Abstract
Context . A turbulent cascade naturally generates a hierarchy of structures within molecular clouds. Filament networks arise from this turbulent cascade, with fibers suggested to be the first (tran)sonic components formed out of it. Consequently, the properties of cores forming within fibers may be inherited from the larger spatial scale of the filaments, where the dissipation of turbulence occurs. Aims . We aim to investigate the diffuse gas kinematics and its interaction with the dense gas composing fibers. We also aim to characterize the diffuse material via the HNC molecular tracer. Methods . We used high-resolution (4.5 arcsec or ~2000 au) large-scale ALMA+IRAM-30 m mosaics to survey five star-forming regions in Orion, as part of the EMERGE Early ALMA Survey. Our sample includes low- (OMC-4 South and NGC 2023), intermediate (OMC-3 and LDN 1641N) and high-mass (Flame nebula) star-forming regions, covering a wide range of stellar activity, cloud morphology, and evolutionary stages. We observed our targets in HNC (1–0) as probes of the diffuse gas in the regions and compared our findings to the N 2 H + (1–0) emission tracing the dense gas. We systematically investigated the emission of the diffuse gas in relation to the dense structures identified in N 2 H + , focusing on its kinematics and velocity field. Results . The high resolution (2000 au) observations reveal that HNC traces lukewarm, diffuse (
5 × 10 21 cm −2 ) material around dense fibers. The properties of the diffuse gas appear to be similar across our sample, despite the wide range of different environments. We find that compared to the quiescent and subsonic gas inside fibers, the diffuse gas is more turbulent (ℳs = 2.9). We observe the HNC nonthermal velocity dispersion to be roughly constant and homogeneous where the dense gas is detected (ℳ s ~ 2), and rising along its edges up to ℳ s ~ 5. Understanding the dissipation process is crucial marking the transition between the dense subsonic gas and diffuse turbulent material occurs. We investigated the turbulence dissipation through the statistical analysis of the HNC velocity gradients. While the majority of the gas shows ∇ V lsr ≤ 5 km s −1 pc −1 with roughly constant values, we identified high-shear regions showing higher gradients with ∇ V lsr ≥ 10 km s −1 pc −1 concentrated in small features of 0.1–0.3 pc in size located near the dense gas. Despite their small size, these high-shear structures appear to be major contributors of the turbulence dissipation in our targets. Conclusions . Our results suggest that in the case of Orion, the transition to coherence occurs at the fiber level. This interpretation is supported by the turbulence being effectively dissipated before the formation of cores and during the formation of these first dense gas structures.