Astronomers from Leiden University (the Netherlands) and the University of Arizona (USA) 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’ (vector-APP) coronagraph was installed at the 6.5-m Magellan Clay telescope in Chile in May 2015, and the first observations demonstrated an unprecedented contrast performance very close to the star, where planets are more likely to reside. These results will be presented by PhD student Gilles Otten (Leiden) this Monday afternoon to the scientific community at the "In the Spirit of Lyot" conference held in Montreal at the Marriott Chateau Champlain Hotel.

Almost 2000 exoplanets have been detected to date, but only a handful 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.

The vector-APP coronagraph[1] uses the wavelike nature of light to cancel out the starlight whilst 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 3D patterning techniques [2]. This technique creates two images of the star, for which dark D-shaped regions are located on opposite sides of each star image (see Figure 1). 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, 2015 a vector-APP coronagraphic device saw first light (at 3.9 microns wavelength, in the infrared range of the spectrum) at the MagAO[3] instrument, attached to the 6.5-m diameter 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.

Frans Snik (Leiden University), who invented the principle behind the new vector-APP coronagraph, says:“ 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 adds:“We knew that we were in business as soon as we saw the first picture on the screen in the telescope control room.”

Jared Males (NASA Sagan Fellow, University of Arizona) is excited about the opportunities of the vector-APP: “ 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. Matthew Kenworthy (Leiden) concludes: “ This new coronagraph technology is also excellent news for the extremely large telescopes currently under construction. Thermal infrared instruments such as the Mid-infrared E-ELT Imager and Spectrograph (METIS) can exploit this new technology and discover new worlds. With a vector-APP coronagraph in the next generation of telescopes, we can search for planets around nearby stars with unprecedented sensitivity.”

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. The first data from an experimental device already shows the viability of this novel approach (see Figure 2).

Support from the William F. and Elizabeth Lucas Junior Faculty Astronomy Award and the NASA Origins of Solar Systems program made this exciting commissioning possible at the MagAO instrument in Chile. This work was performed in part under contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute.

Quelle: NOVA