Infrared Space Observatory Mid‐Infrared Spectra of Reflection Nebulae

Abstract

We present 5È15 km imaging spectroscopy of the reÑection nebulae vdB 17 (NGC 1333), vdB 59 (NGC 2068), vdB 101, vdB 111, vdB 133, and vdB 135, obtained with the infrared camera and circular variable Ðlter wheel on the Infrared Space Observatory (ISO). These nebulae are illuminated by stars with K, implying UV (j \ 400 nm) to total stellar Ñux ratios of T eff \ 3,600È19,000 F (j:400nm) /F total \ 0.01È0.87. We detect the infrared emission features (IEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 km, broad emission features at 6È9 km and 11È13 km, and 5È15 km continuum emission, from the interstellar medium in vdB 17, vdB 59, and vdB 133 and place upper limits on the emission (F (j:400nm) /F total \ 0.22È0.87), from the interstellar medium in vdB 101, vdB 111, and vdB 135 Our goal (F (j:400nm) /F total \ 0.01È0.20). is to test predictions of models attributing the IEFs to polycyclic aromatic hydrocarbons (PAHs). Inter-stellar models predict PAHs change from singly ionized to neutral as the UV intensity, decreases. G 0 , The ratio of PAH emission at 6È10 km to PAH emission at 10È14 km is expected to be 10 times higher in ionized than in neutral PAHs. We observe no spectroscopic di †erences with varying We analyze T eff. the spectra of vdB 17 and vdB 59 as a function of distance from the star to see how the spectra depend on within each source. The only quantitative di †erence we Ðnd is a broadening of the 7.7 km IEF at G 0 within vdB 17. We observe only a 40% change in the 6È10 km to 10È14 km Ñux ratio over G 0 \ 20È60 G 0 \ 20È6 ] 104.