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

ESO Education and Public Relations Dept.

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

Science Contacts:
Sylvie Vauclair
Laboratoire d'Astrophysique de Toulouse-Tarbes
CNRS/Université 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, Gérard
Vauclair, Alain Hui Bon Hoa, and Stéphane Charpinet (LATT, Toulouse,
France), François Bouchy (IAP, Paris, France), and Michaël Bazot
(University of Porto, Portugal).

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