| Abstract
| - 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 2O 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.
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