One of the major challenges to identification of the 3.3 , 6.2 , 7.7 , 8.6 , and 11.3 \micron 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 infrared emission bands of vdB 133 , a UV-poor reflection nebula .

It is also shown that single-photon heating of “ astronomical ” PAHs in reflection nebulae near stars as cool as T _ { eff } = 3000 { K } can result in observable emission at 6.2 , 7.7 , 8.6 , and 11.3 \micron .
Illustrative mid-IR emission spectra are also calculated for reflection nebulae illuminated by cool stars with T _ { eff } = 3500 , 4500 , 5000 { K } .
These will allow comparison with future Space Infrared Telescope Facility ( SIRTF ) observations of vdB 135 ( T _ { eff } = 3600 { K } ) , vdB 47 ( T _ { eff } = 4500 { K } ) , and vdB 101 ( T _ { eff } = 5000 { K } ) .

It is also shown that the dependence of the 12 \micron IRAS emission relative to the total far-IR emission on the effective temperature of the exciting star is consistent with the PAH model expectation for 3000 { K } \leq T _ { eff } \leq 30000 { K } .