2003ApJ...598L..35C

We utilized the unique 6.5 m Multiplie Mirror Telescope deformable secondary adaptive optics (AO) system to produce high-resolution (FWHM=0".3), very high Strehl mid-infrared (9.8, 11.7, and 18 µm) images of the post-asymptotic giant branch star AC Her. The very high (98%±2%) Strehls achieved with mid-IR AO led naturally to an ultrastable point-spread function (PSF) independent of air mass, seeing, or location on the sky. We find no significant difference between AC Her's morphology and our unresolved PSF calibration stars (µ UMa and α Her) at 9.8, 11.7, and 18 µm. Our current observations do not confirm any extended mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 µm images that suggested the presence of a resolved ∼0".6 edge-on circumbinary disk. We conclude that AC Her has no extended mid-IR structure on scales greater than 0".2 (R<75 AU). These first results of mid-IR AO science are very encouraging for future high-accuracy mid-IR imaging with this technique.