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.
Drifters
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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