Andrew yee 2012-04-29 14:51:33
National Astronomical Observatory of Japan
April 21, 2006
Synergy of Technologies Reveal Origin of Dust Around Nearby Star
1. Beta Pictoris as a Laboratory for Planet Formation
Planets form in disks of gas and dust that surround new born stars. Such
disks are called proto-planetary disks. The dust in these disks become rocky
planets like Earth and the inner cores of giant gas planets like Saturn.
This dust is also a repository of elements that form the basis of life.
Proto-planetary disks disappear as stars mature, but many stars have what
are called debris disks. Astronomers hypothesize that once objects such as
asteroids and comets are born from the proto-planetary disk, collisions
among them can produce a secondary dust disk.
The most well-known example of such dust disks is the one surrounding the
second brightest star in the constellation Pictor, meaning “painter’s
easel”. This star, known as Beta Pictoris or Beta Pic, is a very close
neighbor of the Sun, only sixty light years away, and therefore easy to
study in great detail.
Beta Pic is twice as bright as the Sun, but the light from the disk is much
fainter. Astronomers Smith and Terrile were the first to detect this faint
light in 1984, by blocking the light from the star itself using a technique
called coronagraphy. Since then, many astronomers have observed the Beta Pic
disk using ever better instruments and ground and space-based telescopes to
understand in detail the birth place of planets, and hence life.
2. Combining Technologies
A team of astronomers from the National Astronomical Observatory of Japan,
Nagoya University and Hokkaido University combined several technologies for
the first time to obtain an infrared polarization image of the Beta Pic disk
with better resolution and higher contrast than ever before: a large
aperture telescope (the Subaru telescope, with its large 8.2 meter primary
mirror), adaptive optics technology, and a coronagraphic imager capable of
taking images of light with different polarizations (Subaru’s Coronagraphic
Imager with Adaptive Optics,CIAO).
A large aperture telescope, especially with Subaru’s great imaging quality,
allows faint light to be seen at high resolution. Adaptive optics technology
reduces Earth’s atmosphere’s distorting effects on light, allowing higher
resolution observations. Coronagraphy is a technique for blocking light from
a bright object such as a star, to see fainter objects near it, such as
planets and dust surrounding a star. By observing polarized light, reflected
light can be distinguished from light coming directly from its original
source. Polarization also contains information about the size, shape, and
alignment of dust reflecting light.
With this combination of technologies, the team succeeded in observing Beta
Pic in infrared light two micrometers in wavelength at a resolution of a
fifth of an arcsecond. This resolution corresponds to being able to see an
individual grain of rice from one mile away or a mustard seed from a
kilometer away. Achieving this resolution represents a huge improvement over
comparable previous polarimetric observations from the 1990’s that had only
resolutions of about one and a half arcseconds.
3. Collisions of Planetesimals and Bacteria-Size Dust Bunnies
The new results strongly suggest that Beta Pic’s disk contains
planetesimals, asteroid or comet-like objects, that collide to generate dust
that reflects starlight.
The polarization of the light reflected from the disk can reveal the
physical properties of the disk such as composition, size, and distribution.
An image of all the two micrometer wavelength light shows the long thin
structure of the disk seen nearly edge on (Figure 1). The polarization of
the light shows that ten percent of the two micrometer light is polarized.
The pattern of polarization indicates that the light is a reflection of
light that originated from the central star (Figure 2).
An analysis of how the brightness of the disk changes with distance from the
central shows a gradual decrease in brightness with a small oscillation
(Figure 3). The slight oscillation in brightness corresponds to variations
in the density of the disk. The most likely explanation is that denser
regions correspond to where planetesimals are colliding. Similar structures
have been seen closer to the star in earlier observations at longer
wavelengths using Subaru’s COoled Mid-Infrared Camera and Spectrograph
(COMICS) and other instruments.
A similar analysis of how the amount of polarization changes with distance
from the star shows a decrease in polarization at a distance of one hundred
astronomical units (an astronomical unit is the distance between Earth and
the Sun) (Figure 4). This corresponds to a location where the brightness
also decreases, suggesting that at this distance from the star there are
As the team investigated models of the Beta Pic disk that can explain both
the new and old observations, they found that the dust in Beta Pic’s disk is
more than ten times larger than typical grains of interstellar dust. Beta
Pics dust disk is probably made of micrometer sized loose clumps of dust and
ice like miniscule bacteria-size dust bunnies.
Together, these results provide very strong evidence that the disk
surrounding Beta Pic is generated by the formation and collision of
planetesimals. The level of detail of this new information solidifies our
understanding of the environment in which planets form and develop.
Motohide Tamura who leads the team says: “few people have been able to study
the birth place of planets by observing polarized light with a large
telescope. Our results show that this is a very rewarding approach. We plan
on extending our research to other disks, to get a comprehensive picture of
how dust transforms into planets.”
These results were published in the April 20, 2006, edition of the
Team Members: Motohide Tamura, Hiroshi Suto, Lyu Abe (NAOJ), Misato Fukagawa
(Nagoya University, California Institute of Technology), Hiroshi Kimura,
Tetsuo Yamamoto (Hokkaido University)
This research was supported by the Ministry of Education, Culture, Sports,
Science and Technology of Japan through a Grant-in-Aid for Scientific
Research on Priority Areas for the “Development of Extra-solar Planetary