Tracing high energy radiation with molecular lines near deeply embedded protostars
/ Authors
/ 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 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.
Journal: Astronomy and Astrophysics