Tracing high energy radiation with molecular lines near deeply embedded protostars

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(CN) ≈ a few ×10 −11 -10 −8 , x(NO) ≈ 10 −9 -10 −8 and x(CO + ) ≈ 10 −12 -10 −10 . SO + has abundances of a few ×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. 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 X ≈ 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.

The HIFI spectral survey of AFGL 2591 (CHESS)

“…We obtained abundances of [NO] = 5 × 10 −8 and a very similar value was estimated by Stäuber et al (2007), [NO] = 3.1 × 10 −8 . However, our calculations are based on only four lines from two different energy levels, which causes uncertainties that are higher by a factor of ∼8.…”

“…We obtained an abundance of 1 × 10 −9 with the uncertainty within a factor of 5. Stäuber et al (2007) estimated a higher CN abundance of 2.3 × 10 −8 by using a different physical model and assuming a distance of d = 1 kpc. These authors also tested models with a jump in abundance at 100 K and estimated an outer abundance of 10 −9 and an inner abundance of 6 × 10 −7 .…”

“…However, the two models were statistically indistinguishable. Stäuber et al (2007) based their calculations on seven hyper-fine components from E up ∼ 33 K. Our estimations of the CN abundance are based on several CN lines from three different energy levels of E up ∼ 33, 82, and 114 K and are thus more robust. The overall CN abundance uncertainty is estimated to be a factor of 3.…”

Dense molecular cocoons in the massive protocluster W3 IRS5: a test case for models of massive star formation

“…Instead, we assign the line near 353.741 GHz to 33 SO 2 19 4,16 −19 3,17 transition, as was suggested by Stäuber et al (2007). The anomalous CO + line ratio reported by ) is explained by blending of the CO + line with 33 SO 2 at 353.741 GHz.…”

Hydrides in young stellar objects: Radiation tracers in a protostar-disk-outflow system

“…However, evidence of extended high-energy irradiation ) and in particular the observation of CO + , with an abundance four orders of magnitude larger than predicted, in W3 IRS5 and other YSOs with the JCMT (Stäuber et al 2007) raised the expectation that the effects of irradiation may be more dramatic in asymmetric reality. Bruderer et al (2009) modeled and interpreted the CO + anomaly as FUV radiation originating in the YSOs and irradiating the walls of outflow cavities.…”