While searching
for planet-sized bodies that might accompany the nearby star system Epsilon
Indi, astronomers using the Gemini South telescope in Chile made a related
but unexpected detection.
Widely observed
by telescopes on the ground and in space, Epsilon Indi was known to host
an orbiting companion, called Epsilon Indi B, which was discovered last
year and is the nearest known specimen of a brown dwarf. Brown dwarfs
are very small, cool stars thirty to forty times more massive than Jupiter
but of similar size. Despite all the observing, it took the combination
of Gemini's powerful infrared capabilities and the extremely sensitive
spectrograph/imager called PHOENIX (without adaptive optics) to reveal
the more elusive body.
"Epsilon Indi
Ba is the closest confirmed brown dwarf to our solar system," says Dr.
Gordon Walker (University of British Columbia, Vancouver, Canada), who
led the research team that includes Dr. Suzie Ramsay Howat (UK Astronomy Technology Centre, Edinburgh, UK).
Dr. Walker explains, "With the detection of Epsilon Indi Bb, we now know
that Epsilon Indi Ba has a close companion that appears to be another,
even cooler brown dwarf. One certainty is that the Epsilon Indi system
is even more interesting than we previously thought."
The team of scientists who detected Epsilon Indi Bb using the Gemini
South Telescope on Cerro Pachón, Chile, were the first to report
this finding, which was published in the IAU
Circular Volume 8818. Subsequently, the VLT (Very Large Telescope)
announced that scientists had actually observed the object five days earlier
(using adaptive optics), and their finding is reported at http://xxx.lanl.gov/abs/astro-ph?0309256.
"When the target was acquired and we saw that there were clearly two objects
close together, we initially thought it must be the wrong object. Epsilon Indi Ba,
formerly called Epsilon Indi B, had been observed before
and in those observations, no one noticed the companion object. It
was a tremendous surprise for us," says Dr. Kevin Volk (Gemini Observatory,
La Serena, Chile) who was actually making the observation at the Gemini
South telescope along with Dr. Robert Blum (CTIO, La Serena, Chile).
The serendipitous nature of the detection took the science
team--whose members are from Canada, the U.K., the U.S. and Chile--by surprise.
Dr. Blum elaborates, "We then found that the companion, named Epsilon Indi
Bb, is invisible in the methane band where previous Gemini observations
had been taken. The coolest brown dwarfs are very faint and hard to detect,
but there may be vast numbers of them--which makes this detection important."
Epsilon Indi is the fifth brightest star in the southern
constellation of Indus and is located about 11.8 light years away from
our solar system. The star is similar to but cooler than our sun.
The projected separation as seen on the sky between Epsilon Indi and Indi
Ba is approximately 1500 AUs (one AU or Astronomical Unit is the average
distance between the Earth and the Sun or about 93 million miles/150 million
kilometers), and the distance between Epsilon Indi Ba and the newly discovered
Epsilon Indi Bb is at least 2.2 AUs.
"Because this system is so close to us, it appears to move
quite rapidly in the sky," says Dr. Volk. "We were able to confirm
our detection--and rule out a more distant background object--within a
few weeks since we could detect the motion of the system relative to the
background stars relatively quickly."
As the facts surrounding the detection become clearer with
additional spectroscopic data, the research team expects that important
details about Epsilon Indi Bb will be revealed. "Unfortunately, the
window for observing this system is nearly closed for this year, so we will
have to wait until early next year when we can see this system again in
the morning sky," says Dr. David Balam (University of Victoria, Canada).
The data recently obtained from Gemini show that Epsilon
Indi Bb is cooler and less massive than Epsilon Indi Ba as demonstrated
by its significantly lower brightness and deep methane absorption.
Methane absorption is a key indicator for low mass objects since gaseous
methane can only exist in the lower temperature environments of the atmospheres
of brown dwarfs and planets where the gas can exist.
"Methane absorption was the key to the detection," says Dr.
Walker, "because Dr. Volk happened to catch sight of Epsilon Indi Bb through
one of the 'windows' between the methane absorption bands. Because
the absorption bands block longer wavelength infrared light, Epsilon Indi
Bb was visible when viewed at shorter infrared wavelengths."
Epsilon Indi Ba and Bb are members of a recently discovered
type of astronomical object--the "T" class brown dwarfs. These T-dwarfs
have diameters approximately equal to Jupiter but with more mass.
Spectra of Epsilon Indi Ba, taken with PHOENIX by Dr. Verne Smith (University
of Texas, El Paso) and collaborators, show the Epsilon Indi Ba has 32 times
the mass of Jupiter and a 1500-degree surface temperature. It is spinning
about three times faster than Jupiter. Epsilon Indi Bb has less mass,
is cooler, but is still much more massive and hotter than Jupiter.
Like Jupiter, the T-dwarfs do not have enough mass to make energy the way
the sun does from nuclear fusion. Epsilon Indi Ba and Bb are glowing
from heat resulting from the mass pushing down on the interior.
PHOENIX, the instrument that is responsible for producing
the data, is a near-infrared, high-resolution spectrometer that was built
by the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona, and
was commissioned on Gemini South in 2001. Dr. Ken Hinkle (NOAO, Tucson,
Arizona) said, "PHOENIX was designed for exactly this type of research.
It is the first high-resolution infrared spectrograph on a Gemini telescope,
and the first high-resolution infrared spectrograph on any southern hemisphere
telescope."
Dr. Phil Puxley, Associate Director of Gemini South, adds,
"Gemini's infrared optimization makes the 8-meter twin telescopes ideal
for capturing such serendipitous discoveries. Finds like this are exactly
what Gemini is designed to do and this sort of exciting work demonstrates
the potential of Gemini's science."
Epsilon Indi is visible with the naked eye from June to December
in the southern hemisphere. It can be detected with the locator map
available at http://www.gemini.edu/science/epsilonindi-images.html,
which also contains other images and illustrations.
