July 3, 2006
Bewildering Diversity of Protoplanetary Disk Shapes
A close look at the protoplanetary disk around a young star by two teams of astronomers using the Subaru telescope on Mauna Kea has led to the unexpected discovery of two banana-shaped arcs facing each other.
The disk, which surrounds the star HD142527, also shows a gap that could be the tumultuous birthplace of a planet, and an extended arc that could have formed during a recent encounter with a stellar neighbor. This discovery adds yet more variety to the bewildering diversity of protoplanetary disk shapes -- ranging from donuts to spirals -- that astronomers are finding as they study the birthing grounds of planets around other stars.The astronomers used two different instruments on Subaru to observe the disk around HD 142527. A team from Nagoya University, the National Astronomical Observatory of Japan/Graduate University for Advanced Studies (NAOJ/Sokendai and Kobe University observed the protoplanetary disk using the Coronagraphic Imager with Adaptive Optics (CIAO) in the near-infrared at 1.65 and 2.2 microns with a resolution of 0.13 arcseconds. This allowed the team to see details of the disk on a scale comparable to the orbit of Uranus and Neptune in our own solar system.
Adaptive optics technology minimized the effect of Earth's atmosphere to improve the image quality. Coronagraphy, which hid the central star to make fainter material around it easier to detect, also contributed to the successful observations.
Another set of observations made by researchers from the University of Tokyo, Japan Aerospace Exploration Agency (JAXA), NAOJ/Sokendai, and Ibaraki University focused on the protoplanetary disk in mid-infrared wavelengths of 18.8 and 24.5 microns using Subaru's Cooled Mid-Infrared Camera and Spectrograph (COMICS).
The images, with spatial resolutions of 0.5 arcseconds and 0.6 arcseconds, show radiation emitted by the disk out to beyond 100 astronomical units, or three times the distance between Neptune and the Sun. This is the first time that a protoplanetary disk has been detected in the mid-infrared to such a distance.
The mid-infrared observations also extend closer in toward the star and reveal a clear gap between two separate structures: a compact disk about 80 astronomical units in radius and an extended disk that echoes the banana-split shape seen in the near-infrared observations and reaches out to a radius of 170 astronomical units.
For both the near-infrared and mid-infrared images, the difference in brightness in opposite sides of the extended disk is due to the tilt of the disk. The side farther from us is fainter in the near-infrared. In the mid-infrared, it is brighter.
The mid-infrared observations also showed both the size of dust grains in the disk and their temperature. From this information, the team was able to determine that grains of dust in the disk are growing to sizes that are larger than is typical of the dust found between stars.
Before obtaining these detailed images, astronomers expected to find smooth disks around young stars. Yet, recent observations of disks around the stars GG Tauri and AB Aurigae have changed the picture. GG Tauri has a donut-shaped disk, and the disk around AB Aurigae is distinctly spiral-shaped. HD142527's "banana split" construction now seems to be a variation on the theme of diverse protoplanetary disks.
The most likely explanation for the "banana-split" shape of HD 142527 is the presence of another object orbiting the star, a much dimmer companion star or possibly a planet. The extended arc is most likely due to the gravitational tug of a passing star sometime in the last thousand years. Because astronomers expect most stars to be born in groups along with other stars, many features of HD142427's newly charted disk may be common to other stars born with companions.
The new images are the first images of HD142527's protoplanetary disk ever obtained, and among the very few examples of successful direct imaging of a protoplanetary disk from an Earth-based telescope. HD142527 lies only about 650 light-years from Earth, yet despite this star's proximity, turbulence in our own planet's atmosphere makes clear images of its faint protoplanetary disk extremely difficult to get. The successful observations that led to these results relied on the size, stability and location of the Subaru telescope and its instruments, along with the use of its adaptive optics and coronagraphic technology.
Protoplanetary Disks and the Advantages of Infrared Observations
To understand how planets form it is important to learn about protoplanetary disks. These accumulations of gas and dust surround young stars and are the birthing grounds for planets. As a star is born and grows, the disk forms out of the the same material as the star: gas with a small dust component.
Over time, the dust in protoplanetary disks accumulates into larger objects, which eventually create protoplanets. These collide to form planets. Recently astronomers have surveyed stars that are about a million years old to understand the dusty environments in which planets form. Infrared observations are particularly powerful tools to help characterize the detailed structures around such stars.
Protoplanetary disks emit light in many wavelengths, including visible, infrared and millimeter wavelengths. Infrared wavelengths carry information about the structure, temperature and other physical properties of the disk and its dust particles. Yet, even with infrared observations, there are still challenges in observing them. Protoplanetary disks are faint compared to the stars they surround, so obtaining images of them can be difficult.
Protoplanetary disks reflect near-infrared light from the central star. With the use of adaptive-optics technology, near-infrared observations can reveal the detailed structure of the disk at high resolution. However, since the light doesn't directly originate from the disk, it doesn't carry information about the temperature and density of the disk.
At longer mid-infrared wavelengths, the resolution drops but light emitted by the disk itself can be observed to get information about the disk temperature. Since the central star is also fainter at longer wavelengths, it is easier to study regions closer to the star at mid-infrared wavelengths. Combining observations at both near- and mid-infrared wavelengths gives a more comprehensive picture of protoplanetary disks.
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