72 research outputs found
3D-simulation of the Outer Convection-zone of an A-star
The convection code of Nordlund & Stein has been used to evaluate the 3D,
radiation-coupled convection in a stellar atmosphere with Teff=7300K, logg=4.3
and [Fe/H]=0.0, corresponding to a main-sequence A9-star. I will present
preliminary comparisons between the 3D-simulation and a conventional 1D stellar
structure calculation, and elaborate on the consequences of the differences.Comment: 6 pages, 2 figures to appear in "The A Star Puzzle", IAU Symp. 224,
J. Zverko, W. W. Weiss, J. Ziznovsky & S. J. Adelman eds., Cambridge Univ.
Pres
Improved phenomenological equation of state in the chemical picture
I present an overview of an equation of state, being developed in the
chemical picture, and based on the very successful MHD equation of state. The
flexibility of the chemical picture combined with the free-energy minimization
procedure, makes it rather straight-forward, albeit laborious, to include new
effects in the model free-energy, simply by adding new terms.
The most notable additions to the original MHD equation of state, are
relativistic effects, quantum effects, improved higher order Coulomb terms and
a long list of molecules other than the H2 and H2+ treated so far.Comment: 7 pages, 3 figure
A Grid of 3D Stellar Atmosphere Models of Solar Metallicity: I. General Properties, Granulation and Atmospheric Expansion
Present grids of stellar atmosphere models are the workhorses in interpreting
stellar observations, and determining their fundamental parameters. These
models rely on greatly simplified models of convection, however, lending less
predictive power to such models of late type stars.
We present a grid of improved and more reliable stellar atmosphere models of
late type stars, based on deep, 3D, convective, stellar atmosphere simulations.
This grid is to be used in general for interpreting observations, and improve
stellar and asteroseismic modeling.
We solve the Navier Stokes equations in 3D and concurrent with the radiative
transfer equation, for a range of atmospheric parameters, covering most of
stellar evolution with convection at the surface. We emphasize use of the best
available atomic physics for quantitative predictions and comparisons with
observations.
We present granulation size, convective expansion of the acoustic cavity,
asymptotic adiabat, as function of atmospheric parameters. These and other
results are also available in electronic form.Comment: 16 pages, 12 figures. Accepted for publication in ApJ, 201
Improvements to Stellar Structure Models, Based on a Grid of 3D Convection Simulations. I. -Relations
Relations between temperature, T, and optical depth, tau, are often used for
describing the photospheric transition from optically thick to optically thin
in stellar structure models. We show that this is well justified, but also that
currently used T(tau) relations are often inconsistent with their
implementation. As an outer boundary condition on the system of stellar
structure equations, T(tau) relations have an undue effect on the overall
structure of stars. In this age of precision asteroseismology, we need to
re-assess both the method for computing and for implementing T(tau) relations,
and the assumptions they rest on. We develop a formulation for proper and
consistent evaluation of T(tau) relations from arbitrary 1D or 3D stellar
atmospheres, and for their implementation in stellar structure and evolution
models. We extract radiative T(tau) relations, as described by our new
formulation, from 3D simulations of convection in deep stellar atmospheres of
late-type stars from dwarfs to giants. These simulations employ realistic
opacities and equation of state, and account for line-blanketing. For
comparison, we also extract T(tau) relations from 1D MARCS model atmospheres
using the same formulation. T(tau)-relations from our grid of 3D convection
simulations display a larger range of behaviours with surface gravity, compared
with those of conventional theoretical 1D hydrostatic atmosphere models. Based
on this, we recommend no longer to use scaled solar T(tau) relations. Files
with T(tau) relations for our grid of simulations are made available to the
community, together with routines for interpolating in this irregular grid. We
also provide matching tables of atmospheric opacity, for consistent
implementation in stellar structure models.Comment: 18 pages, 7 figures, 2 tables. Accepted for publication in MNRAS,
201
Improvements to stellar structure models, based on a grid of 3D convection simulations. II. Calibrating the mixing-length formulation
We perform a calibration of the mixing length of convection in stellar
structure models against realistic 3D radiation-coupled hydrodynamics (RHD)
simulations of convection in stellar surface layers, determining the adiabat
deep in convective stellar envelopes.
The mixing-length parameter is calibrated by matching averages of
the 3D simulations to 1D stellar envelope models, ensuring identical atomic
physics in the two cases. This is done for a previously published grid of
solar-metallicity convection simulations, covering from 4200 K to 6900 K on the
main sequence, and 4300-5000 K for giants with logg=2.2.
Our calibration results in an varying from 1.6 for the warmest
dwarf, which is just cool enough to admit a convective envelope, and up to 2.05
for the coolest dwarfs in our grid. In between these is a triangular plateau of
~ 1.76. The Sun is located on this plateau and has seen little change
during its evolution so far. When stars ascend the giant branch, they largely
do so along tracks of constant , with decreasing with
increasing mass.Comment: 22 pages, 15 figures, accepted for publication in MNRA
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