IMAGING THE COOL HYPERGIANT NML CYGNI'S DUSTY CIRCUMSTELLAR ENVELOPE WITH ADAPTIVE OPTICS

We present subarcsecond angular resolution, high-Strehl ratio mid-IR adaptive optics images of the powerful OH/IR source and cool hypergiant NML Cyg at 8.8, 9.8, and 11.7 mu m. These images reveal once more the complexity in the dusty envelope surrounding this star. We spatially resolve the physical structures ( radius similar to 0 ''.14, similar to 240 AU adopting a distance of 1.74 kpc) responsible for NML Cyg's deep 10 mu m silicate dust absorption feature. We also detect an asymmetric excess, at separations of similar to 0 ''.3-0 ''.5 (similar to 520-870 AU), northwest from the star. The colors of this excess are consistent with thermal emission of hot, optically thin dust. This excess is oriented in the direction of the Cyg OB2 stellar association, and is likely due to the disruption of NML Cyg's dusty wind with the near-UV radiation flux from the massive hot stars within Cyg OB2. This interaction was predicted in our previous paper to explain the geometry of an inverted photodissociation region observed at optical wavelengths.