Abundances in the Galactic bulge: results from planetary nebulae and giant stars

TL;DR

This study compares planetary nebulae and giant star abundances in the Galactic bulge to understand its chemical evolution, highlighting systematic discrepancies and emphasizing caution in using absolute abundances for such analyses.

Contribution

It provides the largest high-quality dataset of PNe in the Galactic bulge and compares these with giant star data and LMC PNe to identify reliable chemical tracers.

Findings

O and Ne abundances in PNe reflect the interstellar medium at progenitor formation

PN oxygen abundance distribution is shifted lower by 0.3 dex compared to giants

Systematic errors likely cause discrepancies between PNe and giant star abundances

Abstract

Our understanding of the chemical evolution of the Galactic bulge requires the determination of abundances in large samples of giant stars and planetary nebulae (PNe). We discuss PNe abundances in the Galactic bulge and compare these results with those presented in the literature for giant stars. We present the largest, high-quality data-set available for PNe in the direction of the Galactic bulge (inner-disk/bulge). For comparison purposes, we also consider a sample of PNe in the Large Magellanic Cloud (LMC). We derive the element abundances in a consistent way for all the PNe studied. By comparing the abundances for the bulge, inner-disk, and LMC, we identify elements that have not been modified during the evolution of the PN progenitor and can be used to trace the bulge chemical enrichment history. We then compare the PN abundances with abundances of bulge field giant. At the metallicity of the bulge, we find that the abundances of O and Ne are close to the values for the interstellar medium at the time of the PN progenitor formation, and hence these elements can be used as tracers of the bulge chemical evolution, in the same way as S and Ar, which are not expected to be affected by nucleosynthetic processes during the evolution of the PN progenitors. The PN oxygen abundance distribution is shifted to lower values by 0.3 dex with respect to the distribution given by giants. A similar shift appears to occur for Ne and S. We discuss possible reasons for this PNe-giant discrepancy and conclude that this is probably due to systematic errors in the abundance derivations in either giants or PNe (or both). We issue an important warning concerning the use of absolute abundances in chemical evolution studies.