Subaru/COMICS mid-infrared observation of the near-nucleus region of comet 17P/hoimes at the early phase of an outburst

Mid-infrared 8-25 μm imaging and spectroscopic observations of the comet 17P/Holmes in the early phase of its outburst in brightness were performed on 2007 October 25-28 UT using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2-m Subaru Telescope. We detected an isolated dust cloud that moved toward the south-west direction from the nucleus. The 11.2μm peak of a crystalline silicate feature onto a broad amorphous silicate feature was also detected both in the central condensation of the nucleus and an isolated dust cloud. The color temperature of the isolated dust cloud was estimated to be ∼ 200 K, which is slightly higher than the black-body temperature. Our analysis of the motion indicates that the isolated cloud moved anti-sunward. We propose several possibilities for the motion of the cloud: fluffy dust particles in the isolated cloud started to depart from the nucleus due to radiation pressure almost as soon as the main outburst occurred, or dust particles moved by some other anti- sunward forces, such as a rocket effect and photophoresis when the surrounding dust coma became optically thin. The origin and the nature of the isolated dust cloud are discussed in this paper.