Do the Infrared Emission Features Need Ultraviolet Excitation? The Polycyclic Aromatic Hydrocarbon Model in UV‐poor Reflection Nebulae

Abstract

One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6, and 11.3 μm interstellar infrared (IR) emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the recent detection of these bands in regions with little ultraviolet (UV) illumination, since small, neutral PAH molecules have little or no absorption at visible wavelengths and therefore require UV photons for excitation. We show here that our astronomical PAH model, incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases, can closely reproduce the observed IR emission bands of vdB 133, a UV-poor reflection nebula. We also show that single-photon heating of astronomical PAHs in reflection nebulae near stars as cool as Teff = 3000 K can result in observable emission at 6.2, 7.7, 8.6, and 11.3 μm. Illustrative mid-IR emission spectra are calculated for reflection nebulae illuminated by cool stars with Teff = 3500, 4500, and 5000 K. These will allow comparison with future Space Infrared Telescope Facility observations of vdB 135 (Teff = 3600 K), vdB 47 (Teff = 4500 K), and vdB 101 (Teff = 5000 K). The dependence on the effective temperature of the exciting star of the observed 12 μm IRAS emission (relative to the total far-IR emission) is consistent with the PAH model for 3000 K ≤ Teff ≤ 30,000 K.