"Van Gogh" simulations give new insight into turbulent stars(Forwarded)

Royal Astronomical Society
London, U.K.

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Robert Massey
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AND
Anita Heward
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NATIONAL ASTRONOMY MEETING PRESS ROOM (16 - 20 APRIL ONLY):
Tel: +44 (0)1772 892 613
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RAS National Astronomy Meeting web site:
http://nam2007.uclan.ac.uk

CONTACTS:

Dr Falk Herwig
Keele Astrophysics Group
Keele University
Staffordshire
Tel: +44 (0)1782 584211

PRESS INFORMATION NOTE: RAS PN 07/19 (NAM 15)

EMBARGOED FOR 00:01 BST, WEDNESDAY, 18 APRIL 2007

VAN GOGH SIMULATIONS GIVE NEW INSIGHT INTO TURBULENT STARS

Stunning simulations that give a multi-dimensional glimpse into the
interior of stars show that material bubbling around the convection zone
induces a rich spectrum of internal gravity waves in the stable layers
above and below.

The swirling simulations, which are reminiscent of Van Gogh's star
paintings, show the interior of a star during the Asymptotic Giant Branch
(AGB) phase, the final stage of a low- and intermediate-mass star's life
before it becomes a white dwarf. It is during this phase that many of the
heavier elements, such as carbon and sodium, are created inside the stars.
The core of the star is surrounded by shells of helium and hydrogen, in
which nuclear fusion periodically switches off and on in a process called
Helium-Shell Flashes, triggering increased bursts of mixing and heating.
The models focus on the turbulent convection zone in the helium shell and
show how it interacts with the stable, stratified layers above and below.

Dr Falk Herwig said, "Until recently we've only had one-dimensional models
of the interior. That's very different from being able to work out in
detail and 2 or 3 dimensions what's going on deep inside and seeing how
the different layers interact. The three-dimensional simulation is so
complicated that we have been using as much computing time as some of the
large cosmological simulations. This is the first time that we've been
able to see and actually measure the internal gravity waves that are
caused by the convective motions in the unstable layer."

The simulations show that there is minimal penetration of material into
adjoining layers from the convection zone but gravitational mode
oscillations with a dominating horizontal velocity component, induced at
the convective boundaries do cause some of mixing across layers.

Helium-shell flash convection is dominated by large convective cells that
are centered in the lower half of the convection zone. The animations show
entropy or temperature fluctuations which start off as small bumps at the
bottom of the convection zone and expand upwards, developing mushroom-like
instabilities that merge into large-scale features. The stable regions
above and below are filled with horizontal waves, which are induced almost
as soon as the convection plumes start to grow. This means that the
gravity waves are not created by the plumes actually hitting the
boundaries, but are induced by the build-up of pressure above the plumes.

NOTES FOR EDITORS

The 2007 RAS National Astronomy Meeting is hosted by the University of
Central Lancashire. It is sponsored by the Royal Astronomical Society and
the UK Science and Technology Facilities Council.

This year the NAM is being held together with the UK Solar Physics (UKSP)
and Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring
meetings. 2007 is International Heliophysical Year.

FURTHER INFORMATION

Entropy

Entropy is proportional to the ratio of pressure and density and here a
measure of the buoyancy of the fluid elements. A volume element with
entropy excess compared to the average at that radial position will rise.
In other words, a negative entropy gradient leads to convective
instability.

IMAGES

Stills from the animations can be found at
http://www.astro.keele.ac.uk/~fherwig/shydro.html

ANIMATIONS

3-Dimensional animations can be found at
http://www.astro.keele.ac.uk/~fherwi...mcr3-Vy-x7.gif
Caption: Three-dimensional animations are used to study mixing processes
at and across convective boundaries. The animation shows vertical velocity
in the top 1200km of helium-shell flash convection (blue is up velocity
and yellow is down).

2-Dimensional animations can be found at
http://www.astro.keele.ac.uk/~fherwi...ble_dtime2.mpg
http://www.astro.keele.ac.uk/~fherwi...PdT_dtime2.mpg
Caption: The two dimensional hydrodynamics simulation shows entropy
fluctuations: the top and bottom stable layer sandwich a convectively
unstable layer which is driven by high energy injection from nuclear
burning.

Further information and DivX versions of the animations can be found at
http://www.astro.keele.ac.uk/~fherwig/shydro.html

Credits:
The simulations have been performed by F. Herwig, in collaboration with P.
Woodward (U Minnesota) and B. Freytag (ENS Lyon).