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.
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.
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.
Artist's conception of the Epsilon Indi system showing Epsilon Indi and the brown-dwarf binary companions. Due to the perspective of the brown dwarf companions, the relative sizes are not represented in this illustration. Artwork by Jon Lomberg. "Gemini Observatory Illustration""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."
The Gemini Observatory
is an international collaboration that has built two identical 8-meter
telescopes. The Frederick C. Gillett Gemini Telescope is located at Mauna
Kea, Hawai`i (Gemini North) and the other telescope at Cerro Pachón
in central Chile (Gemini South), and hence provide full coverage of both
hemispheres of the sky. Both telescopes incorporate new technologies that
allow large, relatively thin mirrors under active control to collect and
focus both optical and infrared radiation from space.
The Gemini Observatory
provides the astronomical communities in each partner country
with state-of-the-art astronomical facilities that allocate observing
time in proportion to each country's contribution. In addition to
financial support, each country also contributes significant scientific
and technical resources. The national research agencies that form
the Gemini partnership include: the US National Science Foundation
(NSF), the UK Particle Physics and Astronomy Research Council (PPARC),
the Canadian National Research Council (NRC), the Chilean Comisión
Nacional de Investigación Cientifica y Tecnológica (CONICYT),
the Australian Research Council (ARC), the Argentinean Consejo Nacional
de Investigaciones Científicas y Técnicas (CONICET) and
the Brazilian Conselho Nacional de Desenvolvimento Científico
e Tecnológico (CNPq). The Observatory is managed by the Association
of Universities for Research in Astronomy, Inc. (AURA) under a cooperative
agreement with the NSF. The NSF also serves as the executive agency for
the international partnership.