Magnetic fields in planetary nebulae detected through non-thermal radio continuum emission

TL;DR

This paper investigates magnetic fields in planetary nebulae by identifying non-thermal radio emission as a tracer using low-frequency LOFAR observations at 144 MHz combined with archival higher-frequency data. By cross-matching HASH PN catalog entries with LoTSS DR3 sources and calculating multi-frequency spectral indices, the authors identify 30 PNe with non-thermal emission (spectral index $< -0.1$), most of which are bipolar and extended beyond $20$ arcsec. They discuss several physical mechanisms for the non-thermal emission, including magnetized central-star winds and wind–shock interactions, and show that different PNe may host magnetic fields and acceleration sites with morphologies that differ from purely thermal emission. The study concludes that non-thermal emission and magnetic fields are more common in bipolar, likely binary-influenced PNe and emphasizes the utility of low-frequency surveys to unveil these processes, while acknowledging that a single universal mechanism cannot explain all observed cases.

Abstract

Context. Planetary nebulae are shells ejected by low- and intermediate-mass stars. The slow wind ejected by the asymptotic giant branch star is compressed by a fast stellar wind to produce an expanding gaseous shell surrounding a hot bubble. The shell is a source of thermal radio emission which shows a spectral index between -0.1 and 2. Only two planetary nebulae are known to show non-thermal radio emission indicating magnetic fields and non-thermal electrons. Aims. The aim of this paper is verification of presence of magnetic fields of planetary nebulae. Magnetic fields can have a significant influence on shaping planetary nebulae. Methods. We observed a sample of northern planetary nebulae in radio continuum at 144 MHz with the Low Frequency Array. We combined our observations with archival observations at higher frequencies. Results. The spectral indices in 30 planetary nebulae were below -0.1, indicating non-thermal radio emission. The majority of this sample consists of bipolar planetary nebulae, which are known to originate from binary central stars. Most of the nebulae have sizes larger than 20 arcsec. Magnetic fields and nonthermal emission may be common in smaller planetary nebulae, but can be suppressed by thermal emission. Our results suggest that different mechanisms can be responsible for the origin of magnetic fields and non-thermal emission in planetary nebulae.

Figures (5)

• Figure 1: Separations of the PNe and LoTSS DR3 sources.
• Figure 2: Examples of 144 MHz radio (contours) and optical (background) emission: Abell 53, IRAS 19086+0603, KLSS 2-7, M 1-79, NGC 6826, PaEk 1. The beam size is indicated by a circle in the bottom left corner.
• Figure 3: Sizes of PNe dominated by thermal and non-thermal emission.
• Figure 4: Morphology of PNe dominated by thermal and non-thermal emission. R - round, B - bipolar, E - elliptical, I - irregular.
• Figure 5: Spectral indices of the non-thermal emission in PNe.