Beta Pic’s Dust Bunnies

An artists image of a scientifically accurate model of Beta Pictoris and its disk. Note that the actual observing data do not fully extend to the central star due to the use of a coronagraphic mask.
Credit: Subaru Telescope

A combination of observing strategy and advanced technology has produced the most detailed picture yet of a dust disk surrounding a nearby star. Observations of the disk surrounding the star Beta Pictoris by a team of researchers from the National Astronomical Observatory of Japan, Nagoya University and Hokkaido University suggest that asteroid and comet-like objects are colliding to produce fluffy icy dust-balls the size of bacteria.

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 a 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.

 

The dust disk surrounding the star Beta Pictoris seen in infrared light 2 micrometers in wavelength. The disk appears long and thin because we are looking at it edge-on. While observing, a coronagraph blocked light from the star. The black circle in this image masks out areas that may have been affected by light from the star. The size of the circle corresponds to the size of the Solar System (100 astronomical units). Some scattered light from the telescope’s secondary mirror remains in the image right above the black circle. North is up, East is left.
Credit: Subaru Telescope

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.

Polarization vectors superimposed among the image of the Beta Pictoris disk showing the direction of the light’s polarization. Most of the vectors are perpendicular to the direction towards the central star,indicating that the light originated from the star and then reflected off particles in the disk. The light is polarized by about 10%.
Credit: Subaru Telescope

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 fewer planetesimals.

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.

If starlight reflects off dust that evenly surrounds it, the polarization vectors would look like the left image. If it reflects off a thin disk the polarization would look like the right image. The polarization pattern of the Beta Pic disk resembles the right image.
Credit: Subaru Telescope

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 Astrophysical Journal.