Andrew Yee
January 13th 06, 05:41 AM
McDonald Observatory
University of Texas-Austin
Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060
Science Contacts:
Neal Evans, UT-Austin
(512) 471-4396
Dan Jaffe, UT-Austin
(512) 471-3425
Philip Myers, Smithsonian Astrophysical Observatory
(617) 495-7295
11 January 2006
Astronomers Use Spitzer Space Telescope to Challenge Brown Dwarf Formation
Models
WASHINGTON, D.C. -- A group of astronomers led by Neal Evans of The
University of Texas at Austin has used NASA's Spitzer Space Telescope to
show that brown dwarfs form like stars -- by pulling in matter from a
collapsing gas cloud and forming disks of potentially planet-forming
material around themselves -- and that such disks are common around young
brown dwarfs.
The formation process that creates brown dwarfs has long been a mystery.
In terms of their mass, "brown dwarfs are found in the 'no-man's land'
between stars and planets," Evans says. "They have masses too small to be
a star, but too large to be a planet."
Two lines of evidence point to a star-like formation process for brown
dwarfs: the presence of disks found around brown dwarfs, and the discovery
of extremely dim objects forming inside clouds of gas and dust -- objects
too dim to be protostars.
Both lines of evidence come from Spitzer observations by Evans' team, a
group of about 60 astronomers from various institutions, called the "Cores
to Disks" or "c2d" Spitzer Legacy Project.
"None of these objects could have been found without the unprecedented
sensitivity of the instruments on the Spitzer Space Telescope," Evans
says.
First, the new c2d Spitzer observations, combined with supporting
observations from a number of ground-based telescopes, show that a
substantial number of brown dwarfs are surrounded by disks of dusty
material, similar to those found around forming stars.
C2d team members Katelyn Allers, Jacqueline Kessler-Silacci, Daniel Jaffe,
and Lucas Cieza of The University of Texas at Austin found about a dozen
disk-surrounded brown dwarfs in the southern-hemisphere constellations
Chamaeleon, Lupus, and Ophiuchus. Some of the brown dwarfs have a mass of
five to 10 Jupiters, and are only a few million years old -- young,
astronomically speaking.
The disk discoveries were made by comparing observations of these objects
at different wavelengths. The brown dwarfs were first studied in
near-infrared light using the four-meter Blanco Telescope at the National
Science Foundation's Cerro Tololo Interamerican Observatory in Chile.
Astronomers used the near-infrared information to predict how much
mid-infrared light they should give off. The Spitzer observations showed
that the objects gave off much more mid-infrared light than expected. This
can be explained by the presence of a disk around the brown dwarf. Disks
are made up of dust, which absorbs light radiated from the brown dwarf and
re-emits it at lower energies -- that is, in the particular mid-infrared
wavelengths detectable by Spitzer.
The group also found these objects are less massive than the smallest
stars. "You can't weigh these brown dwarfs directly," Allers says. "We
used theoretical models to figure out that they may have masses as low as
five to 10 Jupiter masses."
"The disks around the brown dwarfs are analogous to the disks around very
young Sun-like stars," Allers says, "disks that we believe provide the raw
materials for planets." In fact, Daniel Apai of the University of Arizona
and his collaborators announced in October 2005 that they had found
evidence that disks around more massive brown dwarfs might form planets.
Allers' discoveries broaden the original finding of a disk around the more
massive brown dwarf OTS 44 by Kevin Luhman of Penn State, announced in
February 2005, and his more recent discovery of a less massive
disk-surrounded brown dwarf. Today's c2d announcement shows these are part
of a wide-spread phenomenon -- not oddballs, but the norm.
The presence of these disks around brown dwarfs challenges one idea for
their formation, namely ejection caused by gravitational interactions
inside a region of star formation densely packed with stars. These results
conflict with that theory in three ways: First, computer models show that
it would be difficult for ejected brown dwarfs to keep their disks.
Second, one of Allers' brown dwarfs is in a wide binary system, which is
difficult for the ejection model to produce. Finally, neither Lupus nor
Chamaeleon are forming stars in the dense clusters the ejection model
requires.
The discovery of a substantial number of disks around even very low mass
brown dwarfs increases the likelihood that the alternative formation
scenario applies: that brown dwarfs form more or less like stars do, by
accreting matter from a collapsing cloud of gas and dust -- or, in the
jargon of star-formation researchers, a "core."
"These results suggest an origin for brown dwarfs similar to that of
stars: a collapsing 'core' of gas and dust," Evans says. "If this is
right, we should see evidence for very low mass objects in cores."
Mass is hard to measure in the very early stages of brown-dwarf formation.
But astronomers know that forming objects give off light in amounts
related to their mass and the rate at which they are accreting new
material onto themselves. So a low mass, accreting object would be very
faint.
Evidence for such tiny, dim objects exists. The first discovery of a very
dim object (called L1014-IRS) forming inside what was previously thought
to be a "starless core" in early Spitzer images was made by c2d team
member Chadwick Young of UT-Austin (now at Nicholls State University) and
collaborators and announced in November 2004. Now, c2d team members have
found about a dozen very faint objects that may be brown dwarfs in this
earlier disk phase, embedded in cores of gas and dust. Once again, this
shows that L1014-IRS, like OTS 44, is not an oddball, but the norm.
The new examples were found by c2d team members Tyler Bourke, Tracy Huard,
and Philip Myers of the Harvard-Smithsonian Center for Astrophysics;
Michael Dunham of UT-Austin; and Jens Kauffmann of the Max-Planck-Institut
für Radioastronomie. These findings suggest a new class of objects is
emerging. Dubbed "Very Low Luminosity Objects," or "VeLLOs," they have
less than one-tenth the Sun's luminosity.
These are unlikely to be stars in a very early stage of formation.
"Accreting protostars are much more luminous than they will be when they
become stars," Evans says. "So finding such a low luminosity in these
objects is surprising. It implies that the product of the current mass and
the rate at which mass is being added is unusually low."
These studies show that the VeLLOs embedded in what were thought of as
"starless cores" may be earlier stages of the disk-surrounded brown dwarfs
found by Katelyn Allers and her c2d collaborators. In fact, further
studies by Bourke and Huard show strong evidence for a disk around
L1014-IRS, as announced in October 2005.
"Cores to Disks" is one of six Spitzer Legacy Science Projects selected in
November 2000 to complete major surveys with Spitzer. The c2d team was
awarded 400 hours of Spitzer observations, and produces data freely
available to all astronomers.
The Spitzer Space Telescope is managed for NASA by the Jet Propulsion
Laboratory, a division of Caltech, in Pasadena, Calif. Science operations
are conducted at the Spitzer Science Center at Caltech, also in Pasadena.
IMAGE CAPTIONS:
[Image 1:
http://mcdonaldobservatory.org/images/news/releases/2006/spectrum_300dpi.jpg
(327KB)]
Blue dots are Spitzer observations in mid-infrared of the lowest-mass
brown dwarfs studied (~ 10 Jupiter masses). The black curve is the
predicted model of the brown dwarf emission. The green curve is the model
of emission of a disk-surrounded brown dwarf -- which the data fit.
Katelyn Allers, UT-Austin/NASA/JPL-Caltech (SSC)
[Image 2:
http://www.spitzer.caltech.edu/Media/releases/ssc2005-06/index.shtml]
Artists concept of a disk around a brown dwarf. NASA/JPL-Caltch/T. Pyle
(SSC)
[Image 3:
http://mcdonaldobservatory.org/images/news/releases/2006/L1521F%20Starless%20Core_300dpi.jpg
(698KB)]
L1521F is a dense "core" in Taurus that may contain a brown dwarf. Philip
Myers, Harvard-Smithsonian CfA/NASA/JPL-Caltech (SSC)
University of Texas-Austin
Contact:
Rebecca A. Johnson
ph: 512-475-6763 fax: 512-471-5060
Science Contacts:
Neal Evans, UT-Austin
(512) 471-4396
Dan Jaffe, UT-Austin
(512) 471-3425
Philip Myers, Smithsonian Astrophysical Observatory
(617) 495-7295
11 January 2006
Astronomers Use Spitzer Space Telescope to Challenge Brown Dwarf Formation
Models
WASHINGTON, D.C. -- A group of astronomers led by Neal Evans of The
University of Texas at Austin has used NASA's Spitzer Space Telescope to
show that brown dwarfs form like stars -- by pulling in matter from a
collapsing gas cloud and forming disks of potentially planet-forming
material around themselves -- and that such disks are common around young
brown dwarfs.
The formation process that creates brown dwarfs has long been a mystery.
In terms of their mass, "brown dwarfs are found in the 'no-man's land'
between stars and planets," Evans says. "They have masses too small to be
a star, but too large to be a planet."
Two lines of evidence point to a star-like formation process for brown
dwarfs: the presence of disks found around brown dwarfs, and the discovery
of extremely dim objects forming inside clouds of gas and dust -- objects
too dim to be protostars.
Both lines of evidence come from Spitzer observations by Evans' team, a
group of about 60 astronomers from various institutions, called the "Cores
to Disks" or "c2d" Spitzer Legacy Project.
"None of these objects could have been found without the unprecedented
sensitivity of the instruments on the Spitzer Space Telescope," Evans
says.
First, the new c2d Spitzer observations, combined with supporting
observations from a number of ground-based telescopes, show that a
substantial number of brown dwarfs are surrounded by disks of dusty
material, similar to those found around forming stars.
C2d team members Katelyn Allers, Jacqueline Kessler-Silacci, Daniel Jaffe,
and Lucas Cieza of The University of Texas at Austin found about a dozen
disk-surrounded brown dwarfs in the southern-hemisphere constellations
Chamaeleon, Lupus, and Ophiuchus. Some of the brown dwarfs have a mass of
five to 10 Jupiters, and are only a few million years old -- young,
astronomically speaking.
The disk discoveries were made by comparing observations of these objects
at different wavelengths. The brown dwarfs were first studied in
near-infrared light using the four-meter Blanco Telescope at the National
Science Foundation's Cerro Tololo Interamerican Observatory in Chile.
Astronomers used the near-infrared information to predict how much
mid-infrared light they should give off. The Spitzer observations showed
that the objects gave off much more mid-infrared light than expected. This
can be explained by the presence of a disk around the brown dwarf. Disks
are made up of dust, which absorbs light radiated from the brown dwarf and
re-emits it at lower energies -- that is, in the particular mid-infrared
wavelengths detectable by Spitzer.
The group also found these objects are less massive than the smallest
stars. "You can't weigh these brown dwarfs directly," Allers says. "We
used theoretical models to figure out that they may have masses as low as
five to 10 Jupiter masses."
"The disks around the brown dwarfs are analogous to the disks around very
young Sun-like stars," Allers says, "disks that we believe provide the raw
materials for planets." In fact, Daniel Apai of the University of Arizona
and his collaborators announced in October 2005 that they had found
evidence that disks around more massive brown dwarfs might form planets.
Allers' discoveries broaden the original finding of a disk around the more
massive brown dwarf OTS 44 by Kevin Luhman of Penn State, announced in
February 2005, and his more recent discovery of a less massive
disk-surrounded brown dwarf. Today's c2d announcement shows these are part
of a wide-spread phenomenon -- not oddballs, but the norm.
The presence of these disks around brown dwarfs challenges one idea for
their formation, namely ejection caused by gravitational interactions
inside a region of star formation densely packed with stars. These results
conflict with that theory in three ways: First, computer models show that
it would be difficult for ejected brown dwarfs to keep their disks.
Second, one of Allers' brown dwarfs is in a wide binary system, which is
difficult for the ejection model to produce. Finally, neither Lupus nor
Chamaeleon are forming stars in the dense clusters the ejection model
requires.
The discovery of a substantial number of disks around even very low mass
brown dwarfs increases the likelihood that the alternative formation
scenario applies: that brown dwarfs form more or less like stars do, by
accreting matter from a collapsing cloud of gas and dust -- or, in the
jargon of star-formation researchers, a "core."
"These results suggest an origin for brown dwarfs similar to that of
stars: a collapsing 'core' of gas and dust," Evans says. "If this is
right, we should see evidence for very low mass objects in cores."
Mass is hard to measure in the very early stages of brown-dwarf formation.
But astronomers know that forming objects give off light in amounts
related to their mass and the rate at which they are accreting new
material onto themselves. So a low mass, accreting object would be very
faint.
Evidence for such tiny, dim objects exists. The first discovery of a very
dim object (called L1014-IRS) forming inside what was previously thought
to be a "starless core" in early Spitzer images was made by c2d team
member Chadwick Young of UT-Austin (now at Nicholls State University) and
collaborators and announced in November 2004. Now, c2d team members have
found about a dozen very faint objects that may be brown dwarfs in this
earlier disk phase, embedded in cores of gas and dust. Once again, this
shows that L1014-IRS, like OTS 44, is not an oddball, but the norm.
The new examples were found by c2d team members Tyler Bourke, Tracy Huard,
and Philip Myers of the Harvard-Smithsonian Center for Astrophysics;
Michael Dunham of UT-Austin; and Jens Kauffmann of the Max-Planck-Institut
für Radioastronomie. These findings suggest a new class of objects is
emerging. Dubbed "Very Low Luminosity Objects," or "VeLLOs," they have
less than one-tenth the Sun's luminosity.
These are unlikely to be stars in a very early stage of formation.
"Accreting protostars are much more luminous than they will be when they
become stars," Evans says. "So finding such a low luminosity in these
objects is surprising. It implies that the product of the current mass and
the rate at which mass is being added is unusually low."
These studies show that the VeLLOs embedded in what were thought of as
"starless cores" may be earlier stages of the disk-surrounded brown dwarfs
found by Katelyn Allers and her c2d collaborators. In fact, further
studies by Bourke and Huard show strong evidence for a disk around
L1014-IRS, as announced in October 2005.
"Cores to Disks" is one of six Spitzer Legacy Science Projects selected in
November 2000 to complete major surveys with Spitzer. The c2d team was
awarded 400 hours of Spitzer observations, and produces data freely
available to all astronomers.
The Spitzer Space Telescope is managed for NASA by the Jet Propulsion
Laboratory, a division of Caltech, in Pasadena, Calif. Science operations
are conducted at the Spitzer Science Center at Caltech, also in Pasadena.
IMAGE CAPTIONS:
[Image 1:
http://mcdonaldobservatory.org/images/news/releases/2006/spectrum_300dpi.jpg
(327KB)]
Blue dots are Spitzer observations in mid-infrared of the lowest-mass
brown dwarfs studied (~ 10 Jupiter masses). The black curve is the
predicted model of the brown dwarf emission. The green curve is the model
of emission of a disk-surrounded brown dwarf -- which the data fit.
Katelyn Allers, UT-Austin/NASA/JPL-Caltech (SSC)
[Image 2:
http://www.spitzer.caltech.edu/Media/releases/ssc2005-06/index.shtml]
Artists concept of a disk around a brown dwarf. NASA/JPL-Caltch/T. Pyle
(SSC)
[Image 3:
http://mcdonaldobservatory.org/images/news/releases/2006/L1521F%20Starless%20Core_300dpi.jpg
(698KB)]
L1521F is a dense "core" in Taurus that may contain a brown dwarf. Philip
Myers, Harvard-Smithsonian CfA/NASA/JPL-Caltech (SSC)