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

TL;DR

This study compares elemental abundances in Galactic planetary nebulae and their progenitor stars, revealing iron and sulfur depletion due to grain formation and nitrogen, carbon enrichment from stellar evolution, with implications for galactic chemical evolution.

Contribution

It provides new quantifications of element depletion and enrichment in planetary nebulae compared to their progenitors, using combined literature and APOGEE data.

Findings

Iron is mostly depleted in grains in PNe.

Sulfur shows mild depletion in PNe.

Nitrogen is enriched in PNe, especially with hot bottom burning.

Abstract

We explore the elemental abundances in Galactic planetary nebulae (PNe) compared with those of their stellar progenitors (Red Giant Branch and Asymptotic Giant Branch, RGB and 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$\pm$0.486 dex, and we disclose a weak correlation between iron depletion and the [O/H] abundance, D[Fe/H]$=(6.6003\pm2.443)\times{\rm [O/H]} +(1.972\pm0.199)$. Sulfur may also be mildly depleted in PNe, with $<$D[S/H]$>=0.179\pm0.291$ dex. We also found an indication of nitrogen enrichment for PNe $<$E[N/H]$>$=0.393$\pm$0.421 dex, with maximum enrichment (0.980$\pm$0.243) occurring for the PNe whose progenitors have gone through the HBB. The carbon enrichment is $<$E[C/H]$>$=0.332$\pm$0.460 dex when measured for the general PN populations. Our results will be relevant for future Galactic and extragalactic studies comparing nebular and stellar samples.