Abundances of Planetary Nebulae and Evolved Stars: Iron and Sulfur Depletion, and Carbon and Nitrogen Enrichment, in Low- and Intermediate-mass Stellar Populations in the Milky Way

We research the elemental abundances in Galactic planetary nebulae (PNe) compared with those of their stellar progenitors (red giant branch and asymptotic giant branch, AGB, stars), to explore and quantify the expected—i.e., due to AGB evolution or condensation onto grains—differences. We gleaned the current literature for the nebular abundances while we used the APOGEE DR 17 survey data for the stellar sample. We examined the elements in common between the nebular and stellar samples, namely, C, N, O, Fe, and S. We confirm that iron in PNe is mostly entrapped in grains, with an average depletion 〈D[Fe/H]〉 = 1.741 ± 0.486 dex, and we disclose a weak correlation between iron depletion and the [O/H] abundance, D[Fe/H] = (6.6003 ± 2.443) × [O/H] + (1.972 ± 0.199). Sulfur may also be mildly depleted in PNe, with 〈D[S/H]〉 = 0.179 ± 0.291 dex. We also found an indication of nitrogen enrichment for PNe 〈E[N/H]〉 = 0.393 ± 0.421 dex, with maximum enrichment (0.980 ± 0.243) occurring for the PNe whose progenitors have gone through the hot-bottom burning. The carbon enrichment is 〈E[C/H]〉 = 0.337 ± 0.463 dex when measured for the general PN populations. Our results will be relevant for future Galactic and extragalactic studies comparing nebular and stellar samples.