Presentation #351.06 in the session Planetary Nebulae, Supernova Remnants — iPoster Session.
We report the first definitive detections of three near-infrared emission lines of heavy trans-iron elements: [Te IV] 1.0843, [Xe III] 1.0210, and [Xe V] 1.0762 μm (in vacuo), in several planetary nebulae. The observations were obtained with the Habitable-zone Planet Finder (HPF; Mahadevan et al. 2014, SPIE, id. 91471G) on the Hobby-Eberly Telescope in west Texas (HET; Ramsey et al. 1998, SPIE, 3352, 34). Te and Xe are among many trans-iron nuclides that can be produced by slow neutron captures (the “s-process”) in planetary nebula progenitors during the Asymptotic Giant (AGB) stage, resulting in elevated element abundances that contribute to galactic chemical enrichment. We detected the [Te IV] and [Xe III] lines in about 50% of our sample of two dozen nebulae, some of which were selected due to prior evidence of s-process enrichments in other spectral regions. [Xe V] is detected in only 3 objects, but this is not surprising since Xe+4 should be present only in nebulae with very hot central stars. Accurate ionic abundances were determined using new collision strengths and other atomic data calculated by one of us (M.A.B.). According to a preliminary determination of the elemental abundances, typical enrichment factors of Te and Xe are about 5 – 7, with Xe displaying slightly larger enhancements than Te. These values are generally consistent with the predictions of Karakas and Lugaro (2016, ApJ, 825, 26) and Battino et al. (2019, MNRAS, 489, 1082) for AGB nucleosynthesis in stars with initial masses of order 2-3 solar masses and near-solar metallicities. Te and Xe, elements 52 and 54 respectively, lie close to the second or “heavy” peak of the s-process at atomic numbers 56-60), and well beyond the first or “light-s” peak at elements 38-42. The newly detected lines, together with measurements of enrichments in lighter s-process products such as Kr (element 36; e.g. Sterling et al. 2015, ApJS, 218, 25), can be used to constrain details of the s-process in AGB stars. This work received support from NSF grants AST-1715332 and 1412928.