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During the common envelope (CE) phase, a giant star in a binary systemoverflows its Roche lobe and unstable mass transfer leads to a spiral-in of thecompanion, resulting in a close binary system or in a merger of the stellarcores. Dynamo processes during the CE phase have been proposed as a mechanismto generate magnetic fields that are important for forming magnetic whitedwarfs (MWDs) and for shaping planetary nebulae. Here, we present the firstmagnetohydrodynamics simulations of the dynamical spiral-in during a CE phase.We find that magnetic fields are strongly amplified in the accretion streamaround the $1M_\odot$ companion as it spirals into the envelope of a $2M_\odot$RG. This leads to field strengths of 10 to 100 kG throughout the envelope after120 d. The magnetic field amplification is consistent with being driven by themagnetorotational instability. The field strengths reached in our simulationmake the magnetic field interesting for diagnostic purposes, but they aredynamically irrelevant. They are also too small to explain the formation of thehighest fields found in MWDs, but may be relevant for luminous red novae, anddetecting magnetic fields in these events would support the scenario asproposed here.

Magnetic Field Amplification During the Common Envelope Phase