|
 |
 |
 
By Michael Liu and Trent Dupuy, Institute for Astronomy, University
of Hawaii
|
 |
| This image shows a faint brown dwarf in
our solar neighborhood, about 110 light years from Earth.
The inset image shows a high angular resolution image
obtained with the laser guide star adaptive optics system
on the Keck II Telescope, showing that the brown dwarf
is actually a binary system. About 15 percent of brown
dwarfs near Earth are found in binaries. Credits: Xiaohui
Fan and the Sloan Digital Sky Survey Collaboration; Michael
Liu, IfA, University of Hawaii. |
Understanding what stars are made of and how they shine is
a surprisingly recent discovery for humanity. The underlying
cause is that the intense temperatures and pressures in their
central regions are sufficient to fuse hydrogen into helium,
and in the process energy is generated and released at a star’s
surface as light. Only by the mid-twentieth century was it
established that stars are these giant hydrogen fusion reactors,
and astronomers were for the first time able to write down
equations describing their hidden interiors. The energy
output could be precisely predicted from these equations, and
the relatively new theory of quantum physics provided the explanation
for the properties of the light, or spectra, emitted from stars.
These theoretical models of stars underlie most of modern astronomy
research today and represent one of the great intellectual
triumphs of the last century.
Despite these successes, understanding the inner workings of
stars and planets remains one of astronomy’s fundamental
challenges. Unlike stars, planets do not generate their own
internal energy, as core hydrogen fusion is only possible for
objects with a mass greater than about eight percent the mass
of the Sun. (Jupiter, the most massive planet in the Solar
System, has about 1/1000 of the Sun’s mass, and the Earth
has about 1/300,000 the Sun’s mass.) This means that
gas-giant planets like Jupiter are continually evolving over
their lifetime, starting from an early hot, enlarged state
and then cooling and shrinking with time. In addition, the
gaseous atmospheres of planets are much colder than those of
stars. Stars are hot enough to obliterate molecules and ionize
the constituent atoms, creating a plasma fluid that is relatively
simple to describe with basic physics. However, the colder
atmospheres of planets harbor molecules and even clouds of
particulate matter, or dust, both of which are much more difficult
to predict theoretically.
At face value, stars and planets appear to be two very different
classes of objects, separated by more than a factor of ten
in mass and a factor of million in luminosity, and having radically
different emergent spectra. In the 1960s, theorists Takenori
Nakano and Shiv Kumar independently hypothesized a class
of objects that are intermediates between planets and stars. The objects are now called brown dwarfs. These "missing links" were
conceived of as objects with insufficient masses to reach the
internal temperatures and pressures needed to become fusion
reactors, yet they could be about 100 times more massive than
the gas giants Jupiter and Saturn. As such, brown dwarfs would
have surface temperatures between the coolest stars and the
hottest planets. Even without internal energy generation, they
could emit some light because residual heat from the time of
their formation would be gradually released as they aged and
cooled.
The first brown dwarf was conclusively identified in 1995,
emitting infrared radiation like the coolest stars and yet
its atmospheric properties were similar to the planet Jupiter,
marked by a very low temperature and complex molecules. Since
then, astronomers have identified hundreds of brown dwarfs
within 100 light-years of Earth, including many with observations
from the Keck Telescopes. In fact, we believe that nature produces
about as many brown dwarfs as stars, but because brown dwarfs
emit so little energy they are much harder to find in the sky.
The coolest known brown dwarfs have surface temperatures comparable
to the inside of a pizza oven (800 degrees Fahrenheit) more
than 9,000 F cooler than the surface of the Sun. The study
of brown dwarfs has become an extremely active area of research
in the last decade, because they represent the lowest mass
products of star formation and they can provide critical insights
into the physics of low-temperature atmospheres.
 |
| Trent Dupuy is a Ph D student at the Institute
for Astronomy at the University of Hawaii. He works with
Dr. Liu and the Keck telescopes to study brown dwarfs.
Credit: K. Teramura, IfA, University of Hawaii. |
Shortly after the first brown dwarfs were identified, observations
with the Hubble Space Telescope found that some of them occur
as binary systems, namely two brown dwarfs bound together through
their mutual gravitational attraction and orbiting each other
in a fashion similar to how the Earth orbits the Sun. Overall,
about 15 percent of brown dwarfs are binaries. This is not
surprising, since we know that about 50 percent of stars are
binaries and we believe brown dwarfs form in a similar fashion
to stars (though the reason as to why the brown dwarf binaries
are rarer is a topic for another article).
Astronomy’s Sharpest Eyes
We have been conducting a long-term observational study of
nearby brown dwarf binaries, with the dual goals of finding
more of these systems and then carefully scrutinizing them
to understand their complex physical properties. Such observations
are very challenging, because nearly all brown dwarf binaries
are separated by only a small angle on the sky. With ordinary
astronomical imaging instruments, it is not possible to resolve
the two components of the binaries.
 |
| With the power of the Keck telescopes behind
his research, Dr. Michael Liu is confident that the next
few years will be an exciting time for studying brown
dwarfs. Credit: K. Teramura, IfA, University of Hawaii. |
To do this research, we require the unique capabilities of
the Keck II Telescope. Since 2005, this telescope has been
equipped with a powerful laser-guided adaptive optics (AO)
system that corrects for the blurring of astronomical images
caused by turbulence in Earth’s atmosphere. While astronomers
have been using AO technology for nearly two decades on various
telescopes, including on the Keck II Telescope since 1999,
brown dwarfs have always been far too faint for traditional
AO systems, which only work with bright stars. The Keck laser
system creates an "artificial star" in the sky, which
can then be pointed at the brown dwarfs to produce unprecedentedly
sharp infrared images.
Keck II was the first large (eight to ten meter) ground-based
telescope to deploy a laser-guided AO system, and this capability
has been a transformational technology for many areas of astronomical
research. The physics of light is such that the theoretical
limit for the sharpness of an image from a telescope is inversely
proportional to the diameter of the telescope’s primary
mirror — bigger telescopes can make sharper images. However,
the typical image quality of ground-based telescope is usually
much worse, since the Earth’s turbulent atmosphere blurs
light from stars. This problem can be overcome by equipping
ground-based telescopes with AO. Since the Kecks are the largest
optical/infrared telescopes anywhere in the world (or in space),
Keck II AO produces the sharpest images ever achieved, three
to four times sharper than those produced by Hubble. The images
produced by Keck have an angular resolution as good as 1/20
of an arc second, about 1/40,000 the diameter of the full moon.
If a person’s vision were as sharp as the Keck adaptive
optics system, he would be able to read a magazine that was
about a mile away. In fact, the positional accuracy achieved
with such sharp images is equivalent to hitting a bull’s-eye
on a dartboard that is 8,000 miles away.
Two Are Better Than One
While appearing to be very simple systems, it turns out that
brown dwarfs paired together provide remarkably useful laboratories
for learning about the physics of low-temperature objects.
The reason for this can be understood through an analogy with
biology. Human beings occur in all different shapes, sizes,
appearances, and personalities. One potent method for exploring
the origins of these differences is by studying twins, namely
two people born with the same genetic composition. Like an
individual person, single, non-binary brown dwarfs free-floating
in space are difficult to study since such objects possess
a wide and unknown mix of ages, masses and compositions. However,
for binary systems ("brown dwarf twins"), we believe
that they formed together at the same time and out of the same
natal gaseous material, which is useful information when studying
these complex objects.
 |
| Keck II shows off its sodium laser, which
creates an artificial star. Monitoring the artificial star's twinkling allows astronomers to calculate and then remove
the blurring effects of atmospheric turbulence from the
images of objects such as brown dwarfs. Credit: Laurie Hatch. |
Furthermore, as first shown by Johannes Kepler in the 17th
century, the total mass of any binary system can be determined
by precisely measuring the orbit’s size and how long
it takes for the two objects to complete one orbital cycle — the
orbital period. Thus, binary brown dwarfs provide a unique
opportunity to directly measure the masses of ultracool objects.
In fact, mass is the fundamental parameter that governs
the life-history of any free-floating object (star, planet,
or brown dwarf). Over the last decade, astronomers have measured
the energy outputs and temperatures for hundreds of brown dwarfs.
However, the most important property of all — the mass — is
the hardest one to measure.
Patience is a Virtue
Measuring masses of brown dwarf binaries is a challenging undertaking,
requiring both technical sophistication (high-resolution imaging)
and personal fortitude (specifically, patience). The former
is needed because of the very small angular separations of
brown dwarf binaries on the sky, and thus only very sharp images
can precisely monitor the motion of the two components around
each other. Patience is needed because the orbital motion is
very slow. Typical orbital periods are estimated to be a decade
of more.
In collaboration with Dr. Michael Ireland of the University
of Sydney, we have been regularly monitoring about three dozen
binaries with Keck adaptive optics since laser-guided AO became
operational in 2005. The Hubble Space Telescope originally
discovered many of our binaries, with observations dating as
far back as the year 1998, but only a single epoch of imaging
was obtained. Observing with Keck allows us to obtain many
more epochs and with higher precision. Then, combining the
Keck and Hubble datasets lets us precisely measure the size
and duration of the orbits over a very long time baseline,
thereby determining the masses of the binaries.
Thus far, our team has completed measuring the masses of two
brown dwarf binaries. While two may not seem like a large number
compared to the hundreds of brown dwarfs known, such precise
measurements are very hard-earned, and each new result has
great scientific value. We have already doubled the number
of brown dwarfs with dynamical mass determinations, and in
fact our results are the first two measurements ever for such
cold objects.
One binary, known as 2MASS 1534-2952AB, is composed of two "methane" brown
dwarfs, the coolest type of brown dwarf, which are characterized
by the presence of methane gas in their atmospheres. This is
the first mass measurement for this type of object. Our team
found that the total mass of 2MASS 1534-2952AB is only six
percent of the Sun’s mass, and each of its constituent
brown dwarfs has a mass of about three percent of the Sun’s
mass (about 30 times the mass of Jupiter). The other binary
system, HD 130948BC, is a pair of slightly warmer "dusty" brown
dwarfs with a total mass of only 11 percent of the Sun’s
mass and individual masses of about 5.5 percent of the Sun’s
mass.
 |
| This infrared image of the very low-temperature
binary 2MASS 1534-2952AB was obtained with the Laser
Guide Star Adaptive Optics system on the Keck II Telescope.
This binary is composed of two methane brown dwarfs,
the coolest class of free-floating objects identified
to date. The two components each have temperatures of
about 1300 degrees Farenheit and emit about 1/100,000
the energy of the Sun. With a total mass of only 60 Jupiter
masses, this is the lightest object ever directly weighed
outside the Solar System. Credit: Dr. Michael Liu, IfA,
University of Hawaii. |
The two binaries, located in the constellations of Libra (the
Scales) and Bootes (the Herdsman), are about 45 and 60 light-years
from Earth. The orbital configurations of the two binaries
are very similar — both have orbital separations of about
two astronomical units (AU), where one AU is the distance from
Earth to the Sun or 93 million miles. This is somewhat larger
than the 1.5 AU distance between Mars and the Sun. Their orbital
periods are about 10-15 years compared with the two years it
takes for Mars to orbit the Sun; the longer period is a direct
result of the low masses of these objects, leading to much
weaker mutual gravitational attraction than the Sun-Mars system.
Theoretical models attempt to predict the masses of brown dwarfs
based on their energy output and temperature. But when we compared
our mass measurements to the theoretical predictions, they
did not agree. For example, the surface temperature of 2MASS
1534-2952AB was much cooler than expected given its current
level of energy output, while HD 130948BC was much warmer.
Our study of HD 130948BC turned out to be especially puzzling.
This brown dwarf binary system is actually part of a triple
system in which the primary star (HD 130948A) is a young, Sun-like
star that is about 600 million years old. The only quantity
more difficult to measure than mass for astronomical objects
is their age. However, when a Sun-like star is young, it displays
energetic phenomena such as intense activity in its upper atmosphere
that enables us to gauge its age accurately. Therefore, assuming
the Sun-like star and brown dwarf binary formed at the same
time (which is a very conservative assumption), we can use
the Sun-like star as a "clock" to determine the
age of its companion binary brown dwarf. Because of this unique
set of information, HD 130948BC is now the gold standard for
testing predictions of theoretical models of brown dwarfs.
We find that the energy output theoretically predicted for
the system seriously disagrees with observations: the system
is emitting two to three times more energy than expected by
models.
While theoretical predictions of brown dwarfs come close to
matching our observations, something is obviously not quite
right with the theory — either in determining the temperatures,
in predicting the energy output, or perhaps both. This is a
disturbing and potentially far-reaching result. These same
theoretical models are used to infer the properties (masses,
temperatures, and ages) of the hundreds of other known brown
dwarfs that are not in binary systems. In addition, these models
are used to predict the properties of gas-giant extrasolar
planets found around other stars, such as those recently directly
imaged around the stars Fomalhaut and HR 8799. Clearly, something
is missing from our understanding of the coolest objects that
do not generate their own internal energy, from brown dwarfs
to exoplanets.
 |
| This image shows a simulation of observations
of a star with two low mass companions observed with
the current AO system and NIRC2 camera in the J band
[left], and with the future NGAO system [right]. The
more separated companion is detected on each AO image
(including the current AO). The closer companion is only
visible using the NGAO system, which provides a better
sensitivity and angular resolution. Credit: F. Marchis,
UC-Berkeley. |
What Lies Ahead
Our work to date has been both thrilling and puzzling. Thrilling
in the sense that the Keck II telescope has allowed us to carry
out the most detailed studies of brown dwarf binaries to date,
and yet puzzling in that our mass measurements have pointed
to problems with our current understanding of the inner workings
of low temperature objects. For us, these conundrums serve
as inspiration to measure masses for more brown dwarfs in the
coming years in order to better understand the successes and
failings of current theory.
By 2010, the Keck I Telescope will also be equipped with a
laser-guided AO system, which will be more advanced than the
existing Keck II system. Looking ahead in the more distant
future, however, Keck is now actively in the planning stages
for its Next Generation Adaptive Optics system or NGAO. Working
closely with astronomers and engineers throughout the Keck
community, the new system will employ multiple laser guide
stars to provide even sharper, more accurate images. It will
even extend AO capability beyond the infrared to a much wider
range of wavelengths. With these powerful new tools, we
expect to make great leaps in understanding the inner workings
of nature’s coolest, faintest objects. 
Click here to return to the main
page.
|
|
|
 |
|
