Hydrides in Young Stellar Objects: Radiation tracers in a protostar-disk-outflow system

TL;DR

This study investigates hydride molecules in a high-mass star-forming region, revealing their distribution and excitation conditions, and demonstrating their potential as tracers of UV-irradiated gas in protostellar environments.

Contribution

First detection of certain hydrides in W3 IRS5, with analysis showing their association with outflow walls and UV irradiation, advancing understanding of cosmic chemistry in star formation.

Findings

Detection of CH, NH, H3O+, SH+, H2O+, and OH+ in W3 IRS5.

H2O+ and OH+ exhibit absorption and weak emission, indicating high-density regions near the protostar.

Molecular abundances match models of UV-irradiated outflow walls.

Abstract

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 (about 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.