Optical Luminescence from Reflection Nebulae?

Abstract

It has been clearly demonstrated by Sellgren (1984) that the so-called “unidentified” infrared emission bands plus a hot NIR continuum seen in reflection nebulae cannot be caused by grains in thermal equilibrium with the central stars. Instead she suggested thermal emission from very small grains temporary heated to ~ 1000K by single UV quanta. Following this idea, Leger and Puget (1984) found evidence that polycyclic aromatic hydrocarbons (PAH:s)are responsible for the IR emission bands. These authors also noted that the optical properties for graphite cannot be used for these tiny monolayer grains. This is in fact an important statement - otherwise the model would not be able to explain the observed band to continuum ratio for e.g. the 3.3ym band: There is no doubt that this band is caused by the C-H stretch vibration. The oscillator strength depends on the detailed structure of the molecule, but the absorption cross-section of a single C-H bond should not exceed 10~19 cm² (Duley and Williams, 1979). If we further consider carbon grains with volume, V, and assume the number of CH bonds to be less than the number of C atoms, we estimate the total cross-section of the grain in the band to be ≦ 10⁴V cm². The absorption cross-section in the continuum is at least 1.5 x 10⁴v cm² at 3.3μm for carbon grains (Koike et al., 1980). Consequently we would expect to observe a band to continuum ratio of at most 0.7. This is in clear conflict with the observations.