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Removing Atmospheric Blur

New adaptive optics have removed atmospheric blurring at the 6.5-meter Magellan telescope in Chile to give it twice the resolution of the Hubble space telescope. Already the new Magellan system (MagAO) has imaged a type of planet not found before and mapped an unexpected distribution of dust in a protoplanetary disk. As the new adaptive technology is more widely installed, particularly on instruments such as future 25-meter and 39-meter telescopes, much more information on the habitability of planets around other stars will be gathered.

planet discovery  

A discovery image of planet HD 106906b in thermal infrared light from MagAO/Clio2, processed to remove the bright light from its host star, HD 106906 A. The planet is more than 20 times farther away from its star than Neptune is from our Sun. (Image: Vanessa Bailey)

Astronomers at the University of Arizona, Arcetri Observatory in Italy, and at the Carnegie Observatory developed the adaptive optics technology that achieves the new high resolution. University of Arizona professor Laird Close, the project’s principal scientist, explained “We can, for the first time, make deep images that resolve objects just 0.02 arcseconds across.” (This could be compared to resolving a baseball diamond on the Moon).

To obtain this degree of correction of atmospheric turbulence for visible light, the team developed a curved glass mirror 85 cm across and 1.6 mm thick that floats on a magnetic field.9.2 meters above the Magellan primary mirror. The shape of this adaptive secondary mirror can be changed at 585 points on its surface 1000 times a second.

offon 

On the left is a “normal” photo of the theta 1 Ori C binary star in red light. The middle image shows the same object, but with the MagAO adaptive optics system turned on. Eliminating the atmospheric blurring, the resulting photo becomes about 17 times sharper, turning a blob into a crisp image of a binary star pair. These are claimed to be the highest resolution photos taken by a telescope. (Photo: Laird Close, University of Arizona)

MagAO's development was funded by the National Science Foundation's Major Research Instrumentation program, its Telescope System Instrumentation Program, and an Advanced Technologies and Instrumentation Award.

Direct imaging of the planet was carried out in the infrared using MagAO and the Clio2 camera. This enabled detection of the weak heat emitted by the exoplanet with minimal interference by light from the hotter parent star. Clio2 was optimized for thermal infrared wavelengths where giant planets are brightest compared to their host stars, explained UA astronomy professor and Clio principal investigator Philip Hinz, who directs the UA Center for Astronomical Adaptive Optics.

The international team detecting the new planet 300 light years from Earth, HD 106906b, was led by Vanessa Bailey a fifth-year graduate student in the UA Department of Astronomy. Eleven times Jupiter's mass, the planet orbits the star HD 106906 at a distance of 650 AU. The mass ratio between star and planet of greater than 100:1 indicates that HD 106906 is a planet and not a second member of a binary system. With an age of 13 million years it still glows from the heat of formation. However, at a temperature of 1,500 deg C, the planet is much cooler than its host star, causing it to radiate primarily in the infrared. The team was able to confirm that the planet is moving together with its host star by examining Hubble Space Telescope data taken eight years prior for another research program.

This star system is also of particular interest because researchers can still detect the remnant “debris disk” of material left over from planet and star formation. “Systems like this one, where we have additional information about the environment in which the planet resides, have the potential to help us disentangle the various formation models,” Bailey pointed out. "This system is especially fascinating because no model of either planet or star formation fully explains what we see." It is thought that planets close to their stars, like Earth, coalesce from small asteroid-like bodies born in the primordial disk of dust and gas that surrounds a forming star. However, this process acts too slowly to grow giant planets far from their star. Another proposed mechanism is that giant planets can form from a fast, direct collapse of disk material. However, primordial disks rarely contain enough mass in their outer reaches to allow a planet like HD 106906b to form.

debris silhoutte 

A MagAO image of the Orion 218-354 debris disk silhouette after removal of light from the central star. The image on the left shows the dark silhouette of the disk against the bright background light of the Orion nebula. The image on the right shows the pattern imposed on the nebular light passing through the disk, indicating the distribution of gas and dust. (Photo: Kate Follette/UA)

The amount of dust in various regions of a debris disk was measured in one of the rare “silhouette” disks in Orion, around the faint star Orion 218-354. Because the disk lies face-on in front of the bright Orion nebula, we see the dark shadow cast by the dust. This was imaged in the bright 656 nm hydrogen alpha emission line from the background nebula by the MagAO. The Magellan VisAO camera was used in its simultaneous differential imaging mode (SDI) to minimize the light from the star—giving, for the first time, a clear look at the inner regions of the disk's silhouette. This allowed the international team to trace the variation of absorption (hence mass) across the disk. “We were surprised to find that the amount of attenuated light from the nebula increased gradually, rather than sharply, toward the star”, noted Arizona graduate student Kate Follette.“It seems as though the outer parts of this large disk have less dust than we would have expected”. Follette is the lead author of a letter to the Astrophysical Journal describing the research.