The evolution of planetary nebulae VI. On the chemical composition of the metal-poor PN G135.9+55.9

TL;DR

This study refines the oxygen abundance measurement of the metal-poor planetary nebula G135.9+55.9 using new observations and advanced hydrodynamic models, revealing it has extremely low oxygen content and unusual elemental ratios.

Contribution

It introduces a combined approach of integral field spectroscopy, ultraviolet observations, and time-dependent hydrodynamic modeling to accurately determine chemical abundances in evolved planetary nebulae.

Findings

Oxygen abundance is about 1/80 of the Galactic disk value.

Departures from thermal equilibrium significantly affect temperature and emission lines.

C/O and Ne/O ratios are unusually high, similar to other halo objects.

Abstract

The actual value of the oxygen abundance of the metal-poor planetary nebula PN G135.9+55.9 has frequently been debated in the literature. We wanted to clarify the situation by making an improved abundance determination based on a study that includes both new accurate observations and new models. We made observations using the method of integral field spectroscopy with the PMAS instrument, and also used ultraviolet observations that were measured with HST-STIS. In our interpretation of the reduced and calibrated spectrum we used for the first time, recent radiation hydrodynamic models, which were calculated with several setups of scaled values of mean Galactic disk planetary nebula metallicities. For evolved planetary nebulae, such as PN G135.9+55.9, it turns out that departures from thermal equilibrium can be significant, leading to much lower electron temperatures, hence weaker emission in collisionally excited lines. Based on our time-dependent hydrodynamic models and the observed emission line [OIII]5007, we found a very low oxygen content of about 1/80 of the mean Galactic disk value. This result is consistent with emission line measurements in the ultraviolet wavelength range. The C/O and Ne/O ratios are unusually high and similar to those of another halo object, BoBn-1.