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We present 5 deg^2 (~250 pc^2) HCN, HNC, HCO+, and CO J=1-0 maps of the OrionB GMC, complemented with existing wide-field [CI] 492 GHz maps, as well as newpointed observations of rotationally excited HCN, HNC, H13CN, and HN13C lines.We detect anomalous HCN J=1-0 hyperfine structure line emission almosteverywhere in the cloud. About 70% of the total HCN J=1-0 luminosity arisesfrom gas at A_V < 8 mag. The HCN/CO J=1-0 line intensity ratio shows a bimodalbehavior with an inflection point at A_V < 3 mag typical of translucent gas andUV-illuminated cloud edges. We find that most of the HCN J=1-0 emission arisesfrom extended gas with n(H2) ~< 10^4 cm^-3, even lower density gas if theionization fraction is > 10^-5 and electron excitation dominates. This resultexplains the low-A_V branch of the HCN/CO J=1-0 intensity ratio distribution.Indeed, the highest HCN/CO ratios (~0.1) at A_V < 3 mag correspond to regionsof high [CI] 492 GHz/CO J=1-0 intensity ratios (>1) characteristic oflow-density PDRs. Enhanced FUV radiation favors the formation and excitation ofHCN on large scales, not only in dense star-forming clumps. The low surfacebrightness HCN and HCO+ J=1-0 emission scale with I_FIR (a proxy of the stellarFUV radiation field) in a similar way. Together with CO J=1-0, these linesrespond to increasing I_FIR up to G0~20. On the other hand, the bright HCNJ=1-0 emission from dense gas in star-forming clumps weakly responds to I_FIRonce the FUV radiation field becomes too intense (G0>1500). The different powerlaw scalings (produced by different chemistries, densities, and line excitationregimes) in a single but spatially resolved GMC resemble the variety ofKennicutt-Schmidt law indexes found in galaxy averages. As a corollary forextragalactic studies, we conclude that high HCN/CO J=1-0 line intensity ratiosdo not always imply the presence of dense gas.

astronomy and astrophysics

HCN emission from translucent gas and UV-illuminated cloud edges  revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as  tracer of the dense gas reservoir for star formation