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Localized SiO emission triggered by the passage of the W51C SNR shock

The region towards W51C is a convincing example of interaction between a supernova remnant and a surrounding molecular cloud. Large electron abundances have been reported towards the position W51C-E located in this interaction region, and it was proposed that the enhanced ionization fraction was due to cosmic ray particles freshly accelerated by the SNR shock. We present PdB interferometer observations of the H$^{13}$CO$^+$(1-0) and DCO$^+$(2-1) emission lines centered at position W51C-E. These observations confirm the previous scenario of cosmic-ray induced ionization at this location. In addition, SiO(2-1) emission has been successfully mapped in the close vicinity of W51C-E, with a spatial resolution of 7". The morphology and kinematics of the SiO emission are analyzed and strongly suggest that this emission is produced by the passage of the SNR primary shock. Put in conjunction with the enhanced ionization fraction in this region, we give a consistent picture in which the W51C-E position is located downstream of the shock, where a large reservoir of freshly accelerated particles is available.