2016MNRAS.462..532M

We present 12 mm Mopra observations of the dense (>10³ cm^–3) molecular gas towards the north-east 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 an SNR-molecular cloud interaction region. Shock-disruption is evident from broad NH₃ (1,1) spectral linewidths 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) x 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.