Astronomers 'listen' to an exoplanet-host star and find its birthplace (Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Dr. Henri Boffin
ESO Press Officer
Phone: +49 89 3200 6222

Science Contacts:
Sylvie Vauclair
Laboratoire d'Astrophysique de Toulouse-Tarbes
CNRS/Universite de Toulouse, France
Phone: +33 (0)5 61 33 29 50

For Immediate Release: 15 April 2008

ESO Science Release 09/08

The Drifting Star

Astronomers 'listen' to an exoplanet-host star and find its birthplace

By studying in great detail the 'ringing' of a planet-harbouring star, a
team of astronomers using ESO's 3.6-m telescope have shown that it must have
drifted away from the metal-rich Hyades cluster. This discovery has
implications for theories of star and planet formation, and for the dynamics
of our Milky Way.

The yellow-orange star Iota Horologii, located 56 light-years away towards
the southern Horologium ("The Clock") constellation, belongs to the
so-called "Hyades stream", a large number of stars that move in the same
direction.

Previously, astronomers using an ESO telescope had shown that the star
harbours a planet, more than 2 times as large as Jupiter and orbiting in 320
days (ESO 12/99).

But until now, all studies were unable to pinpoint the exact characteristics
of the star, and hence to understand its origin. A team of astronomers, led
by Sylvie Vauclair from the University of Toulouse, France, therefore
decided to use the technique of 'asteroseismology' to unlock the star's
secrets.

"In the same way as geologists monitor how seismic waves generated by
earthquakes propagate through the Earth and learn about the inner structure
of our planet, it is possible to study sound waves running through a star,
which forms a sort of large, spherical bell," says Vauclair.

The 'ringing' from this giant musical instrument provides astronomers with
plenty of information about the physical conditions in the star's interior.

And to 'listen to the music', the astronomers used one of the best
instruments available. The observations were conducted in November 2006
during 8 consecutive nights with the state-of-the-art HARPS spectrograph
mounted on the ESO 3.6-m telescope at La Silla.

Up to 25 'notes' could be identified in the unique dataset, most of them
corresponding to waves having a period of about 6.5 minutes.

These observations allowed the astronomers to obtain a very precise portrait
of Iota Horologii: its temperature is 6150 K, its mass is 1.25 times that of
the Sun, and its age is 625 million years. Moreover, the star is found to be
more metal-rich than the Sun by about 50%.

"These results show the power of asteroseismology when using a very precise
instrument such as HARPS," says Vauclair. "It also shows that Iota Horologii
has the same metal abundance and age as the Hyades cluster and this cannot
be a coincidence."

The Hyades is an ensemble of stars that is seen with the unaided eye in the
Northern constellation Taurus ("The Bull"). This open cluster, located 151
light-years away, contains stars that were formed together 625 million years
ago.

The star Iota Horologii must have thus formed together with the stars of the
Hyades cluster but must have slowly drifted away, being presently more than
130 light-years away from its original birthplace. This is an important
result to understand how stars move on the galactic highways of the Milky
Way.

This also means that the amount of metals present in the star is due to the
original cloud from which it formed and not because it engulfed planetary
material. "The chicken and egg question of whether the star got planets
because it is metal-rich, or whether it is metal-rich because it made
planets that were swallowed up is at least answered in one case," says
Vauclair.

More information

The astronomers' study is being published as a Letter to the Editor in
Astronomy and Astrophysics ("The exoplanet-host star iota Horologii: an
evaporated member of the primordial Hyades cluster", by S. Vauclair et al.).
The team is composed of Sylvie Vauclair, Marion Laymand, Gerard Vauclair,
Alain Hui Bon Hoa, and Stephane Charpinet (LATT, Toulouse, France), Franis
Bouchy (IAP, Paris, France), and Michael Bazot (University of Porto,
Portugal).

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