University of Arizona astronomers have used a new technique called nulling
interferometry to probe a dust disk around a young nearby star for the first
time. They not only confirmed that the young star does have a protoplanetary
disk — the stuff from which solar systems are born — but discovered a
gap in the disk, which is strong evidence of a forming planet.

“It’s very exciting to find a star that we think should be forming planets,
and actually see evidence of that happening,” said UA astronomer Philip
Hinz.

“The bottom line is, we not only confirmed the hypothesis that this young
star has a protoplanetary disk, we found evidence that a giant,
Jupiter-like
protoplanet is forming in this disk,” said Wilson Liu, a doctoral student
and research assistant on the project.

“There’s evidence that this star is right on the cusp of becoming a
main-sequence star,” Liu added. “So basically, we’re catching a star that
is
right at the point of becoming a main-sequence star, and it looks like it’s
caught in the act of forming planets.”

Main-sequence stars are those like our sun that burn hydrogen at their
cores.

Earlier this year, Hinz and Liu realized that observations of HD 100546 at
thermal, or mid-infrared, wavelengths showed that the star had a dust disk.

Finding faint dust disks is “analogous to finding a lighted flashlight next
to Arizona Stadium when the lights are on,” Liu said.

The nulling technique combines starlight in such a way that it is canceled
out, creating a dark background where the star’s image normally would be.
Because HD 100546 is such a young star, its dust disk is still relatively
bright, about as bright as the star itself. The nulling technique is needed
to distinguish what light comes from the star, which can be suppressed, and
what comes from the extended dust disk, which nulling does not suppress.

Hinz and UA astronomers Michael Meyer, Eric Mamajek, and William Hoffmann
took the observations in May 2002. They used BLINC, the only working
nulling
interferometer in the world, along with MIRAC, a state-of-the-art
mid-infrared camera, on the 6.5-meter (21-foot) diameter Magellan telescope
in Chile to study the roughly 10-million-year-old star in the Southern
Hemisphere sky.

Typically, dust in disks around stars is uniformly distributed, forming a
continuous, flattened, orbiting cloud of material that is hot on the inner
edge but cold most of the distance to the frigid outer edge.

“The data reduction was complicated enough that we didn’t realize until
later that there was an inner gap in the disk,” Hinz noted.

“We realized the disk appeared about the same size at warmer (10 micron)
wavelengths and at colder (20 micron) wavelengths. The only way that could
be is if there’s an inner gap.”

The most likely explanation for this gap is that it is created by the
gravitational field of a giant protoplanet =AD an object that could be
several
times more massive than Jupiter. The researchers believe the protoplanet
may
be orbiting the star at perhaps 10 AU. (An AU, or astronomical unit, is the
distance between Earth and the sun. Jupiter is about 5 AU from the sun.)

Astronomers from the Netherlands and Belgium had previously used the
Infrared Space Observatory to study HD 100546, which is 330 light-years
from
Earth. They detected comet-like dust around the star and concluded that it
might be a protoplanetary disk. But the European space telescope was too
small to clearly see dust surrounding the star.

Hinz, who developed BLINC, has been using the nulling interferometer with
two 6.5-meter telescopes for the past three years for his survey of nearby
stars in search of protoplanetary systems. In addition to the Magellan
telescope that covers the Southern Hemisphere, Hinz uses the 6.5-meter
UA/Smithsonian MMT atop Mount Hopkins, Ariz., for the Northern Hemisphere
sky.=20

Hinz developed BLINC as a technology demonstration for the Terrestrial
Planet Finder mission, which is managed for NASA by the Jet Propulsion
Laboratory, Pasadena, Calif. NASA, which funds Hinz’ survey, supports
research on solar-system formation under its Origins program and is
developing nulling interferometry for Terrestrial Planet Finder.

“Nulling interferometry is very exciting because it is one of only a few
technologies that can directly image circumstellar environments,” Liu said.

Using MIRAC, the camera developed by William Hoffmann and others, was
important because it is sensitive to mid-infrared wavelengths, Hinz said.
Astronomers will have to look in mid-infrared wavelengths, which correspond
to room temperatures, to find planets with liquid water and possible life,
he said.

Hinz’ survey includes HD 100546 and other “Herbig Ae” stars, which are
nearby young stars generally more massive than our sun, but are not yet
main sequence stars powered by nuclear fusion.

Hinz and Liu plan to observe increasingly mature star systems, searching
for
ever-fainter circumstellar dust disks and planets, as they continue to
improve nulling interferometry and adaptive optics technologies. Adaptive
optics is a technique that eliminates the effects of Earth’s shimmering
atmosphere from starlight.

Hinz and others at UA Steward Observatory are designing a nulling
interferometer for the Large Binocular Telescope, which will view the sky
with two 8.4-meter (27-foot) diameter mirrors on Mount Graham, Ariz., in
2005.

The UA team is reporting the research in Astrophysical Journal Letters and
also will present a paper on the research at the American Astronomical
Society meeting in Atlanta, Ga., in January 2004.

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