviewe
Sputtering yields exceeding 1000 by 80keV Xe irradiation of Au nanorods
Using experiments and computer simulations, we find that 80 keV Xe ion irradiation of Au nanorods can produce sputtering yields exceeding 1000, which to our knowledge are the highest yields reported for sputtering by single ions in the nuclear collision regime. This value is enhanced by more than an order of magnitude compared to the same irradiation of flat Au surfaces. Using MD simulations, we show that the very high yield can be understood as a combination of enhanced yields due to low incoming angles at the sides of the nanowire, as well as the high surface-to-volume ratio causing enhanced explosive sputtering from heat spikes. We also find, both in experiments and simulations, that channeling has a strong effect on the sputtering yield: if the incoming beam happens to be aligned with a crystal axis of the nanorod, the yield can decrease to about 100
Numerical simulations of surface convection in a late M-dwarf
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD)
simulations of time-dependent compressible convection, we have studied the
dynamics and thermal structure of the convective surface layers of a
prototypical late-type M-dwarf (Teff~2800K log(g)=5.0, solar chemical
composition). The RHD models predict stellar granulation qualitatively similar
to the familiar solar pattern. Quantitatively, the granular cells show a
convective turn-over time scale of ~100s, and a horizontal scale of 80km; the
relative intensity contrast of the granular pattern amounts to 1.1%, and
root-mean-square vertical velocities reach 240m/s at maximum. Deviations from
radiative equilibrium in the higher, formally convectively stable atmospheric
layers are found to be insignificant allowing a reliable modeling of the
atmosphere with 1D standard model atmospheres. A mixing-length parameter of
alpha=2.1 provides the best representation of the average thermal structure of
the RHD model atmosphere while alternative values are found when fitting the
asymptotic entropy encountered in deeper layers of the stellar envelope
alpha=1.5, or when matching the vertical velocity field alpha=3.5. The close
correspondence between RHD and standard model atmospheres implies that
presently existing discrepancies between observed and predicted stellar colors
in the M-dwarf regime cannot be traced back to an inadequate treatment of
convection in the 1D standard models. The RHD models predict a modest extension
of the convectively mixed region beyond the formal Schwarzschild stability
boundary which provides hints for the distribution of dust grains in cooler
(brown dwarf) atmospheres.Comment: 19 pages, 16 figures, accepted for publication in A&
Hydrodynamical model atmospheres and 3D spectral synthesis
We discuss three issues in the context of three-dimensional (3D)
hydrodynamical model atmospheres for late-type stars, related to spectral line
shifts, radiative transfer in metal-poor 3D models, and the solar oxygen
abundance. We include a brief overview about the model construction, taking the
radiation-hydrodynamics code CO5BOLD (COnservative COde for the COmputation of
COmpressible COnvection in a BOx of L Dimensions with L=2,3) and the related
spectral synthesis package Linfor3D as examples.Comment: 6 pages, 2 figures, to appear in the Proceedings of the
ESO/Lisbon/Aveiro Workshop "Precision Spectroscopy in Astrophysics", eds. L.
Pasquini, M. Romaniello, N.C. Santos, and A. Correi
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
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.Comment: to appear in the proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
The response of a turbulent accretion disc to an imposed epicyclic shearing motion
We excite an epicyclic motion, whose amplitude depends on the vertical
position, , in a simulation of a turbulent accretion disc. An epicyclic
motion of this kind may be caused by a warping of the disc. By studying how the
epicyclic motion decays we can obtain information about the interaction between
the warp and the disc turbulence. A high amplitude epicyclic motion decays
first by exciting inertial waves through a parametric instability, but its
subsequent exponential damping may be reproduced by a turbulent viscosity. We
estimate the effective viscosity parameter, , pertaining to
such a vertical shear. We also gain new information on the properties of the
disc turbulence in general, and measure the usual viscosity parameter,
, pertaining to a horizontal (Keplerian) shear. We find that,
as is often assumed in theoretical studies, is approximately
equal to and both are much less than unity, for the field
strengths achieved in our local box calculations of turbulence. In view of the
smallness () of and we conclude
that for the timescale for diffusion
or damping of a warp is much shorter than the usual viscous timescale. Finally,
we review the astrophysical implications.Comment: 12 pages, 18 figures, MNRAS accepte
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
- …