Neutron-Capture Elements in Planetary Nebulae: Identification of Two Near-Infrared Emission Lines as [K[CLC]r[/CLC] [CSC]iii[/CSC]] and [S[CLC]e[/CLC] [CSC]iv[/CSC

Abstract

I propose that two previously unidentified lines at 2.199 and 2.287 μm seen in the spectra of planetary nebulae (PNs) are fine-structure transitions of Kr+2 and Se+3. These are the cosmically most abundant elements with Z > 32. The ionic stages and originating energy levels of the observed transitions are expected to be significantly populated under nebular conditions, and these elements—especially the noble gas Kr—are unlikely to be strongly depleted out of the gas phase into grains. Furthermore, their concentrations can be enhanced by s-processing, which occurs in the interiors of the PN progenitor stars. The observed line strengths are consistent with modest (factor of a few) overabundances. In support of the identification of the 2.199 μm line as [Kr III] 3P1-3P2, PNs that display this line also show [Kr III] 1D2-3P2 6828 A emission. I identify the 2.287 μm line as [Se IV] 2P3/2-2P1/2. These identifications suggest a new range of possibilities for line identifications in the infrared. In addition, the observability of emission lines from n-capture species introduces a novel approach for studying advanced evolutionary stages and nucleosynthesis in the progenitor stars and for more fully delineating the role of PNs as agents of galactic chemical enrichment.