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Context: Ultraviolet radiation (UV) influences the physics and chemistry ofstar-forming regions, but its properties and significance in the immediatesurroundings of low-mass protostars are still poorly understood. Aims: We aimto extend the use of the CN/HCN ratio, already established for high-massprotostars, to the low-mass regime to trace and characterize the UV fieldaround low-mass protostars on $\sim 0.6\times0.6$ pc scales. Methods: Wepresent $5'\times5'$ maps of the Serpens Main Cloud encompassing 10 protostarsobserved with the EMIR receiver at the IRAM 30 m telescope in CN 1-0, HCN 1-0,CS 3-2, and some of their isotopologues. The radiative-transfer code RADEX andthe chemical model Nahoon are used to determine column densities of molecules,gas temperature and density, and the UV field strength, $G_\mathrm{0}$.Results: The spatial distribution of HCN and CS are well-correlated with CO 6-5emission that traces outflows. The CN emission is extended from the centralprotostars to their immediate surroundings also tracing outflows, likely as aproduct of HCN photodissociation. The ratio of CN to HCN total column densitiesranges from $\sim$1 to 12 corresponding to G$_0$ $\approx$ $10^{1}-10^{3}$ forgas densities and temperatures typical for outflows of low-mass protostars.Conclusions: UV radiation associated with protostars and their outflows isindirectly identified in a significant part of the Serpens Main low-massstar-forming region. Its strength is consistent with the values obtained fromthe OH and H$_2$O ratios observed with Herschel and compared with models ofUV-illuminated shocks. From a chemical viewpoint, the CN to HCN ratio is anexcellent tracer of UV fields around low- and intermediate-mass star-formingregions.

Signatures of UV radiation in low-mass protostars I. Origin of HCN and CN emission in the Serpens Main region