The planetary nebula population of M33 and its metallicity gradient: A look into the galaxy's distant past

TL;DR

This study analyzes the planetary nebulae in M33 to understand its chemical composition and metallicity gradient, revealing a consistent gradient over time and providing insights into the galaxy's evolutionary history.

Contribution

First detailed spectroscopic analysis of 102 PNe in M33, establishing the metallicity gradient and its implications for galaxy evolution.

Findings

O/H and Ne/H are tightly correlated, indicating oxygen evolution is minimal.

Alpha-element abundances match HII regions, suggesting little enrichment since PN progenitors formed.

The radial oxygen gradient is consistent with that from HII regions, showing no significant flattening over time.

Abstract

The Planetary Nebula (PN) population of M33 is studied via multi-fiber spectroscopy with Hectospec at the MMT. In this paper we present the spectra of 102 PNe, whereas plasma diagnostic and chemical abundances were performed on the 93 PNe where the necessary diagnostic lines were measured. About 20% of the PNe are compatible with being Type I; the rest of the sample is the progeny of an old disk stellar population, with main sequence masses M<3M${_\odot}$ and ages t$>$0.3 Gyr. By studying the elemental abundances of the PNe in the M33 disk we were able to infer that: (1) there is a tight correlation between O/H and Ne/H, broadly excluding the evolution of oxygen; (2) the average abundances of the $\alpha$-elements are consistent with those of \hii regions, indicating a negligible global enrichment in the disk of M33 from the epoch of the formation of the PN progenitors to the present time; (3) the radial oxygen gradient across the M33 disk has a slope of -0.031$\pm$0.013 dex kpc$^{-1}$, in agreement, within the errors, with the corresponding gradient derived from HII regions. Our observations do not seem to imply that the metallicity gradient across the M33 disk has flattened considerably with time. We report also the discovery of a PN with Wolf-Rayet features, PN039, belonging the class of late [WC] stars