# # Program information file # PROGRAM_ID 2024A051 PROGRAM_TITLE Improving the Accuracy of Nebular Selenium Abundances with SpeX PROGRAM_INV1 Nicholas Sterling PROGRAM_INV2 Harriet Dinerstein PROGRAM_INV3 William Vacca PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG Low- and intermediate-mass [1-8 Msun] asymptotic giant branch [AGB] stars synthesize half of the neutron[n]-capture element [atomic number Z > 30] nuclei in the Universe, via slow n-capture nucleosynthesis [the 's-process']. Selenium [Z=34] is by far the most widely-observed n-capture element in ionized nebulae, due to the strong [Se IV] 2.2864 micron emission line, and thus is especially valuable for assessing s-process enrichments in large samples of planetary nebulae [PNe]. However, there is evidence that Se abundances derived from the [Se IV] line are systematically underestimated by existing ionization equilibrium models. To empirically correct Se ionization balance solutions, and thus improve the accuracy of abundance determinations, it is necessary to detect multiple Se ions in nebulae with a wide range of ionization levels. We propose to SpeX in the SXD mode to simultaneously observe [Se III] 1.0992 and [Se IV] 2.2864 microns in six PN with different central star temperatures and hence degrees of ionization. Observations with SpeX in June 2023 detected both Se ions in three of four PNe, demonstrating the feasibility of the proposed program. Other n-capture elements [e.g., Kr, Te, and Xe] with isolated lines will also be detected, maximizing the scientific return. We will use the measured Se ionic abundances to benchmark grids of photoionization models and compute new Se ionization equilibrium solutions. The results will substantially improve the accuracy of Se enrichments in PNe, and hence their AGB progenitors, including in large samples where only one ion is observed. The results will also be applicable to Se abundance determinations in kilonovae and other ionized nebulae. PROGRAM_ABSTRACT_END