Ammonia excitation imaging of shocked gas towards the W28 gamma-ray source HESS J1801-233

Abstract: We present 12 mm Mopra observations of the dense (>10^3 cm^-3 ) molecular gas towards the north-east (NE) of the W28 supernova remnant (SNR). This cloud is spatially well-matched to the TeV gamma-ray source HESS J1801-233 and is known to be a SNR-molecular cloud interaction region. Shock-disruption is evident from broad NH3 (1,1) spectral line-widths in regions towards the W28 SNR, while strong detections of spatially-extended NH3(3,3), NH3(4,4) and NH3(6,6) inversion emission towards the cloud strengthen the case for the existence of high temperatures within the cloud. Velocity dispersion measurements and NH3(n,n)/(1,1) ratio maps, where n=2, 3, 4 and 6, indicate that the source of disruption is from the side of the cloud nearest to the W28 SNR, suggesting that it is the source of cloud-disruption. Towards part of the cloud, the ratio of ortho to para-NH3 is observed to exceed 2, suggesting gas-phase NH3 enrichment due to NH3 liberation from dust grain mantles. The measured NH3 abundance with respect to H2 is ~(1.2+/-0.5)*10^-9, which is not high, as might be expected for a hot, dense molecular cloud enriched by sublimated grain-surface molecules. The results are suggestive of NH3 sublimation and destruction in this molecular cloud, which is likely to be interacting with the W28 SNR shock.