Direct Images of Disks Unravel Mystery of Planet Formation
February 17, 2011
The fruits of the SEEDS (Strategic Explorations of Exoplanets and Disks with Subaru) Project, led by Motohide Tamura at NAOJ (National Astronomical Observatory of Japan), are accumulating. Composed of over 100 scientists and 25 institutions, the international consortium of researchers supporting the project has announced another set of stunning findings obtained with the recently commissioned Subaru instrument HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics), an upgraded version of its predecessor CIAO (Coronographic Imager with Adaptive Optics). Their initial announcement of a significant discovery came in December, 2009: an exoplanet candidate around a Sun-like star. Now they are announcing another remarkable discovery: direct and sharp images of the protoplanetary disks of two young stars that reveal how planets may have formed within them. No other telescopes, whether ground-based or in space, have ever penetrated so close to a central star, showing the details of its disk.
The Significance and Challenges of Research on Protoplanetary Disks
One key to understanding planetary formation lies in the protoplanetary disks of young stars, which provide the initial conditions for the development of planets. These flattened structures of dense gas and dust evolve as by-products of star formation and become the womb for the maturation of planets (Conventional Schematic Diagrams of the Evolution of a Protoplanetary Disk around a Sun-like Star). However, scientists have not determined the actual details of how planets originate and mature. The detection of over 500 exoplanets that orbit around stars outside of our solar system has heightened interest in disks as a source of planetary formation. Did planets grow from the collision of bodies of rock and/or ice (planetesimals) or from gravitational instability in their disks? How does the development of exoplanets help us explain how the Earth formed?
The answers are difficult to obtain because disks are challenging to study. Both the small angular size of disks as well as the apparent dimness of a disk relative to its bright central star pose barriers to detailed observations of disks. In addition, available spatial resolution has only permitted the study of the outer envelope of a disk's structure. Finally, the scale size for observations is much larger than the familiar scale of our solar system, a scale that even the highest resolution telescopes have had difficulty in accessing so far.
Specialized instruments help scientists meet these challenges. A coronagraph facilitates observation of dim objects around a star by masking its extremely bright light while adaptive optics (AO) enhance spatial resolution by compensating for the blurring effects of the Earth’s atmosphere.
Subaru Telescope has been in the forefront of developing instruments designed for planet-hunting. In late 2009 Subaru Telescope replaced its earlier coronograph CIAO with HiCIAO that features not only a 188-element AO system and a stellar coronograph thats block out the central star's light but also various advanced techniques to enhance observation of the fine features inside a disk.
The Strategic Approach of the SEEDS Project
The SEEDS Project uses HiCIAO's planet-hunting technology to study exoplanets and their processes of formation. Begun in 2009 and led by Motohide Tamura of NAOJ, the project is one of the first large-scale undertakings approved by Subaru to implement a strategic, coherent approach to exploring the universe with its telescope. The consortium of project supporters has grown to include an international group of scientists and institutions as well as a variety of experts, including those in the areas of data analysis and high resolution imaging.
The SEEDS Project’s New Discoveries about Protoplanetary Disks
The SEEDS' Project has yielded new discoveries about protoplanetary disks that contribute to our understanding of how planets may form. They focus on observations of two young stars.
Details of the Disk of AB Aur
One of the primary targets was the very young star AB Aur in the constellation Auriga ("the Charioteer"). It is only about one million years old and 460 light years away from Earth. The research group has succeeded in directly imaging the fine details of AB Aur's disk. This is the first, finest, and sharpest image of a disk ever taken for this or any other objects.
Figure 1 shows the recent images of AB Aur taken by HiCIAO compared with the image taken by CIAO in 2004. The HiCIAO images display high spatial resolution and high contrast that reveals fine details of the disk’s inner structure that are on a scale similar to Neptune’s orbit in our solar system. Such precision of features near its central star contrasts with their masking in the earlier CIAO image.
The bottom two images in Figure 1 show close-ups of the inner disk, displaying the richness of its features. Double rings with some intricate bright and dark patterns are readily visible; they are tilted relative to each other, and strangely, their centers do not coincide with the position of the central star. A gap between these rings is rather strikingly void of material.
The disk's irregularities point to the presence of a giant planet that may sweep up material between the rings and cause irregularities in them with its gravitational force. Unfortunately this planet is not visible because disk material still covers it, extinguishing the planet's light.
A Gap in the Disk of the Star LkCa 15
The research group also targeted the star LkCa 15 for an examination of its disk. The star LkCa 15 is located toward the constellation Taurus, has a weight similar to the Sun's, and is several million years old. Its disk has been observed for some time. Although spectral energy distributions have indicated a gap in the disk, no direct imaging has clearly confirmed the presence of the gap-until now. The SEEDS group used HiCIAO for their observations, which succeeded in capturing a high-resolution image of the LkCa 15 disk.
Figure 2 shows an image of the disk of LKCa 15, which is masked in the dark area. The faint feature below the masked star is part of the disk illuminated by the central star. The opposite segment of the disk is not visible. The void between these features is the gap between the disk and the central star, which has a scale similar to Neptune's orbit in our solar system. The lack of material in the vicinity of the central star implies that a giant planet is sweeping up the disk's leftover materials that the central star did not swallow. Although the image might seem a bit blurry to readers, this is the first clear example of a truncated structure of a protoplanetary disk.
Implications for Planet Formation
These two results are the first to show features within disks where the planets are actually being born, and on a scale similar to that of our solar system. The direct imaging strongly indicates the existence of Jupiter-like giant planets that have affected the structure of the disks. Theorists were surprised to discover planets already formed within one million years. They had thought that giant planets such as Jupiter and Saturn in our solar system as well as giant exoplanets would take several tens of million years to form. The findings from the current research give them a tighter boundary condition for developing a theory of planetary formation. And the SEEDS Project will continue to search for and study exoplanets over the next five years, contributing even more to solving the mysteries of planet formation.
These discoveries are reported in the Astrophysical Journal Letters(Volume 729, page 17, 2011 March 10 issue and Volume 718, page 87, 2010 August 1 issue)
Institutions and Institutional Affiliations of Researchers
National Astronomical Observatory of Japan, The Graduate University for Advanced Studies (SOKENDAI), Max Planck Institute for Astronomy (Germany), Hokkaido University, Tohoku University, Ibaraki University, Saitama University, The University of Tokyo, Tokyo Institute of Technology, Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency, Kanagawa University, Nagoya University, Nagoya City University, Osaka University, Kobe University, The Open University of Japan, Princeton University, University of Hawaii, Jet Propulsion Laboratory, Academia Sinica Institute of Astronomy and Astrophysics ASIAA (Taiwan), University of Nice Sophia Antipolis (France), University of Hertfordshire (UK), Eureka Scientific and Goddard Space Flight Center, University of Washington, The College of Charleston.
This work is supported by a Grant-in-Aid for Specially Promoted Research from the Japan's Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Figure 1: Near-infrared (1.6 micron) images of AB Aur.
The top panels compare images taken by HiCIAO and CIAO. Both images have a field of view of 7.5" by 7.5".
For this object, 1" (one arc second) corresponds to 144 AU in real scale (144 times the distance between Earth and Sun).