Discovery of three magnetic helium-rich hot subdwarfs with SALT

Magnetic fields with strengths ranging from 300 to 500\,kG have recently been discovered in a group of four extremely similar helium-rich hot subdwarf (He-sdO) stars. In addition to their strong magnetic fields, these He-sdO stars are characterised by common atmospheric parameters, clustering around = 46\,500\,K, a $ g/ cm s $ close to 6, and intermediate helium abundances. Here we present the discovery of three additional magnetic hot subdwarfs, J123359.44$-$674929.11, J125611.42$-$575333.45, and J144405.79$-$674400.93. These stars are again almost identical in terms of atmospheric parameters, but, at $B 200\,kG, their magnetic fields are somewhat weaker than those previously known. The close similarity of all known He-sdOs implies a finely tuned formation channel. We propose the merging of a He white dwarf with a H+He white dwarf. A differential rotation at the merger interface may initiate a toroidal magnetic field that evolves via a magnetic dynamo to produce a poloidal field. This field is either directly visible at the surface or might diffuse towards the surface if initially buried. We further discuss a broad absorption line centred at about 4630\ that is common to all magnetic He-sdOs. This feature may not be related to the magnetic field but instead to the intermediate helium abundances in these He-sdO stars, allowing the strong He ii line to be perturbed by collisions with hydrogen atoms.