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Monday, May 31, 1999
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The University of Chicago
The Institute for Advanced Study
Japan Participation Group
The John Hopkins University
Max-Planck-Institut für Astronomie
U.S. Naval Observatory
University of Washington
Apache Point Observatory
Happy, Sleepy, Grumpy... METHANE?
Sky Survey Scientists Discover New Celestial Dwarfs
Chicago, Ill. -- Scientists of the Sloan Digital Sky Survey announced
today (May 31) that early data from the Survey have revealed a new type
of astronomical object, smaller than a star but larger than a
planet. Until now, only one such object had ever been detected in the
Early this spring, while searching Sky Survey data for unusual
objects such as the universe's most distant quasars, graduate student
Xiaohui Fan and astronomer Michael Strauss, of Princeton University,
found a faint but extremely red dot of light in the night sky.
Subsequent spectroscopic observations revealed that the object was not a
distant quasar but instead an equally fascinating find -- a nearby cool,
brown dwarf with properties between those of a planet and a star.
Until their discovery, only one of this type of "cool substellar
object," known as Gliese 229B, discovered in 1995, had ever been
observed. However, unlike Gliese 229B, which is a close companion to a
star, the new object was not orbiting a star, but occurred as a
free-floating object about 30 light years away in the constellation
Then, last week, Sky Survey astronomers Zlatan Tsvetanov and Wei
Zheng of The Johns Hopkins University, along with fellow JHU astronomer
David Golimowski, confirmed the observation of another ultra-red,
free-floating object of comparable mass in the constellation Virgo, with
a spectrum so similar that Golimowski, a co-discoverer of Gliese 229B,
described it as a clone.
"The JHU result shows that the Princeton discovery was not simply
beginner's luck but that these objects may be quite common," said
astronomer Tom Geballe of the Gemini observatory in Hawaii. "They are
still so new to astronomy that they require a new vocabulary. The name
`methane dwarf' has emerged, because of the dramatic presence of bands
of methane in their spectra. Methane is characteristic of giant planets,
like Jupiter, but it never appears in normal stars -- they are much too
hot -- or even in most brown dwarfs."
Objects whose mass falls between that of stars and planets are
extraordinarily elusive. Because they lack sufficient mass to generate
the nuclear reactions that make stars shine, they cool down from their
formation temperature and become too red and too dim to show up in most
searches of the sky. And, unlike planets, which are found in association
with stars, these objects may occur as isolated objects in interstellar
space. Thus, they are very hard to detect.
Astronomers have previously observed objects with masses lower than
the minimum required to sustain nuclear reactions, the criterion that
makes an object a star. These lower-mass objects are called "brown
dwarfs," a name that reflects their temperature: red dwarfs are cooler
than white ones, and brown dwarfs are cooler still. The term "methane
dwarf" distinguishes the three known very cool brown dwarfs, which all
show methane in their spectra, from the population of hotter brown
dwarfs that lack this signature.
Astronomers also know that low-mass objects -- those with masses
smaller than the sun -- may be important contributors to the total mass
of the Milky Way. But to understand just how much they contribute,
astronomers must determine their masses -- and how many of them exist.
"Just because we haven't seen these free-floaters before doesn't mean
they are rare," said University of Chicago astronomy graduate student
Constance Rockosi. "But to find them, you need to cover a lot of sky
area and at the same time be able to see very faint objects. The Sky
Survey covers so much celestial territory that at last we will begin to
get a grip on how many there really are."
For now, the mass of the methane dwarfs remains unknown. They must be
smaller than stars, because they are so cool. However, their dimness
depends not only on their mass but on their age: brown dwarfs cool with
age, like embers drawn from a fire. So, although the methane dwarfs are
fainter than other brown dwarfs, they may not be less massive-just much
older. Astronomers estimate their masses at 10 to 70 times the mass of
Both the Princeton and Johns Hopkins groups discovered the new
methane dwarfs while looking for objects so red that they were not even
visible with filters sensitive to other wavelengths. Princeton's Strauss
described the detective work that led to the confirmation of the
object's true identity.
"The object we observed was the reddest object we have found thus far
in 400 square degrees of observing," Strauss said. "We obtained its
optical spectrum with the 3.5-meter telescope at Apache Point
Observatory in New Mexico. The spectrum showed strong absorption by
water, in the form of steam. We immediately realized it might be an
object similar to Gliese 229 B. At the suggestion of Princeton astonomer
Gillian Knapp, we called our colleagues Tom Geballe, of the Gemini
Observatory, and Sandy Leggett of the Joint Astronomy Centre. By
coincidence, they were observing at UKIRT, the United Kingdom Infrared
Telescope in Hawaii, which has one of the best near-infrared
spectrographs in the world. They obtained infrared spectra and showed
that our object was an almost exact twin to Gliese 229 B, with strong
bands of water and methane."
A few weeks later, Geballe and Leggett received a similar call from
the Johns Hopkins group, and, on May 22 obtained the spectrum for the
second candidate object.
"As anticipated," Golimowski said, "the spectrum revealed absorption
features from water vapor and methane. The third known methane dwarf was
now confirmed! Among many new questions, we need to understand why the
spectra of these three objects are so amazingly similar. The two new
free-floaters are ideal targets for searching for even lower-mass
companions or planets."
Fan said Sky Survey collaborators are pleased at the objects'
detection so early in the life of their project. "Even the engineering
test data from the Sky Survey are proving very productive for science,"
The wide-field capability of the Sky Survey telescope and camera are
only part of the technology necessary to advance the discovery rate of
new classes of astronomical objects.
"The capability for precision five-color photometry is just as
important," said Knapp. "The work of Princeton software experts such as
Robert Lupton and Zeljko Ivezic was crucial."
When the Sky Survey begins routine operations, on each clear moonless
night a powerful digital camera will generate 200 gigabytes of data
containing millions of objects. Most of the objects are more or less
ordinary stars; the interesting, one-in-(literally)-a-million objects
must be carefully filtered out from the vast quantities of Sky Survey
data. Thus, another crucial contribution to finding such rare objects as
methane dwarfs so early in the Survey is the extraordinary capability of
the Sky Survey's image-processing software, which enables scientists to
zoom in on unusual classes of objects with high reliability.
The Sloan Digital Sky Survey (SDSS) is a joint project of The
University of Chicago, Fermilab, the Institute for Advanced Study, the
Japan Participation Group, The Johns Hopkins University, the
Max-Planck-Institute for Astronomy, Princeton University, the United
States Naval Observatory, and the University of Washington. Apache Point
Observatory, site of the SDSS, is operated by the Astrophysical Research
Consortium. Funding for the project has been provided by the Alfred
P. Sloan Foundation, the SDSS member institutions, the National
Aeronautics and Space Administration, the National Science Foundation,
the U.S. Department of Energy and the Ministry of Education of Japan.
The SDSS Web site is http://www.sdss.org/.
A plain text version of the press release
is also available.