Home of Drifting Star Found
Using HARPS on ESO's 3.6-m telescope at La Silla, astronomers were able to study in great detail the star Iota Horologii, known to harbour a giant planet. The measurements showed that the star must have drifted from the Hyades cluster where it formed.
Credit: Digital Sky Survey/VirGO

By listening to the "ringing" of a nearby planet-harboring star, astronomers have for the first time identified the birthplace of one of our galaxy's many drifting stars.

The yellow-orange star Iota Horologii, located 56 light-years away near the southern-sky constellation Horologium ("The Clock"), was discovered to harbor a planet about two times the size of Jupiter in 1999.

But until now, scientists were unable to identify the exact characteristics of the star, or where in the galaxy it had formed.

The star currently resides in the "Hyades stream," a large number of stars that move in the same direction, many of which are thought to be so-called "drifting stars" ? stars that were displaced from their birthplace. The new method used by the team of astronomers to identify Iota Horologii's stellar parentage involves studying how sound waves move through a star.

The approach could be used to ID other orphaned stars ? estimated to make up about 20 percent of the stars within 1,000 light-years of the sun ? and shed more light on how these stars move in the galaxy.

Good vibrations

Sylvie Vauclair of the University of Toulouse in France and her team of astronomers used a technique called "asteroseismology" to unlock the elusive properties of the star.

"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," Vauclair explained.

This "ringing" of the star gives scientists information about the physical conditions in the star's interior. With observations taken from the HARPS spectrograph, which is mounted on the European Southern Observatory's 3.6-meter telescope at La Silla, Chile, up to 25 "notes" were identified by the team.

"You can analyze the sound and analyze the harmonies, and you can get the harmonies of the star," Vauclair said. "And different stars have different harmonies."

These notes gave the astronomers a precise portrait of the star: its temperature is 6,150 Kelvin, its mass is 1.25 times that of the Sun, and it is 625 million years old. It is also 1.5 times as metal-rich as the sun, which was the clue astronomers needed to figure out where the star came from ? the Hyades cluster.

"Iota Horologii has the same metal abundance and age as the Hyades cluster," Vauclair said, adding that "the chance is really low that it's a coincidence."

The team's findings are detailed in a Letter to the Editor in the journal Astronomy and Astrophysics.


Other stellar drifters inhabit these so-called "streams" that travel with the same velocity as nearby clusters, with some of them suspected to have formed in the cluster. In the Hyades stream, previous research has shown that "most of the stars have not been formed in the Hyades," Vauclair said. Only about 15 percent were estimated to come from the Hyades cluster.

"This guy [Iota Horologii] would be one of this 15 percent," Vauclair told SPACE.com. "I think it's the first star for which we have this result."

Vauclair says the star must have formed together with the other stars of the Hyades cluster, but then must have slowly drifted away, to its current spot more than 130 light-years from its birthplace.

Previous research has shown that quite a few stars follow unusual trajectories compared to most stars, which orbit the Milky Way's center. It is thought that the wanderers may be gravitationally stirred by the spiral arms of the galaxy, which could deflect the motions of the stars. The stars may eventually get sucked into other arrangements of stars with more normal paths around the galactic center, which is thought to have happened to most of the stars in the Hyades stream, Vauclair said.

Plenty of other clusters have associated streams, and the same method that Vauclair and her team used to ID Iota Horologii could be used to pinpoint where other stars came from, shedding light on how they got to where they are now, she said.

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