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\title{Subaru/COMICS Mid-Infrared Observation of the Near-Nucleus Region
�@�@�@ of Comet 17P/Holmes at the Early Phase of an Outburst} 
\date{}
\author{Watanabe, Jun-Ichi{\small(NAOJ)}, Honda, Mitsuhiko{\small(Kanagawa University)},
Ishiguro, Masateru{\small(NAOJ)}, \\Ootsubo, Takafumi{\small(ISAS/JAXA)}, 
Sarugaku, Yuki{\small(University of Tokyo)}, Kadono, Toshihiko{\small(Osaka University)}, 
\\ Sakon, Itsuki{\small(University of Tokyo)}, Fuse, Tetsuharu, Takato, Naruhisa{\small(NAOJ/Hawaii)},
Furusho, Reiko{\small(NAOJ)}}
\maketitle

A Mid-infrared imaging and spectroscopic observations of
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 at 8.7$\pm$0.3 (''/day) (fig. 1). The 11.2 $\mu$m peak of a crystalline silicate
feature onto a broad amorphous silicate feature was detected
both in the central condensation of the nucleus and the isolated dust
cloud. The color temperature of the cloud is estimated to
be $\sim$ 200K. Our analysis of the motion indicates the isolated cloud
moved anti-sunward(fig. 2). We propose several possibilities for the motion of 
the cloud;fluffy dust particles in the isolated cloud started to 
depart from the nucleus by radiation pressure almost as soon as the 
main outburst occurred, or dust particles moved by some other forces 
such as rocket effect and photophoresis when the surrounding dust coma became 
optically thin. 

\begin{figure}[h]

\begin{center}
\includegraphics[width=0.40\textwidth]{17Pmotion.eps}
\caption{\small 
(Top) Light curve of comet 17P/Holmes. (Bottom) Motion of the 
isolated dust cloud to the central condensation. The
dashed and the solid lines denote the first-order
(a constant velocity model) and second-order fits (a constant
acceleration model), respectively. 
The estimated onset time of the outburst is indicated by arrows.
}
\end{center}
\end{figure}

\begin{figure}[h]
\begin{center}
\includegraphics[width=0.28\textwidth]{17P-N88-20071024-2.ps}
\includegraphics[width=0.28\textwidth]{17P-N88-20071025-2.ps}
\includegraphics[width=0.28\textwidth]{17P-N88-20071026-2.ps}
\includegraphics[width=0.28\textwidth]{17P-N88-20071027-2.ps}
\caption{\small 
�@N8.8 images of 17P on Oct. 25, 26, 27, and 28, 2007
UT. The flux unit is in Jy/pixel. The pixel scale is 0.13''.
The cross indicates the center of cloud and the rectangular region is the aperture for photometry (31 pix $\times$ 31 pix). The isolated cloud was moving toward the anti-sun direction. 
}
\end{center}
\end{figure}

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\begin{thebibliography}{}

\bibitem{} Watanabe, J., Honda, M., Ishiguro, M., Ootsubo, T., Sarugaku, Y., Kadono, T., 
 Sakon, I., Fuse, T., Takato, N., Furusho, R.: 2009, {it Publ. Astron. Soc. Japan}, {\bf 61}, 67\end{thebibliography}

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