A rare
sun-like star that is both young and relatively close to Earth has been found
to be harboring an even weirder object – a failed star locked in a close orbit
around its host, according to a new study.
The
newfound failed star, known as a brown
dwarf, has been dubbed PZ Tel B. It is
separated from its sun-like companion star PZ Tel A by a distance similar to
that between Uranus and the sun in our solar system. [Photo of the brown dwarf.]
"Because
PZ Tel A is a rare star being both close and very young, it had been imaged
several times in the past," said research team member Laird Close, a University
of Arizona professor at Steward Observatory. "So we were quite surprised
to see a new companion around what was thought to be a single star."
An
international team of astronomers, led by Beth Biller and Michael Liu of the
University of Hawaii, made the rare find using the Near-Infrared Coronagraphic
Imager (NICI), on the international 8-meter Gemini-South Telescope in Chile. The
brown dwarf is about 36 times the mass of Jupiter, they found.
The
host star PZ Tel A is essentially a younger version of the sun, but while it
has a similar mass, it is much younger, at only 12 million years of age (about
400 times younger than the sun). The star is located approximately 168
light-years away from the sun.
The
two objects are separated by only 18 astronomical units (AU). One astronomical
unit is the average distance between the Earth and the sun, which is
approximately 93 million miles (about 150 million km).
In
the study, Close reanalyzed images of the parent star from 2003, in which the
star's glare obscured the presence of the brown dwarf. That suggests its orbit
is more elliptical, rather than circular, researchers said.
Why
it's special
Brown
dwarfs are odd objects that fall
somewhere between planets and stars because of their temperature and mass. They
are cooler and lighter than stars, but more massive – and typically hotter - than
planets.
Most
young brown dwarfs and their companions found by direct imaging are at orbital
separations of greater than 50 AUs (about 4.6 billion miles). For comparison,
Pluto's highly elliptical orbit ranges between 30 AU (2.7 billion miles) at its
closest to 49 AU (4.5 billion miles) at its most distant, according to a NASA
fact sheet.
So
the relatively small 18 AU (1.6 billion miles) separation between the brown
dwarf PZ Tel B and its partner is striking, researchers said. The astronomers also
observed PZ Tel B moving quickly outward from its host star in a non-circular
orbital path.
"PZ
Tel B travels on a particularly eccentric
orbit – in the last 10 years, we have
literally watched it careen through its inner solar system," said Beth
Biller, lead author of the study. "This can best be explained by a highly
eccentric, or oval-shaped, orbit."
What
this tells us
The
PZ Tel system is young enough to still possess significant amounts of cold
circumstellar dust, which may have been sculpted by the gravitational
interaction with the young brown dwarf companion.
As
such, the odd binary object system can serve as a laboratory that can help
astronomers study the early stages of solar
system formation, researchers said. The
brown dwarf's mass and orbital motion have significant implications for what
type of planets can form – if at all – in the PZ Tel star system.
Hunting
brown dwarfs
The
newfound brown dwarf was discovered using NICI, which is the most powerful
high-contrast instrument designed for imaging brown dwarfs and alien
planets around other stars. NICI is
capable of detecting companions 1 million times fainter than the host star, at
just one arcsecond separations.
Since
the brown dwarf is so close to its parent star, special techniques were needed
in order to distinguish the faint light of the companion from the light of the
primary star. PZ Tel B is separated by 0.33 arcseconds – an angular measurement
equal to 1/60 of a degree – from PZ Tel B. This is equivalent to a dime that is
seen at a distance of 7 miles (11 km).
The
research team was able to take pictures so close to the star by using an
adaptive optics system and coronagraph to block our excess starlight. They then
applied specialized analysis techniques to the images to detect PZ Tel B and
measure its orbital motion.
An
international team of researchers is currently carrying out a 300-star survey
using NICI, which will be the largest high-contrast imaging survey conducted to
date.
"We
are just beginning to glean the many configurations of solar systems around stars
like the sun," said Michael Liu, NICI campaign leader. "The unique
capabilities of NICI provide us with a powerful tool for studying their
constituents using direct imaging."
The
details of the PZ Tel B discovery are described in a paper being published by
the Astrophysical Journal Letters.
advertisement
