HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC

Context. Massive stars form within dense clumps inside giant molecular clouds (GMCs). Finding appropriate chemical tracers of the dense gas (n(H2 ) > several 104 cm−3 or AV > 8 mag) and linking their line luminosity with the star formation rate is of critical importance. Aims. Determine the origin and physical conditions of the HCN emitting gas at cloud scales and study their relation to those of other molecules. Methods. In the context of the IRAM30m ORION-B large program, we present 5deg2(∼250pc2) HCN, HNC, HCO+, and CO J=1−0 maps of Orion B giant molecular cloud (GMC), complemented with existing wide-field [C i] 492 GHz maps, as well as new pointed observations of rotationally excited HCN, HNC and H13CN lines. We compare the observed HCN line intensities with radiative transfer models including line overlap effects and electron excitation. We study the HCN/HNC isomeric ratio with updated photochemical models. Results. We spectroscopically resolve the HCN J = 1–0 hyperfine structure (HFS) components (partially also the J=2−1 and 3−2 components). We detect anomalous HFS line intensity (and line width) ratios almost everywhere in the cloud. About 70% of the total HCN J=1–0 luminosity (L′(HCN J = 1−0) = 110 K km s−1 pc−2) arises from gas at AV < 8 mag. The HCN/CO J=1−0 line intensity ratio, widely used as a tracer of the dense gas fraction in extragalactic studies, shows a bimodal behavior with an inflection point at AV . 3 mag typical of translucent gas and UV-illuminated cloud edges. The extended nature of this cloud component agrees with the widespread [C i] 492 GHz emission. We find that most of the HCN J = 1–0 emission arises from extended gas with n(H2 ) < 104 cm−3 , even lower if the ionization fraction is χe ≥ 10−5 and electron excitation dominates. This result contrasts with the prevailing view of HCN J = 1–0 emission as a tracer of dense gas and explains the low-AV branch of the HCN/CO J =1–0 line intensity ratio distribution. Indeed, the highest HCN/CO ratios (∼ 0.1) at AV < 3 mag correspond to regions of high [C i] 492 GHz/CO J =1−0 intensity ratios (> 1) characteristic of low-density photodissociation regions and χe & 10−5 . The low-surface brightness HCN J = 1−0 emission (. 1 K km s−1) is close to linearly correlated with the intensity of the far-IR dust emission (IFIR), whereas the spatially compact and bright HCN emission (> 6 K km s−1 ), arising from dense gas in star-forming clumps such as NGC 2024, shows a superlinear relation with IFIR . On the other hand, the HNC J = 1−0 emission shows a single superlinear power law with IFIR , which supports our conclusion (based on chemical and excitation arguments) that HNC is a more appropriate tracer of dense gas with n(H2 ) > 104 cm−3 . These different power law scalings (produced by different gas density and line excitation regimes) in a single but spatially resolved GMC resemble the variety of Kennicutt-Schmidt law indexes found in galaxies. Conclusions. As in most GMCs, the extended component and cloud edges of Orion B are porous to UV radiation from nearby massive stars. This favors the formation and excitation of HCN on large scales, not only in dense star-forming clumps, and leads to a relatively low value of the dense gas mass to total luminosity ratio, L′(HCN J=1−0) = α (HCN) = 29 M⊙ / (K km s−1pc2). As a corollary for extragalactic studies, we conclude that high HCN/CO J=1–0 line intensity ratios do not always imply the presence of dense gas, which may be better traced by HNC than by HCN.