Tracing X-ray and FUV Radiation in the Embedded Phase of Star Formation
Abstract
Molecules containing one (or a few) hydrogen atoms and one heavier atom (hydrides) are predicted to trace ionizing FUV radiation. In some chemical models, FUV emission by the central object or protostar of a star forming region greatly enhances the abundance of some hydrides. Two massive regions, W3 IRS5 and AFGL 2591, have been observed in hydride lines by HIFI onboard the Herschel Space Observatory. They both have Ultra Compact HII regions, indicating the presence of FUV. We derive molecular column densities from ground state absorption lines, radiative transfer modeling and/or rotational diagrams. Models assuming no internal FUV are compared with two-dimensional models including FUV irradiation of outflow walls. The most sensitive molecules to FUV irradiation are CH^+, OH^+, SH^+, and NH^+. They are enhanced in abundance by many orders of magnitude. We confirm that the effect of FUV is clearly noticeable and greatly improves the fit of observations with theory (see figure). Modeling in addition also full line radiative transfer, we report good agreement of a two-dimensional FUV model with line observations of CH^+ in AFGL 2591. It is concluded that CH^+ and OH^+ are good and abundant FUV tracers in star-forming regions. Having confirmed the tracer molecules in cases with known FUV radiation, we search for that radiation in cases where FUV irradiation is questionable, such as younger stellar objects and lower mass objects. We report the results of new Herschel observations of CH^+, OH^+, and CH in 5 massive YSOs of different age, in 6 low-mass objects of class 0 and I, and an intermediate-mass source. The observations are compared with chemical and radiative transfer models. For nearby low-mass YSOs, we discuss also the possibility to indentify protostellar X-ray irradiation. The conclusion is that high-energy irradiation has started in all objects observed, thus at a very early phase of star formation. Its impact on star and planet formation needs to be investigated.
- Publication:
-
The Molecular Universe
- Pub Date:
- May 2011
- DOI:
- 10.48550/arXiv.1101.1746
- arXiv:
- arXiv:1101.1746
- Bibcode:
- 2011IAUS..280P..91B
- Keywords:
-
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- EAS Publications Series, 5th Zermatt conference on Conditions and impact of star formation: New results with Herschel and beyond (invited), submitted