Planetary Nebulae - Bowen's Fluorescent Mechanism

In the spectra of many high excitation planetary nebula, certain permitted lines of OIII and NIII appear, and these are sometimes quite intense. Bowen observed that the OIII lines could be produced by atoms cascading from the 2p3d³P₂ level. Bowen noticed that there was a frequency coincidence between the resonant Ly transition of the HeII and the transition from the 2p² 3P₂ to the 2p3d³P₂ level of OIII, i.e., 303.78Å Ly of HeII and the 3033.693Å and 303.799Å of OIII. Bowen further observed an equally surprising similarity, namely that the final transition of the OIII, i.e., 2p3s ³PÅ-2p³P₂ emitting a Figure 2. Illustration by Hans & Cassidy. Courtesy of Gale Group. Figure 3. Illustration by Hans & Cassidy. Courtesy of Gale Group.


photon of 374.436Å, coincides with the resonance line 374.442Å of the 2p²P_3/2-3d²D_3/2 of NIII which also produces in this ion a similar fluorescent cycle. Detailed investigations and analyses showed that the Bowen fluorescent mechanism was fundamentally correct both qualitatively and quantitatively. It has applications to high excitation gaseous nebulae, quasars, and stellar envelopes.