Two GeV-TeV Supernova Remnants without Associated Neutral and Molecular Clouds

TeV gamma ray emissions have been detected at the directions of supernova remnants (SNRs) W51C and Tycho SNR. We analyze the Hi absorption spectra towards W51C, Tycho SNR and their nearby compact sources. We conclude that W51C is at a distance of about 4.3 kpc and Tycho SNR has a distance of 2.5 ~ 3.0 kpc. Our study detects high-velocity Hi clouds which coincide with star formation region W51B, but finds that the clouds are behind W51B which argues against previous claims that W51C has shocked the high velocity Hi clouds. We argue that Tycho SNR is naked Ia SNR (lack of evidence of interacting with adjacent neutral and molecular cloud). This gives two examples that the very high gamma ray emission from SNRs does not likely originate from SNR-cloud interaction.