Signatures of UV radiation in low-mass protostars I. Origin of HCN and
  CN emission in the Serpens Main region
  Context: Ultraviolet radiation (UV) influences the physics and chemistry of
star-forming regions, but its properties and significance in the immediate
surroundings of low-mass protostars are still poorly understood. Aims: We aim
to extend the use of the CN/HCN ratio, already established for high-mass
protostars, to the low-mass regime to trace and characterize the UV field
around low-mass protostars on $\sim 0.6\times0.6$ pc scales. Methods: We
present $5'\times5'$ maps of the Serpens Main Cloud encompassing 10 protostars
observed 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 and
the 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 6-5
emission that traces outflows. The CN emission is extended from the central
protostars to their immediate surroundings also tracing outflows, likely as a
product of HCN photodissociation. The ratio of CN to HCN total column densities
ranges from $\sim$1 to 12 corresponding to G$_0$ $\approx$ $10^{1}-10^{3}$ for
gas densities and temperatures typical for outflows of low-mass protostars.
Conclusions: UV radiation associated with protostars and their outflows is
indirectly identified in a significant part of the Serpens Main low-mass
star-forming region. Its strength is consistent with the values obtained from
the OH and H$_2$O ratios observed with Herschel and compared with models of
UV-illuminated shocks. From a chemical viewpoint, the CN to HCN ratio is an
excellent tracer of UV fields around low- and intermediate-mass star-forming
regions.
