By meticulously studying the 'ringing' of a nearby planet-harboring star, a team of astronomers using the 3.6-m telescope of the European Southern Observatory (ESO) have demonstrated that the star must have drifted away from the Hyades star cluster. This discovery has implications for the studies of star and planet formation, and for stellar dynamics.

The yellow-orange star, known as Iota Horologii, is located 56 light-years away in the southern constellation Horologium, “The Clock”. Interestingly, it belongs to a category of stars termed the "Hyades stream", a large number of stars that move in the same direction of the Hyades star cluster.

The Hyades is a nearby star cluster, consisting of gravitationally-bound stars, moving in the same direction with similar velocities. The stars of the Hyades are believed to have formed 625 million years ago. The Hyades is located 151 light years away. It is part of the zodiacal constellation Taurus, “The Bull”. The prominent stars of the Hyades form the V-shaped head of the Bull. They are visible to the unaided eye.

Previous research work, conducted using an ESO telescope, had revealed that Iota Horologii is orbited by a planet, over two times larger than Jupiter, and lasts 320 days to orbit its parent star (ESO press release 12/99).

However, all earlier studies were unable to determine the exact characteristics of the star, and hence to find clues to its origin. A team of astronomers, led by Sylvie Vauclair (University of Toulouse), used the technique of 'asteroseismology', a technique similar to the geological studies of seismic waves induced by earthquakes, to unravel 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," said Vauclair.

The 'ringing' from this cosmic musical instrument yields plenty of information about the physical conditions in the interior of the star. To 'listen’ to this stellar music, the astronomers used one of the best devices 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 data 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 about 50% more abundant in metals than the Sun. (In astronomy, metals are all the elements heavier than hydrogen and helium.)

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

Therefore, the star Iota Horologii may have been a member of the Hyades cluster, but could have slowly drifted away, being currently over 130 light-years away from the cluster. This is an interesting conclusion for the studies of the stellar motions in the Milky Way Galaxy.

This also means that the amount of metals present in Iota Horologii 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," concluded Vauclair.

Further Reading

ESO
www.eso.org/

Aymen Mohamed Ibrahem
Senior Astronomy Specialist