Fig. 1: This movie shows the light emerging from the Sun with its
typical granulation pattern. See also the animation
here
Fig. 2:
A vertical slice through the Sun (with the temperature being colour-coded)
and the over-plotted velocity field (arrows). See also the animation
here
For stars like the Sun, the surface is literally bubbling. The energy
that is produced in the stellar interior through nuclear fusion
reaches the surface through convection - the same phenomenon that is
seen in boiling water. The starlight observed by astronomers with
their telescopes is emitted from this "stellar soup". In order to
interpret the starlight correctly in terms of for example the star's
temperature, size, mass and chemical make-up, it is paramount to
understand the physical processes and convective motions occurring in
the surface layers of stars: the stellar atmospheres. Now scientists
at the Max Planck Institute of Astrophysics have made a major
breakthrough by realistically modelling in 3D the surface layers for a
very wide range of stars using powerful supercomputers. These new
computer models will be very extensively used by astronomers studying
stars as well as the Milky Way and planets around other stars.
The main challenge when modelling stars and their atmospheres is how
to properly simulate the convective heat transfer and its interplay
with the emitted stellar radiation. Traditionally, theoretical
one-dimensional (1D) atmosphere models that are assumed not to change
with time have been used. Such models rely on several major
simplifications that are insufficient for describing the complex
phenomenon that is convection, which clearly takes place in 3D and is
constantly evolving. These models are therefore sometimes quite
unrealistic and provide erroneous results. It is like cooking an
elaborate dinner using a single basic ingredient: the basic structure
is in place but something is clearly missing. A major advantage with
such simplified stellar modelling, however, is that it is
computationally quite cheap, making it possible to simulate many
stars.
With the advent of powerful supercomputers it is now possible to
compute three-dimensional (3D) stellar atmosphere models, in which the
convective motions are followed in time and the interactions between
the stellar plasma and the radiation is followed in detail by solving
the hydrodynamical equations coupled with radiative transfer in 3D. In
these sophisticated 3D models the convective motions arise from first
principles, making the multiple free parameters used in 1D modelling
redundant. The predictive power of such 3D models has been
successfully demonstrated, especially for the Sun, which proves that
the new stellar models are highly realistic. This makes them
applicable to the analysis of starlight for a wide range of
investigations. It is reassuring that astronomers now understand how
convection works in stars and that they can compute models that enable
the accurate determination of stellar properties through the radiation
emitted by stars.
An international team of scientists spearheaded by Zazralt Magic at
the MPA has now computed a large number of 3D stellar atmospheres, by
far the largest and most ambitious undertaking in the field. It is
like a splendid menu of some 250 dishes, all prepared with an
elaborate attention to detail. The new 3D stellar models rely on the
best possible input physics such as the equation of state of the
plasma (the relationship between temperature and pressure) and
opacities (how transparent is the plasma for radiation). The whole
stellar surface is not modelled all at once, but rather a small
representative volume in the atmosphere is followed, from which the
complete picture of the star can be reconstructed in a statistical
sense. Typically each computer simulation follows some 10 convective
cells, so-called granules: the material flowing upwards that is heated from
below.
Using the new 3D stellar models, the MPA scientists have found several
new and interesting scaling relations of global properties with
stellar parameters. For example the intensity contrast between the
warm up-flowing material and the cold down-flows is enhanced at lower
metallicity, and the size of the granules scales with the pressure
close to the surface. The entropy jump, density and vertical velocity
are the components of the convective energy transport, and these
properties also scale with stellar parameters in a clear and
understandable way. Comparison of the spatially and temporally
averaged 3D models with classical 1D models reveal distinctive
systematic differences, highlighting the shortcomings of previous
1D-based analyses.
The range of the possible applications of the 3D stellar models is
enormous. Currently, the team is computing a grid of predicted stellar
spectra from each 3D model as a first application, enabling improved
stellar parameter determination and analysis of the chemical make-up
of stars. These in turn will be very beneficial for on-going and
future large surveys of stars in the Milky Way to trace the formation
and history of our Galaxy.
Improved knowledge of the stellar radiation and how it varies across
the stellar surface will be helpful in the determining precise
parameters of exoplanets from transits — the variation in stellar
brightness when a planet passes in front of its host star. The MPA
group also expects major advances in asteroseismology — the ability
to probe the interior of stars using their vibrations — with new
stellar evolutionary models that rely on the 3D atmosphere models. The
new work represents a major step in the fine art of cooking the
"stellar soup", which will be appreciated by a large number of
astronomers around the world.
Zazralt Magic (MPA), Remo Collet (ANU) and Martin Asplund (ANU)
Reference
Z. Magic, R. Collet, M. Asplund, R. Trampedach, W. Hayek,
A. Chiavassa, R. F. Stein and Å. Nordlund,
"The Stagger-grid: A Grid of 3D Stellar Atmosphere Models I. Methods and general properties",
(submitted to A&A)
http://adsabs.harvard.edu/abs/2013arXiv1302.2621M
Further reading
Nordlund, Å., Stein, R. F., & Asplund, M.,
"Solar Surface Convection",
2009, Living Reviews in Solar Physics, 6, 2
