PHL 932: when is a planetary nebula not a planetary nebula?

TL;DR

This study reclassifies the nebula around PHL 932 from a planetary nebula to a Stromgren sphere in the interstellar medium, based on multi-wavelength data analysis, challenging previous classifications and providing insights into its true nature.

Contribution

The paper demonstrates that the nebula around PHL 932 is an interstellar HII region rather than a planetary nebula, based on comprehensive multi-wavelength observations and analysis.

Findings

The nebula is a Stromgren sphere in the interstellar medium.

No evidence of close binarity for PHL 932.

Similar nebulae around other hot stars are also interstellar HII regions.

Abstract

The emission nebula around the subdwarf B (sdB) star PHL 932 is currently classified as a planetary nebula (PN) in the literature. Based on a large body of multi-wavelength data, both new and previously published, we show here that this low-excitation nebula is in fact a small Stromgren sphere (HII region) in the interstellar medium around this star. We summarise the properties of the nebula and its ionizing star, and discuss its evolutionary status. We find no compelling evidence for close binarity, arguing that PHL 932 is an ordinary sdB star. We also find that the emission nebulae around the hot DO stars PG 0108+101 and PG 0109+111 are also Stromgren spheres in the ISM, and along with PHL 932, are probably associated with the same extensive region of high-latitude molecular gas in Pisces-Pegasus.