Tracing high energy radiation with molecular lines near deeply embedded protostars
Abstract
Submillimeter lines of CN, NO, CO+ and SO+, and upper limits on SH+ and N2O 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. Constant fractional abundances derived from radiative transfer modeling of the line strengths are x(CN) ~ a few x 10-11-10-8, x(NO) ~ 10-9-10-8 and x(CO+) ~ 10-12-10-10. SO+ has abundances of a few x 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 > 300 K are much higher than found in models, so that an X-ray enhanced region close to the protostar (r < 500 AU) is more plausible. The observed abundances imply X-ray fluxes for the Class 0 objects of LX ~ 1029-1031 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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