------------------------------------------------------------------------
  TsubGC50MC9.tex PASJ Feb 2009, in press
Date: Wed, 7 Jan 2009 12:08:02 +0900
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¥title{The Galactic Center 50-km s$^{-1}$ Molecular Cloud with An  
Expanding Shell}
¥SetRunningHead{Tsuboi et al.}{50-km s^{-1} Molecular Cloud}
¥author{Masato T¥sc{suboi}}
¥affil{Institute of Space and Astronautical Science, Sagamihara,  
Kanagawa 229-8510}
¥affil{Department of Astronomy, the University of Tokyo, Bunkyo,  
Tokyo 113-0033}
¥author{Atsushi M¥sc{iyazaki}}
¥affil{Mizusawa VERA Observatory, National Astronomical Observatory,  
¥¥ Mizusawa, Oshu, Iwate 023-0861}
¥author{Sachiko K. O¥sc{kumura}}
¥affil{National Astronomical Observatory, Mitaka, Tokyo, 181-8588}

¥KeyWords{Galaxy, molecular cloud, super nova remnant}
¥maketitle

¥begin{abstract}
We performed high-resolution observation of the Galactic Center 50-km s 
$^{-1}$ molecular cloud in the CS $J = 1-0$ line using the Nobeyama  
Millimeter Array (NMA).  The molecular cloud  mainly has three  
different spatial components with large velocity widths up to 60 km s 
$^{-1}$.  The northwest component is located at an apparent contact  
point to the Sgr A east shell and elongated along the boundary of the  
shell. The large velocity width of the component is responsible for  
the interaction with the Sgr A east shell. The molecular gas  
distribution in CS line emission is dissimilar to that observed  
previously in NH$_3$ line emissions. The appearances shows presumably  
the area of CS line emission enhanced by shock. The central and  
southwest components are located just out of the Sgr A east shell and  
far from it, respectively.   However, these components have large  
velocity widths.  We found a well-shaped circular molecular shell with  
expanding motion in the  50-km s$^{-1}$ molecular cloud.  This is  
responsible for the large velocity width. The continuum source in the  
expanding molecular shell has a steep spectrum in the mm-wave although  
this is not identified in the previous 5-GHz map. This source may be  
an SNR with an ionized sheath. From the aspect ratio of the expanding  
molecular shell  of 1.1, the magnetic field in/around the shell is  
estimated to be smaller than 100$¥mu$ Gauss. The weak magnetic field  
is consistent with on-going active star formation in the 50-km s 
$^{-1}$ molecular cloud.  The comparison among CS line emission, low  
frequency continuum, and millimeter continuum toward the 50-km s 
$^{-1}$ molecular cloud suggests a face-on view of the Sgr A region.  
The molecular cloud is located in the Sgr A halo region.

¥end{abstract}