Sub-arcsecond imaging of HCN and CN in the Orion Bar. UV-driven warm Nitrogen chemistry in dense environments

Rotationally excited HCN and CN lines, due to their high critical densities, are often used as tracers of gas densities. ALMA, with its high spectral and spatial resolution, provides unique access to the spatial morphology and kinematics of HCN and CN emission to study the substructures of the molecular cloud edges. We present new sub-arcsecond imaging of HCN 4-3 and CN 3-2 lines to probe the density structure in the Orion Bar. We study their spatial distribution and compare them to other existing tracers inside the PDR. We complemented these data with multiple J lines observed with the IRAM 30m and Herschel telescopes to study HCN and CN excitation near the dissociation front (DF). We find that HCN and CN emission are more spatially extended toward the more UV-shielded cloud than C2H and H2. However, they both peak very close to the DF, in disagreement with previous lower-angular-resolution observations. This morphology can be explained by the enhanced UV-driven chemistry. In dense and irradiated environments, HCN is efficiently formed via H2+CN. Hence, its emission can also probe warmer gas near the DF. CN emission is observed in the atomic PDR, not accounted by stationary gas-phase chemistry due to the very low H2 abundances. We interpret this as photoprocessing of CN-bearing PAHs. We derive high densities near the DF when comparing observations with RADEX and Meudon PDR Code models. Finally, we do not find evidence of small-scale high-density clumps and argue that the observed density gradient could explain the variation of excitation between the different tracers. We resolve two velocity components in HCN emission: 10.5 km/s (velocity of the Bar) and 11.5 km/s. The very structured emission of HCN observed in this redshifted emission component is more compatible with the signature of the propagation of a UV-induced shock compressing the gas than clumps.