High dynamic range direct imaging of exoplanets with an off-axis Antarctic telescope

摘要

This thesis presents an analysis of the potential for high dynamic range direct imaging of exoplanets with a propsed off-axis Antarctic telescope named the Large Antarctic Plateau Clear-Aperture Telescope (LAPCAT). LAPCAT is a proposed 8.4 metre off-axis telescope with a deformable 1 m secondary mirror to be located at Dome C in Antarctica.The low atmospheric temperatures and minimal high altitude turbulence make Dome C a unique site for astronomical observations. The low wind speeds, the absence of dust in the atmosphere and minimal seismic activity make this a very stable site. The off-axis design of LAPCAT will assist in reducing the emissitivity of the secondary mirror and spider arms which are likely to dominate the infra-red background at these low temperatures. Low sky emissivity is also desirable for high contrast direct imaging of faint infrared sources such as exoplanets. The performance due to LAPCAT's off-axis design, adaptive optics system, and Antarctic location is quantified here. Simulations have been run to compare the point spread functions of LAPCAT, two existing mid-latitude on-axis telescopes, and a hypothetical on-axis Antarctic telescope. For comparison I chose the Keck II telescope located at Mauna Kea, Hawaii, and Gemini South situated on Cerro Paranal, Chile. Keck II is an on-axis segmented telescope and Gemini is an on-axis monolithic telescope. Under diffraction-limited, seeing-limited and adaptive-optics-corrected seeing conditions the telescopes' PSFs is compared at six different wavelengths. Simulations were run at 1.25, 1.26, 2.2, 3.4, 5, and 10 μm, using Performance of Adaptive Optics for Large Apertures (PAOLA), an analytical adaptive optics simulation package written in IDL.Having studied the effects of a typical Antarctic atmospheric turbulence profile on the PSF, LAPCAT can be expected to out-perform similar aperture telescopes located at temperate sites. Results demonstrate the intended adaptive optics system for LAPCAT allows the telescope to reach the diffraction limit. LAPCAT is able to detect a 20 MJ 5 Gyr old planet out to 10 pc, and a 5 Gyr planet less than 40 MJ out to 100 pc at 5 μm. For 1 Gyr planets the best observing wavelengths are 5 μm and 10 μm. The results demonstrate that LAPCAT is more sensitive to hot young extrasolar giant planets but is unable to directly image an exoplanet with a mass less than 4 MJ.