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

Context. Planetary nebulae are shells that have been ejected by low- and intermediate-mass stars. The slow wind ejected by an 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 displays a spectral index between −0.1 and 2. Only two planetary nebulae have been known to show non-thermal radio emission, which would indicate magnetic fields and non-thermal electrons. Aims. The aim of this paper is to verify the presence of magnetic fields of planetary nebulae, based on the knowledge that magnetic fields can play a key role in 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 are characterised by sizes of more than 20 arcseconds in diameter. Magnetic fields and non-thermal emission may be common in smaller planetary nebulae, but it could be suppressed by the presence of thermal emission. Our results suggest that a range of different mechanisms can be responsible for the origin of magnetic fields and non-thermal emission in planetary nebulae.