New Infrared Emission Lines of the s-Process Enriched Neutron-Capture Elements Br and Rb in Planetary Nebulae

We report new detections and identifications of infrared emission lines from the neutron-capture elements Br (atomic number 35) and Rb (37) in planetary nebulae. Enhanced abundances of these and other trans-iron elements produced by s-process reactions during the Asymptotic Giant Branch can be observed in the planetary nebulae subsequently expelled by these stars, providing information on the s-process and the role of these stars as global chemical enrichment agents. The observations were made with the high-resolution infrared spectrometer iSHELL (Rayner et al. 2016, SPIE, 9908E, 84R) at NASA’s Infrared Telescope Facility (IRTF) on Maunakea, Hawaii. Detection of [Rb III] 1.356 microns, which is at a wavelength with strong telluric absorption, required conditions of low atmospheric water vapor. We find that Rb⁺⁺ is the majority ion in both NGC 7027 and IC 5117, the two nebulae in which we first detected the next higher ion via [Rb IV] 1.597 micron (Sterling et al. 2016, ApJL, 819, 9). Preliminary analysis of these data indicates elevated elemental abundances of [Rb/H] = +1.5 dex in NGC 7027 and +1.0 in IC 5117. Br is less copiously produced by the s-process than Rb. The [Br V] 1.643 micron line first reported by our group (Madonna et al. 2018, ApJL, 861, 8) samples a minority ion, is undetectable in most planetary nebulae, and yields subsolar values, while anomalously high abundances derived from optical [Br III] lines may be affected by line blends (Madonna et al. 2017, MNRAS, 471, 1341). We have detected [Br IV] lines at 3.810 and 3.344 microns. These two lines yield consistent ionic ratios Br⁺³/H⁺ to within 10% for IC 5117, and the 3.810 micron line in NGC 7027 is consistent within 40% with a weak optical [Br IV] line which is detected in only that planetary nebula (Sharpee et al. 2007, ApJ, 659, 1265). We find preliminary values of [Br/H] = +0.5 for NGC 7027 and IC 5117, while the unenriched nebula Hb 12 is consistent with a solar abundance. Ionic abundances were calculated using new atomic data including transition probabilities and collision strengths. Future ionization structure calculations will improve the accuracy of the elemental abundances. Support for this work was provided by NSF grants AST-1715332 and 1412928.