The drifting star
BY EMILY BALDWIN
ASTRONOMY NOW
Posted: April 16, 2008

By examining the ‘ringing’ of a planet-harboring star, a team of astronomers has shown that it has drifted away from its original birthplace, providing important information about the dynamics of our Milky Way and theories of star and planet formation.

Astronomers using the ESO telescopes already know that the yellow-orange star lota Horologii, located 56 light-years away in the Horologium constellation and belonging to the
‘Hyades stream’, a large number of stars that move in the same direction, harbors a planet more than twice the size of Jupiter. But until now, the secrets of the star's origin and evolution remained hidden. Using a technique called asteroseismology, which is similar to the way geologists monitor seismic waves generated by earthquakes that propagate through the Earth, but instead applied to the star, the astronomers can study the sound waves running through the star. “The star forms a sort of large, spherical bell,” says lead astronomer Sylvie Vauclair from the University of Toulouse, France.

A graphical representation of resonating acoustic waves in the interior of a solar-like star. By recording the behaviour of the reflections, astronomers can deduce the interior properties of the star. Image: ESO.

The ‘ringing’ from this stellar musical instrument is translated into the physical conditions of the star’s interior by listening to the music using the aptly named HARP spectrograph which is mounted on the ESO 3.6 metre telescope at La Silla. Up to 25 different notes were identified in the study, corresponding to wave periods of 6.5 minutes, which allowed the astronomers to paint a very precise picture of lota Horologii: its temperature is 6150 degrees Kelvin, it has a mass 1.25 times that of the Sun and it is 625 million years old. In addition, it is over 50 percent more metal-rich than our Sun.

“Lota Horologii has the same metal abundance and age as the Hyades cluster and this cannot be a coincidence,” says Vauclair. The Hyades is an ensemble of 625 million year old stars that can be seen with the naked eye in the constellation of Taurus. The astronomers deduce, therefore, that the star lota Horologii must have formed together with the stars of the Hyades cluster but has since slowly drifted away to its present location 130 light years away from its original birthplace.

Not only do the results help to understand how the stellar traffic moves on the galactic highways of the Milky Way galaxy, but it 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 later on. "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.