Excitation of Polycyclic Aromatic Hydrocarbon (PAH) Emission: Dependence
  on Size Distribution, Ionization, and Starlight Spectrum and Intensity
  Using physical models, we study the sensitivity of polycyclic aromatic
hydrocarbon (PAH) emission spectra to the character of the illuminating
starlight, to the PAH size distribution, and to the PAH charge distribution.
Starlight models considered range from the emission from a 3 Myr-old starburst,
rich in far-ultraviolet (FUV) radiation, to the FUV-poor spectrum of the very
old population of the M31 bulge. A wide range of starlight intensities is
considered. The effects of reddening in dusty clouds are investigated for
different starlight spectra. For a fixed PAH abundance parameter $q_{\rm PAH}$
(the fraction of the total dust mass in PAHs with $<10^3$ C atoms), the
fraction of the IR power appearing in the PAH emission features can vary by a
factor of two as the starlight spectrum varies from FUV-poor (M31 bulge) to
FUV-rich (young starburst). We show how $q_{\rm PAH}$ can be measured from the
strength of the 7.7$\mu$m emission. The fractional power in the 17$\mu$m
feature can be suppressed by high starlight intensities.
