Friday, October 27, 2006

Circumstellar Disk

Subaru Telescope ENLARGE Image

The Role of a Circumstellar Disk
A star is born when gas collects within a molecular cloud. The gas is mainly in the form of molecular hydrogen. Because gas has angular momentum, it can't land directly onto the surface of a star. Instead, it forms a thin, disk-like structure around a star, and slowly loses momentum as it orbits the star and so that the star can eventually pull it in. Without such a "circumstellar disk", a star could not collect mass from its birth cloud.

Beyond its function as the gas supply for star formation, a circumstellar disk also provides raw material for planets. Material left over from the star formation gradually stick together, making pebbles and rocks. These amass together to form even larger bodies, such as 100-meter-wide planetesimals. All of this material continues to rotate around the star while it grows into ever-larger bodies. Eventually, if conditions are right, this accretion process produces a rocky planet similar to Earth.

Recent observational studies of circumstellar disks have been taking advantage of the thermal emission and scattered light from the solid material in disks. However, in the early epochs of a disk's existence, these solids only comprise about one percent of the total disk mass. The rest is still in the gas phase, and mainly in molecular form (like carbon monoxide). Looking at a disk and studying its carbon monoxide component rather than its dust grains, means we are looking at the gas disk, which is the main component of the disk.

A circumstellar disk only exists for a short time while its central star is collecting gas from it. To understand how a disk evolves, imagine that the entire lifetime of the star was only a hundred years. The circumstellar disk would only exist from three days to a month before it dissipates altogether. A star has only one chance to form a planetary system during the relatively short life of its circumstellar disk. If the ionizing radiation from the star prevents the dust disk from accreting into planets before it dissipates, then the star's chance to become the center of a solar system is lost forever. When and how a disk dissipates, therefore, has direct consequences for the possibility of planetary formation.
Photo-evaporation of a Disk
Top: An image of HD 141569A from HST (Clampin et al. 2003 AJ,126, 385). The inner 200AU of the disk is masked.
Middle: The spectral data combined into a single image showing the spatial distribution of gas and its velocity. The horizontal axis is the distance from the star and the vertical axis is the velocity.
Bottom: A close up of the middle panel. The combination of a high-resolution spectrograph with adaptive optics technology allows access to the inner most part of the disk hidden behind the mask in the HST data.
The size of the central opening in the disk suggests that photo-evaporation is responsible for its clearing. It is also possible that one or more planets could be sweeping up material as they orbit around the star. There is, as yet, no evidence for such planets.

These results will be published in
the Astrophysical Journal in late 2006 or early 2007.
Subaru Telescope Press release 23rd October 2006
Inner Rim of A Molecular Disk Spatially Resolved in Infrared CO Emission Lines.
The Research Group: Miwa Goto (Max Planck Institute for Astronomy, Heidelberg, Germany) Tomonori Usuda (Subaru Telescope, NAOJ) C. P Dullemong (MPIA) Th. Henning (MPIA) H. Linz (MPIA) B. Stecklum (MPIA) Hiroshi Suto (NAOJ)
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