Magnetic Field Amplification During the Common Envelope Phase

TL;DR

This study uses magnetohydrodynamics simulations to show how magnetic fields are amplified during the common envelope phase in binary star systems, which may influence phenomena like luminous red novae.

Contribution

First MHD simulations of the CE phase demonstrating magnetic field amplification via magnetorotational instability during spiral-in.

Findings

Magnetic fields are amplified to 10-100 kG in the envelope.

Amplification driven by magnetorotational instability.

Fields are too weak to explain highly magnetic white dwarfs.

Abstract

During the common envelope (CE) phase, a giant star in a binary system overflows its Roche lobe and unstable mass transfer leads to a spiral-in of the companion, resulting in a close binary system or in a merger of the stellar cores. Dynamo processes during the CE phase have been proposed as a mechanism to generate magnetic fields that are important for forming magnetic white dwarfs (MWDs) and for shaping planetary nebulae. Here, we present the first magnetohydrodynamics simulations of the dynamical spiral-in during a CE phase. We find that magnetic fields are strongly amplified in the accretion stream around 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 after 120 d. The magnetic field amplification is consistent with being driven by the magnetorotational instability. The field strengths reached in our simulation make the magnetic field interesting for diagnostic purposes, but they are dynamically irrelevant. They are also too small to explain the formation of the highest fields found in MWDs, but may be relevant for luminous red novae, and detecting magnetic fields in these events would support the scenario as proposed here.