<i>Infrared Space Observatory</i>Mid‐Infrared Spectra of Reflection Nebulae

Abstract

We present 5-15 μm imaging spectroscopy of the reflection 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 filter wheel on the Infrared Space Observatory (ISO). These nebulae are illuminated by stars with Teff = 3,600-19,000 K, implying UV (λ < 400 nm) to total stellar flux ratios of F(λ<400 nm)/Ftotal = 0.01-0.87. We detect the infrared emission features (IEFs) at 6.2, 7.7, 8.6, 11.3, and 12.7 μm, broad emission features at 6-9 μm and 11-13 μm, and 5-15 μm continuum emission, from the interstellar medium in vdB 17, vdB 59, and vdB 133 (F(λ<400 nm)/Ftotal = 0.22-0.87), and place upper limits on the emission from the interstellar medium in vdB 101, vdB 111, and vdB 135 (F(λ<400 nm)/Ftotal = 0.01-0.20). Our goal is to test predictions of models attributing the IEFs to polycyclic aromatic hydrocarbons (PAHs). Interstellar models predict PAHs change from singly ionized to neutral as the UV intensity, G0, decreases. The ratio of PAH emission at 6-10 μm to PAH emission at 10-14 μm is expected to be 10 times higher in ionized than in neutral PAHs. We observe no spectroscopic differences with varying Teff. We analyze the spectra of vdB 17 and vdB 59 as a function of distance from the star to see how the spectra depend on G0 within each source. The only quantitative difference we find is a broadening of the 7.7 μm IEF at G0 = 20-60 within vdB 17. We observe only a 40% change in the 6-10 μm to 10-14 μm flux ratio over G0 = 20-6 × 104.