Press Release

Closest Brown Dwarf Companion Ever Spotted Around a Star Proves New Perspective on Formation of Low-mass Objects

By SpaceRef Editor
May 21, 2002
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Image caption: This discovery image from the Gemini Observatory represents the closest brown dwarf companion ever directly imaged around a star (named LHS 2397a). This image was obtained on Feb. 7, 2002 with the Gemini North Telescope on Mauna Kea in Hawaii using the University of Hawaii’s Adaptive Optics system Hokupa`a and the QUIRC infrared imager. The resolution of the image is 0.1 arcseconds (equal to the size of a quarter held 32 miles away). The companion is so faint and red that it must have a cool “L7” atmosphere and is therefore only massive enough to be a brown dwarf.

Astronomers using adaptive optics technology on the Gemini North
Telescope have observed a brown dwarf orbiting a low-mass star at a
distance comparable to just three times the distance between the Earth
and Sun. This is the closest separation distance ever found for this
type of binary system using direct imaging.

The record-breaking find is just one of a dozen lightweight binary
systems observed in the study. Together, they provide a new perspective
on the formation of stellar systems and how smaller bodies in the Universe
(including large planets) might form.

“By using Gemini’s advanced imaging capabilities, we were able to
clearly resolve this binary pair where the distance between the
brown dwarf and its parent star is only about twice the distance
of Mars from the Sun,” said team member Melanie Freed, a graduate
student at the University of Arizona in Tucson. With an estimated mass
of 38-70 times the mass of Jupiter, the newly identified brown dwarf is
located just three times the Sun-Earth distance (or 3.0 Astronomical
Units) from its parent star. The star, known as LHS 2397a, is only 46
light-years from Earth. The motion of this object in the sky indicates
that it is an old, very low-mass star.

The previous imaging record for the closest distance between a brown dwarf
and its parent (a much brighter, Sun-like star) was almost five times greater
at 14 AU. One Astronomical Unit (AU) equals the average distance between
the Earth and the Sun or about 150 million kilometers (93 million miles).

Often portrayed as “failed stars,” brown dwarfs are bigger than giant planets
like Jupiter, but their individual masses are less than 8% of the Sun’s mass
(75 Jupiter masses), so they are not massive enough to shine like a star.
Brown dwarfs are best viewed in the infrared because surface heat is released
as they slowly contract. The detection of brown dwarf companions within
3 AU of another star is an important step toward imaging massive planets
around other stars.

This University of Arizona team led by Dr. Laird Close used the Gemini North
Telescope to detect eleven other low mass companions, suggesting that these
low-mass binary pairs may be quite common. The discovery of so many low-
mass pairs was a surprise, given the argument that most very low-mass stars
and brown dwarfs were thought to be solo objects wandering though space
alone after being ejected out of their stellar nurseries during the star
formation process.

“We have completed the first adaptive optics-based survey of stars with
about 1/10th of the Sun’s mass, and we found nature does not discriminate
against low-mass stars when it comes to making tight binary pairs,” said
Close, an assistant professor of astronomy at the University of Arizona.
Dr. Close is the lead author on a paper presented today at the Brown Dwarfs
International Astronomical Union Symposium in Kona, Hawaii, and
he is the principal investigator of the low-mass star survey.

The team looked at 64 low-mass stars (originally identified by John Gizis
of the University of Delaware) that appeared to be solo stars in the lower
resolution images from the 2MASS all-sky infrared survey. Once the team
used adaptive optics on Gemini to make images that were ten times sharper,
twelve of these stars were revealed to have close companions. Surprisingly,
Close’s team found that the separation distances between the low mass stars
and their companions were significantly less than expected.

“We find companions to low-mass stars are typically only 4 AU from
their primary stars, this is surprisingly close together,” said team member
Nick Siegler, a University of Arizona graduate student. “More massive
binaries have typical separations closer to 30 AU, and many binaries are
much wider than this.” The new Gemini observations, Close said,
“imply strongly that low-mass stars do not have companions that
are far from their primaries.” Similar results had been found previously
by a team led by Dr. Eduardo L. Martin of the University of Hawaii
Institute for Astronomy in a survey of 34 very low-mass stars and
brown dwarfs in the Pleiades cluster carried out with the Hubble
Space Telescope. These two surveys together clearly demonstrate
that there is an intriguing dearth of brown dwarfs at separations larger
than 20 AU from very low-mass stars and other brown dwarfs.

The team projects that one out of every five low-mass stars has a companion
with a separation in the range (3-200 AU). Within this separation range,
astronomers have observed a similar frequency of more massive stellar
companions around larger Sun-like stars.

Taken as a whole, these new results suggest that (contrary to theory)
low-mass binaries may form in a process similar to that of more massive
binaries. Indeed, this finding adds to growing evidence from other groups
that the percentage of binary systems is similar for bodies spanning the
range from one solar mass to as little as 0.05 solar masses (or 52 times
Jupiter’s mass). For example, a group led by Neill Reid of the Space
Telescope Science Institute and the University of Pennsylvania has
come to a similar conclusion with a smaller sample of 20 even lower-
mass stars and brown dwarfs observed with the Hubble Space Telescope.

The fact that low-mass stars have any low-mass brown dwarf companions
inside 5 AU is also surprising because the exact opposite is true around Sun-
like stars. Very few Sun-like stars have brown dwarf companions inside this
distance, according to radial velocity studies. “This lack of brown dwarf
companions within 5 AU of Sun-like stars has been called the ‘brown dwarf
desert’,” Close noted. “However, we see there is likely no brown dwarf desert
around low-mass stars.”

These results form important constraints for theorists working to understand
how the mass of a star affects the mass and separation distance of the
companions that form with it. “Any accurate model of star and planet
formation must reproduce these observations,” Close said.

These observations were possible only because of the combination of
the University of Hawaii’s uniquely sensitive Hokupa’a adaptive optics
imaging system and the technical performance of the Gemini telescopes.
The Hokupa’a system sensitivity is due to the curvature wavefront sensing
concept developed by Dr. Francois Roddier. Adaptive optics is an
increasingly crucial technology that eliminates most of the “blurring”
caused by the turbulence in the Earth’s atmosphere (i.e., the twinkling
of the stars). It does this by rapidly adjusting the shape of a special,
smaller flexible mirror to match local turbulence, based on real-time
feedback to the mirror’s support system from observations of the low-
mass star. Hokupa’a can count individual photons (particles of light)
and so can sharpen accurately even very faint (i.e., low-mass) stars.

The near-infrared adaptive optics images made by the 8-meter Gemini
telescope in this survey were twice as sharp as those that can be made at
the same wavelengths by the Earth-orbiting, 2.4-meter Hubble Space
Telescope. The only ground-based survey of its kind, this work required
five nights over one year with the Hokupa’a system at Gemini North.

It is important to note that the distances used here are as measured on
the sky. The real orbital separations may be slightly larger once the full
orbit of these binaries is known in the future.

Other science team members include James Liebert (Steward Observatory,
University of Arizona), Wolfgang Brandner (European Southern Observatory,
Garching, Germany), and Eduardo Martin and Dan Potter (Institute for
Astronomy, University of Hawaii).

The observations reported here are part of an ongoing survey. Initial
results from the first 20 low-mass stars of the survey have been published
in the March 1, 2002, issue of The Astrophysical Journal Letters.

This survey was supported in part by the U.S. Air Force Office of Scientific
Research and the University of Arizona’s Steward Observatory. Hokupa’a is
supported by the University of Hawaii Adaptive Optics Group and the
National Science Foundation.

The Gemini Observatory is an international collaboration that has built
two identical 8-meter telescopes. The telescopes are located at Mauna Kea,
Hawaii (Gemini North) and Cerro Pachn 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 Comision
Nacional de Investigacion Cientifica y Tecnologica (CONICYT), the
Australian Research Council (ARC), the Argentinean Consejo Nacional
de Investigaciones Cientificas y Tecnicas (CONICET) and the Brazilian
Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (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.

For more information, see the Gemini website at:

http://www.us-gemini.noao.edu/media/.

SpaceRef staff editor.