# # Program information file # PROGRAM_ID 2023A049 PROGRAM_TITLE Accurate Selenium Abundances in Ionized Nebulae With SpeX PROGRAM_INV1 Nicholas Sterling PROGRAM_INV2 Harriet Dinerstein PROGRAM_INV3 William Vacca PROGRAM_INV4 Mabel Stephenson PROGRAM_INV5 PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG Low- and intermediate-mass [1--8 Msun] asymptotic giant branch [AGB] stars synthesize roughly half of the neutron[n]-capture element [atomic number Z > 30] nuclei in the Universe. Of these elements, selenium is by far the most widely-observed in ionized nebulae owing to the strong [Se IV] 2.2864 micron emission line, which can be brighter than 10% of the H I Brackett gamma flux. Consequently, Se is particularly valuable for assessing slow n-capture [s-process] enrichments in planetary nebulae [PNe], which are the ejecta of low- and intermediate-mass stars at the end of the AGB. However, there is evidence that elemental Se abundances derived from the [Se IV] line are systematically underestimated. To address this problem, we propose to use SpeX to simultaneously observe [Se III] 1.0992 and [Se IV] 2.2864 microns in four PNe with different central star temperatures. The [Se III] line has previously been detected in only one object, and a better understanding of the Se ionization equilibrium requires measurements in objects with a range in degree of ionization. With a resolution of 2000, the SpeX SXD mode will resolve [Se III] from nearby emission lines and enable detections of other isolated n-capture elements [including Kr, Te, and Xe], maximizing the scientific return of the observations. From our measurements of the abundances of multiple Se ions we will construct a robust procedure to solve the Se ionization balance in nebulae with a range in degree of ionization. This procedure can be applied in the future to determine accurate elemental enrichments of Se in PNe, and hence in their AGB progenitors, in large-sample surveys where only one ion of Se is observed. PROGRAM_ABSTRACT_END