Tracing high energy radiation with molecular lines near deeply embedded protostars

Aims. The aim is to probe high energy radiation emitted by deeply embedded protostars. Methods. Submillimeter lines of CN, NO, CO + and SO + , and upper limits on SH + and N 2 O are observed with the James Clerk Maxwell Telescope in two high-mass and up to nine low-mass young stellar objects and compared with chemical models. Results. Constant fractional abundances derived from radiative transfer modeling of the line strengths are $x({\rm CN}) \approx$ a few $\times$10 -11 –10 -8 , $x({\rm NO}) \approx 10^{-9}$–10 -8 and $x({\rm CO^+}) \approx 10^{-12}$–10 -10 . SO + has abundances of a few $\times 10^{-11}$ in the high-mass objects and upper limits of ≈ 10 -12 –10 -11 in the low-mass sources. All abundances are up to 1–2 orders of magnitude higher if the molecular emission is assumed to originate mainly from the inner region ( ≲ 1000 AU) of the envelope. For high-mass sources, the CN, SO + and CO + abundances and abundance ratios are best explained by an enhanced far-ultraviolet (FUV) field impacting gas at temperatures of a few hundred K. The observed column densities require that this region of enhanced FUV has scales comparable to the observing beam, such as in a geometry in which the enhanced FUV irradiates outflow walls. For low-mass sources, the required temperatures within the FUV models of $T \gtrsim 300$ K are much higher than found in models, so that an X-ray enhanced region close to the protostar ($r \lesssim 500$ AU) is more plausible. Gas-phase chemical models produce more NO than observed, suggesting an additional reduction mechanism not included in current models. Conclusions. The observed CN, CO + and SO + abundances can be explained with either enhanced X-rays or FUV fields from the central source. High-mass sources likely have low opacity regions that allow the FUV photons to reach large distances from the central source. X-rays are suggested to be more effective than FUV fields in the low-mass sources. The observed abundances imply X-ray fluxes for the Class 0 objects of $L_{\rm X} \approx 10^{29}$–10 31 erg s -1 , comparable to those observed from low-mass Class I protostars. Spatially resolved data are needed to clearly distinguish the effects of FUV and X-rays for individual species.