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

Context. Hydrides of the most abundant heavier elements are fundamental molecules in cosmic chemistry. Some of them trace gas irradiated by UV or X-rays.
Aims: We explore the abundances of major hydrides in W3 IRS5, a prototypical region of high-mass star formation.
Methods: W3 IRS5 was observed by HIFI on the Herschel Space Observatory with deep integration (≃2500 s) in 8 spectral regions.
Results: The target lines including CH, NH, H₃O⁺, and the new molecules SH⁺, H₂O⁺, and OH⁺ are detected. The H₂O⁺ and OH⁺ J = 1-0 lines are found mostly in absorption, but also appear to exhibit weak emission (P-Cyg-like). Emission requires high density, thus originates most likely near the protostar. This is corroborated by the absence of line shifts relative to the young stellar object (YSO). In addition, H₂O⁺ and OH⁺ also contain strong absorption components at a velocity shifted relative to W3 IRS5, which are attributed to foreground clouds.
Conclusions: The molecular column densities derived from observations correlate well with the predictions of a model that assumes the main emission region is in outflow walls, heated and irradiated by protostellar UV radiation.

Herschel is an ESA space observatory with science instruments provided by a European-led Principal Investigator consortia and with important participation from NASA.Appendix (page 5) is only available in electronic form at http://www.aanda.org