# # Program information file # PROGRAM_ID 2021A049 PROGRAM_TITLE Assessing Planetary Nebulae as Sources of Neutron-Capture Element Enrichments PROGRAM_INV1 Harriet Dinerstein PROGRAM_INV2 Nicholas Sterling PROGRAM_INV3 William Vacca PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT galactic/interstellar medium PROGRAM_ABSTRACT_BEG We propose to observe infrared emission lines from trans-iron-group [Z > 30] elements in planetary nebulae [PNe]. These nuclides can be synthesized by the s-process [slow neutron captures] during the Asymptotic giant branch [AGB] phase. AGB stars are major agents of galactic enrichment in these species, and by observing PNe we are directly witnessing this enrichment in progress. Our group has pioneered the use of infrared spectroscopy for this purpose, detecting and identifying a dozen new lines from 1.0-2.5 microns from elements with atomic numbers 32 [Ge] to 54 [Xe] and finding enrichments of up to an order of magnitude or larger. These results quantify the contributions of lower-mass stars to the evolving chemical composition of the Galaxy and provide empirical tests of AGB evolutionary models that exhibit dependencies of element yields on initial stellar mass and metallicity. We are using the IRTF to search for lines in spectral regions not accessible on other facilities, i.e. beyond 2.5 microns and regions of high telluric opacity. To date we have discovered four new IR lines with the IRTF from the light neutron-capture elements Br [35] and Rb [37], providing checks on claimed extreme enrichments of these elements based on optical observations. Having observed several PNe so far in a [Rb III] line, in 2021A we propose to observe several additional nebulae from stars in the mass range predicted to have particularly large Rb enrichments [~4-6 M_sun] to confirm this origin. By observing IR lines of [Br II] and [Br IV] in PNe with a range in degree of ionization, we will resolve inconsistencies in abundances from optical lines. Finally, we will measure [Zn/H], an [Fe/H] proxy, in PNe for which we find large enrichments in neutron-capture elements, facilitating comparison with theoretical models of AGB nucleosynthesis that are sensitive to initial [Fe/H]. PROGRAM_ABSTRACT_END