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

One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6, and11.3um interstellar IR emission bands with polycyclic aromatic hydrocarbon(PAH) molecules has been the recent detection of these bands in regions withlittle UV illumination, since small, neutral PAH molecules have little or noabsorption at visible wavelengths and therefore require UV photons forexcitation. We show here that our "astronomical" PAH model, incorporating theexperimental result that the visual absorption edge shifts to longer wavelengthupon ionization and/or as the PAH size increases, can closely reproduce theobserved infrared emission bands of vdB 133, a UV-poor reflection nebula. It is also shown that single-photon heating of ``astronomical'' PAHs inreflection nebulae near stars as cool as T_eff=3000K can result in observableemission at 6.2, 7.7, 8.6, and 11.3um. Illustrative mid-IR emission spectra arealso calculated for reflection nebulae illuminated by cool stars withT_eff=3500, 4500, 5000K. These will allow comparison with future Space InfraredTelescope Facility (SIRTF) observations of vdB 135 (T_eff=3600K), vdB 47(T_eff=4500K), and vdB 101 (T_eff=5000K). It is also shown that the dependence of the 12um IRAS emission relative tothe total far-IR emission on the effective temperature of the exciting star isconsistent with the PAH model expectation for 3000K < T_eff < 30000K.Comment: 12 pages. Submitted to The Astrophysical Journal Letter