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

Abstract

<i>Context. <i/>Hydrides of the most abundant heavier elements are fundamental molecules in cosmic chemistry. Some of them trace gas irradiated by UV or X-rays.<i>Aims. <i/>We explore the abundances of major hydrides in W3 IRS5, a prototypical region of high-mass star formation.<i>Methods. <i/>W3 IRS5 was observed by HIFI on the <i>Herschel<i/> Space Observatory with deep integration (2500 s) in 8 spectral regions. <i>Results. <i/>The target lines including CH, NH, H<sub>3<sub/>O<sup>+<sup/>, and the new molecules SH<sup>+<sup/>, H<sub>2<sub/>O<sup>+<sup/>, and OH<sup>+<sup/> are detected. The H<sub>2<sub/>O<sup>+<sup/> and OH<sup>+<sup/> <i>J<i/> = 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, H<sub>2<sub/>O<sup>+<sup/> and OH<sup>+<sup/> also contain strong absorption components at a velocity shifted relative to W3 IRS5, which are attributed to foreground clouds.<i>Conclusions. <i/>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.