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   PR # 21044
    Blue massive stars can form low density stellar wind cavities surroun
    ded by dense shells, before supernovae explode. Before their remnants
    (SNR) collide  with the dense stellar shells, they may remain in the 
    free expansion phase, and  the reverse shock has not yet reached the 
    center.The shell region is hotter than the interior and can be as hot
    as several keV. The SNR G54.4-0.3, which  interacts partially with a 
    CO shell, is such a candidate.The temperature is too high to be deter
    mined in the ROSAT energy band. A hard X-ray peak in the  Ginga scan 
    profile is located just on the southwestern shell region of the SNR. 
    We propose to observe three areas of the radio shell region of this  
    SNR expected to be hot, based on the Ginga scan and the ROSAT and   
   radio morphologies.          
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