Tracing FUV Radiation in the Embedded Phase of Star Formation

TL;DR

This study demonstrates that hydride molecules like CH$^+$ and OH$^+$ are effective tracers of FUV radiation in the embedded phase of star formation, using observations and models of massive star-forming regions.

Contribution

It provides observational evidence and modeling that confirm the significant impact of FUV radiation on hydride abundances in star-forming regions, highlighting specific molecules as tracers.

Findings

FUV radiation greatly enhances CH$^+$ and OH$^+$ abundances.

Models including FUV irradiation fit observations better.

CH$^+$ and OH$^+$ are effective FUV tracers.

Abstract

Molecules containing one or a few hydrogen atoms and a heavier atom (hydrides) have been predicted to trace FUV radiation. In some chemical models, FUV emission by the central object or protostar of a star forming region greatly enhances some of the hydride abundances. Two massive regions, W3 IRS5 and AFGL 2591, have been observed in hydride lines by HIFI onboard the {\it Herschel Space Observatory}. We use published results as well as new observations of CH$^+$ towards W3 IRS5. Molecular column densities are derived from ground state absorption lines, radiative transfer modeling or rotational diagrams. Models assuming no internal FUV are compared with two-dimensional models including FUV irradiation of outflow walls. We confirm that the effect of FUV is clearly noticeable and greatly improves the fit. The most sensitive molecules to FUV irradiation are CH$^+$ and OH$^+$, enhanced in abundance by many orders of magnitude. Modeling in addition also full line radiative transfer, Bruderer et al (2010b) achieve good agreement of a two-dimensional FUV model with observations of CH$^+$ in AFGL 2591. It is concluded that CH$^+$ and OH$^+$ are good FUV tracers in star-forming regions.