Planetary Nebulae in the Small Magellanic Cloud

TL;DR

This study models the planetary nebulae in the Small Magellanic Cloud to identify their progenitors and understand their chemical evolution, highlighting differences with the Large Magellanic Cloud due to star formation history.

Contribution

It introduces detailed AGB star models with dust formation to interpret PNe observations in the SMC, linking chemical signatures to progenitor mass and evolution.

Findings

Most PNe originate from low-mass, carbon-rich stars.

Some PNe are from massive AGB stars with hot bottom burning.

Differences with LMC PNe are due to distinct star formation histories.

Abstract

We analyse the planetary nebulae (PNe) population of the Small Magellanic Cloud (SMC), based on evolutionary models of stars with metallicities in the range $10^{-3} \leq Z \leq 4\times 10^{-3}$ and mass $0.9 M\odot < M < 8M\odot$, evolved through the asymptotic giant branch (AGB) phase. The models used account for dust formation in the circumstellar envelope. To characterise the PNe sample of the SMC, we compare the observed abundances of the various species with the final chemical composition of the AGB models: this study allows us to identify the progenitors of the PNe observed, in terms of mass and chemical composition. According to our interpretation, most of the PNe descend from low-mass ($M < 2 M\odot$) stars, which become carbon rich, after experiencing repeated third dredge-up episodes, during the AGB phase. A fraction of the PNe showing the signature of advanced CNO processing are interpreted as the progeny of massive AGB stars, with mass above $\sim 6 M\odot$, undergoing strong hot bottom burning. The differences with the chemical composition of the PNe population of the Large Magellanic Cloud (LMC) is explained on the basis of the diverse star formation history and age-metallicity relation of the two galaxies. The implications of the present study for some still highly debated points regarding the AGB evolution are also commented.