The Path To Planet Formation
November 28, 2012

Subaru Telescope Used To Study Protoplanetary Disk

April Flowers for - Your Universe Online

A new study, using the capabilities of the Subaru Telescope, has captured a clear image of the protoplanetary disk of the star UX Tau A.

An international team of researchers from the National Astronomical Observatory of Japan (NAOJ) and the Japanese universities of Kobe, Hyogo, and Saitama released a detailed study of the disk's characteristics, suggesting that its dust particles are large in size and non-spherical in shape. The results of this observation show that the dust particles are colliding with and adhering to each other. This process will eventually lead to the formation of planets.

The Subaru Strategic Exploration of Exoplanets and Disks Project's (SEEDS) mission is to explore hundreds of nearby stars in an attempt to directly image extrasolar planets and protoplanetary/debris disks. The current team of researchers used the Subaru High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) to observe UX Tau A — a young star in the Taurus constellation's molecular cloud or "star nursery" - as part of the SEEDS initiative. The HiCIAO is mounted on the Subaru Telescope. Using HiCIAO, the team was able to detect the "circumstellar" disk of gas and dust around the star. Such a disk is called a protoplanetary disk when it is the site of planet formation.

Using near-infrared wavelengths, the team made a detailed study of UX Tau A. To find out the distribution of dust particles that scattered the infrared light, they measured the polarization of the light. Such distribution gives important information about planetary formation in disks. The dust particles make up only a tiny fraction of the protoplanetary disk, however, they can develop in to planetesimals — solid objects less than a kilometer in diameter. These eventually form into planets.

The light from UX Tau A's disk is strongly polarized. The angle of polarization shows a concentric pattern relative to the central star.

"The objects we have observed so far show a high degree of polarization no matter what the angle is. However, the polarization of this particular object ranges widely from 2 to 66 % as the polarization angle changes. It was a pleasant challenge to explain this characteristic," said Yoichi Itoh of the University of Hyogo.

Only 0.1 microns in size, the dust particles in the disk originally came from interstellar space. Smaller than the observed wavelengths, small grain particles can produce a high degree of polarization regardless of their location. If the grain size is closer to the size of the wavelength, the scattering performance is very different. The challenge, however, is that these principles do not account for the current observations of this disk.

Itoh explained, "Only particles with a non-spherical shape and a size of 30 microns, which is much larger than the near-infrared wavelength that was used for the observation, can explain the features of our observation."

Dust particles collided and adhered to each other in the disk, growing to 30 microns in size. The research team feels fortunate that they were able to witness the dust particles at this critical phase in the formation of a fully-grown planet in the protoplanetary disk.

The findings of this study will be published in an upcoming issue of Publications of the Astronomical Observatory of Japan.