Do the Infrared Emission Features Need Ultraviolet Excitation?

Abstract

We present the results of imaging spectroscopy of the reflection nebula vdB 133, obtained with the infrared camera and circular variable filter wheel on the Infrared Space Observatory. Our observations reveal the infrared emission features (IEFs), at 6.2, 7.7, 8.7, 11.3, and 12.7 μm, and associated 5-15 μm continuum emission. The stellar system illuminating vdB 133 has the lowest ratio of ultraviolet (shortward of 0.4 μm) to total flux of any stars demonstrated to date to excite the IEFs and associated continuum emission from adjacent interstellar dust, as opposed to circumstellar dust. The low fraction of UV flux from this system poses a problem for existing models for the emission mechanism and emitting material, which all require substantial UV radiation for the excitation of the IEFs and associated continuum.

Based on observations with the Infrared Space Observatory, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.