22 Jun 2015

A new optical component developed by astronomers at the UA and Leiden University in the Netherlands pushes the capability of detecting alien planets closer to their host stars than ever before.

Astronomers from Leiden University in the Netherlands and the University of Arizona have successfully commissioned a new type of optic that can reveal the image of an exoplanet next to its parent star. The "vector Apodizing Phase Plate," or vector-APP, coronagraph was installed at the 6.5-m Magellan Clay telescope in Chile in May, and the first observations demonstrated an unprecedented contrast performance very close to the star, where planets are more likely to reside.
Almost 2,000 exoplanets have been detected to date, but only a few of those have been imaged directly. Exoplanets are typically more than a million times fainter than and are lost in the glare of their parent star as seen from Earth. To directly image exoplanets and to characterize their atmospheres, astronomical instruments at the world's largest telescopes use coronagraphs to suppress the overwhelming halo of light from the star.
Building on technology developed at the UA's Department of Astronomy, the vector-APP coronagraph uses the wavelike nature of light to cancel out the starlight while allowing the planet’s light to shine through.
This manipulation is implemented through a complex phase pattern that can only be manufactured using advanced liquid crystal 3-D patterning techniques. This technique creates two images of the star, for which dark, D-shaped regions are located on opposite sides of each star image. In this way, the whole region around the star can be scrutinized for planets. By combining several layers of liquid crystals, the device can be used over a wide range of wavelengths, including the infrared where the contrast between planet and star is more favorable.
On May 6, a vector-APP coronagraphic device saw first light in the infrared range of the spectrum at the Magellan Adaptive Optics, or MagAO, instrument, attached to the Magellan Clay telescope in Chile. The telescope's integrated adaptive optics system provided the instrument with sharp images of stars, which were consequently split up and modified by the coronagraph to exhibit dark holes in which much fainter planets could be imaged than without the vector-APP coronagraph.
"With this breakthrough, we're approaching the theoretically optimum angular resolution while performing high contrast imaging of exoplanets," said Jared Males, a NASA Sagan Fellow at the UA Department of Astronomy. "With this coronagraph, the MagAO system and its Clio infrared camera will be able to search for planets orbiting at just 0.5 AU from alpha Centauri A, the closest sunlike star."
One AU, or astronomical unit, is the average distance between the Earth and the sun.
"This allows astronomers to look for planets closer to their stars, probing distances like those of the planets in our own solar system," Males explained. "With this new coronagraph we are now looking for planets around nearby stars. We have the capacity to directly detect, or rule out, planets smaller than Jupiter. "
The advanced liquid crystal technology that the team adopted also permitted the production of extreme vector-APP designs that are not possible with more traditional manufacturing technologies. These new designs produce dark holes that cover the full 360 degrees around the target stars.
Frans Snik of Leiden University, who invented the principle behind the new vector-APP coronagraph, said: "It is fantastic to see that after all our design work and lab testing, this new approach works perfectly at the telescope on the very first night."
Gilles Otten, a doctoral student at Leiden involved in the project, added: "We knew that we were in business as soon as we saw the first picture on the screen in the telescope control room."
Matthew Kenworthy, also at Leiden, concluded: "This new coronagraph technology is also excellent news for the extremely large telescopes currently under construction. With a vector-APP coronagraph in the next generation of telescopes, we can search for planets around nearby stars with unprecedented sensitivity."

Image: (Top) Double image of the star beta Centauri taken through an experimental version of the vector-APP coronagraph installed at MagAO. Located 350 light-years from Earth, beta Centauri is a system of two companion stars orbiting each other at about four times the average distance between the Earth and the sun. Both images of beta Centauri contain a dark region that covers the complete 360 degrees around the central star. In both cases, the binary companion to beta Centauri is easily detected.
(Down) Double image of the star Eta Crucis taken through the vector-APP coronagraph installed at MagAO. The two main images of the star exhibit D-shaped dark holes on complementary sides. In this way, the whole region around the star can be scrutinized for planets.