Title: Properties of Unidentified 9 Micron Band in M17

Authors:
Yasuhiro Takahashi, Takashi Onaka, Itsuki Sakon(The University of Tokyo), Yoshiko K. Okamoto(Ibaraki University), Hirokazu Kataza(ISAS/JAXA), Hidehiro Kaneda(Nagoya University)

Abstract:
Understanding the evolution of dust is one of the most important issues in today's infrared astronomy. In this study, we focus on the changing in properties of dust in massive star forming region, where dust is illuminated by harsh radiation fields. M17 is a well-known massive star forming region located at the distance of 1.6kpc. We have carried out the N-band low-resolution spectroscopic observations of the compact HII region in the southern bar of M17 using Cooled Mid-infrared Camera and Spectrometer (COMICS) onboard Subaru telescope. The slit was settled so that it goes across both the infrared core and the diffuse nebulae. In the diffuse nebulae, a 11.2um PAH feature is detected. On the other hand, in the infrared core, where a harsh environment is suggested from the relatively high ionic line ratios of [SIV]/[NeII] and [SIV]/[ArIII], we found almost no signs of PAH features but, instead, a broad unidentified 9um band with a FWHM of 1um, which was originally reported by Peeters et al. (2005). We found this new broad 9um band is emitted only from the region within ~0.015pc from the center of the infrared core, which suggests that the emitting process and/or the formation process of the carrier might be closely related with the harsh radiation environment of the massive star forming region. Further analyses in collaboration with the laboratory experiments are important to identify the carrier of the unidentified 9um band and to understand their formation process.