When the dust clears: New observations of solar systems in the making
New observations with the SUBARU Telescope in Hawaii show the protoplanetary disks surrounding two young stars in unprecedented detail. This is the first time that disk structures comparable in size to our own solar system have been resolved this clearly, revealing features such as rings and gaps that are associated with the formation of giant planets. The observations are part of a systematic survey to search for planets and disks around young stars using HiCIAO, a state-of-the-art high-contrast camera designed specifically for this purpose.
Figure 1: This image taken with the HiCIAO planet-hunter camera on SUBARU Telescope shows a bright arc of scattered light (white) from the protoplanetary disk around the young star LkCa 15 (center, masked out with a dark circle). The arc’s sharp inner edge traces the outline of a wide gap in the disk. The gap is decidedly lopsided – it is markedly wider on the left side – and has most likely been carved out of the disk by one or more newborn planets that orbit the star.
Planetary systems like our own share a humble origin as mere by-products of star formation. A newborn star's gravity gathers leftover gas and dust in a dense, flattened disk of matter orbiting the star. Clumps in the disk sweep up more and more material, until their own gravity becomes sufficiently strong to compress them into the dense bodies we know as planets. Recent years have seen substantial advances both in observations (mostly indirect) and in theoretical modeling of such »protoplanetary« disks. The two new observations have added intriguing new details, revealing some structures that had never before been seen directly.
One of the two studies targeted the star LkCa 15, which is located around 450 light-years from Earth in the constellation Taurus. At an age of a few million years, LkCa 15 is a young star – the Sun is a thousand times older. From previous observations of its infrared spectrum and its millimeter emissions, scientists had deduced the presence of a large gap in the center of its protoplanetary disk. The new images show starlight gleaming off the disk surface, clearly outlining the sharp edge of the gap for the first time. Most interestingly, the elliptical shape of the gap is not centered on the star, but appears lopsided.
»The most likely explanation for LkCa 15's disk gap, and in particular its asymmetry, is that one or more planets, freshly born from the disk material, have swept up the gas and dust along their orbits,« says Christian Thalmann, who led the study while on staff at the Max Planck Institute for Astronomy (MPIA). Intriguingly, the disk gap is sufficiently large to accommodate the orbits of all the planets in our own Solar System. It is therefore tempting to speculate that LkCa 15 might be in the process of forming an entire planetary system much like our own. »We haven't detected the planets themselves yet«, adds Thalmann. »But that may change soon.«
The second observation, led by Jun Hashimoto (National Observatory of Japan), targeted the star AB Aur in the constellation Auriga, at a distance of 470 light-years from Earth. This star is even younger, with an age of a mere one million years. The observations were the first to show details down to length scales comparable to the size of our own solar system – for comparison: At a distance of 470 light-years, the solar system has the same apparent size as a 1 Euro coin viewed at a distance of more than 10 km. They show nested rings of material that are tilted with respect to the disk's equatorial plane, and whose material, intriguingly, is not distributed symmetrically around the star – irregular features that indicate the presence of at least one very massive planet.
Both observations where made with the HiCIAO instrument at the 8.2 m SUBARU Telescope. Imaging a disk or planet close to a star is an enormous challenge, as it is very difficult to discern the light emitted by those objects in the star's intense glare. HiCIAO meets this challenge by correcting for the distorting influence of the Earth's atmosphere and by physically blocking out most of the star's light.
The observations are part of the SEEDS project, short for »Strategic Explorations of Exoplanets and Disks with SUBARU«. MPIA's managing director, Thomas Henning, one of the project's co-investigators, explains: »SEEDS is a five-year systematic search for exoplanets and protoplanetary disks. We are thrilled about the images the SUBARU telescope has produced as part of this project. Detailed observations like these are the key to understanding how planetary systems, including our own solar system, came into being.« SEEDS involves more than 100 researchers from 25 astronomical institutions in Asia (NAOJ and others), Europe (MPIA and others), and the US (Princeton University and others).
Prof. Dr. Thomas Henning (Co-Investigator, SEEDS)
Max Planck Institute for Astronomy, Heidelberg
Phone: (+49|0) 6221 – 528 200
Dr. Markus Pössel (Public Relations)
Max Planck Institute for Astronomy, Heidelberg
Phone: (+49|0) 6221 – 528 261
Thalmann, C., et al. 2010, »Imaging of a Transitional Disk Gap in Reflected Light: Indications of Planet Formation Around the Young Solar Analog LkCa 15« in Astrophysical Journal Letters 718, p. L87-L91 [ADS Entry]
Hashimoto, J. et al. 2011, »Direct Imaging of Fine Structures in Giant Planet-forming Regions of the Protoplanetary Disk Around AB Aurigae« in Astrophysical Journal Letters 729, p. L17 [ADS entry]
The SEEDS project team consists of about 100 researchers from 25 institutions in Japan, Europe and the US as well as in Taiwan. The participating institutions are: in Japan NAOJ, Graduate University of Advanced Studies, University of Air, Hokkaido University, Tohoku University, Ibaraki University, Saitama University, University of Tokyo, Tokyo Institute of Technology, ISAS, Kanagawa University, Nagoya University, Osaka University, Nagoya City College, Kobe University; in Europe: Max Planck Institute for Astronomy (Germany), University of Hertfordshire (UK), Université de Nice-Sophia Antipolis Parc Valrose (France), CSIC-INTA (Spain); in the US: Princeton University, University of Hawaii, NASA/JPL, NASA/Goddard, University of Washington, the College of Carleston; in Taiwan: Academia Sinica Institute of Astronomy and Astrophysics.
The research is supported by a Grant-in-Aid for Specially Promoted Research 22000005 from MEXT (Ministry of Education, Culture, Sports, Science and Technology of Japan).
What are protoplanetary disks?
Stars and their planets form when molecular clouds containing gas and dust collapse under their own gravity. In the course of this process, the clouds’ densest regions collapse and heat up, eventually forming stars. The matter surrounding each star will, quite generally, orbit the star, resulting in a flattened disk: the protoplanetary disk. As matter within that disk clumps to objects of ever greater size, planets are formed. The exact processes of planet formation – including such issues as the role of turbulence and magnetic fields – are the subject of active research. Observations such as the one reported here are crucial input for current models seeking to explain planet formation.
What do the star names mean?
For all but the very brightest stars, astronomers use a (sometimes bewildering) array of catalogue numbers. »LkCa 15« is entry number 15 in a small survey of stars emitting light characteristic for the chemical element Calcium, undertaken by astronomers at Lick Observatory – hence »Lick Calcium«, or »LkCa« for short. »AB Aur« denotes an entry in the General Catalogue of Variable Stars, first published in 1948 by the Academy of Sciences of the USSR. »Aur« stands for the constellation of Auriga, while AB is an alphabetical identifier (after A...Z, continue with AA, AB, ...) designating the specific star.
Which telescopes and instruments were used?
The observations were made with the SUBARU Telescope, the flagship telescope of the National Astronomical Observatory of Japan, a reflecting telescope whose main mirror is 8,2 metres in diameter. »SUBARU« is Japanese for the Pleiades, perhaps the best-known open star cluster.
The instrument – the »astronomical camera« used was HiCIAO, the »High-Contrast Coronagraphic Imager for Adaptive Optics«. HiCIAO is especially designed for the observations of faint objects near stars, such as exoplanets and protoplanetary disks. It uses state-of-the-art adaptive optics, a technique that can compensate for the disturbances caused as a distant stars’ light passes through the Earth’s atmosphere (the »twinkling« of stars), and it physically blocks out part of the stars’ light (»coronagraph«) to enable imaging of the much fainter objects next to it; remei
In late 2009, HiCIAO produced the first image of an ultracool (»planet-like«) brown dwarf companion around a Sun-like star (cf. this press release).
Figure 2b: A sketch of the three-dimensional shape of the protoplanetary disk around the star LkCa 15. Only the light reflected from the outer disk (shown in yellow) is seen on the HiCIAO images. The other structural features have been inferred from previous indirect observations of the system. The large gap between the inner and the outer disk has most likely been carved out by one or more newborn planets that orbit the star. The planets themselves have not been detected – yet.
Figure 3: This image shows a reconstruction of the geometry of the disk around LkCa 15 (dashed blue lines), superimposed on the HiCIAO image. The bright arc represents light from the central star (LkCa 15) that reflects off the surface of the disk. This kind of light scattering is particularly effective at grazing angles, which is why most of the observed light comes from the near side of the disk.
Figure 4: Recent images of AB Aur taken by HiCIAO (top left), compared with an image taken in 2004 by its predecessor instrument CIAO (top right). The new images give a much more detailed view of the inner regions (bottom left; with explanations bottom right): Intricate bright and dark patterns indicate the presence of different rings of matter. The fact that their centers do not coincide with the position of the star and the other irregularities point to the existence of a massive giant planet which is sweeping up the material between the rings.