Magnetic Archaeology of Early-type Stellar Dynamos

Early-type stars show a bimodal distribution of magnetic field strengths, with some showing very strong fields (greater than or similar to 1 kG) and others very weak fields (less than or similar to 10 G). Recently, we proposed that this reflects the processing or lack thereof of fossil fields by subsurface convection zones. Stars with weak fossil fields process these at the surface into even weaker dynamo-generated fields, while in stars with stronger fossil fields magnetism inhibits convection, allowing the fossil field to remain as is. We now expand on this theory and explore the timescales involved in the evolution of near-surface magnetic fields. We find that mass loss strips near-surface regions faster than magnetic fields can diffuse through them. As a result, observations of surface magnetism directly probe the frozen-in remains of the convective dynamo. This explains the slow evolution of magnetism in stars with very weak fields: these dynamo-generated magnetic fields evolve on the timescale of the mass loss, not that of the dynamo.

Automated summary

Early-type stars exhibit a bimodal distribution of magnetic field strengths, which is related to the processing or lack thereof of fossil fields by subsurface convection zones. Stars that process weak fossil fields at the surface generate even weaker dynamo-generated fields, while stars with stronger fossil fields have magnetism that inhibits convection, allowing the fossil field to remain unchanged. This explains the slow evolution of magnetism in stars with very weak fields, as surface magnetism reflects the frozen-in remains of the convective dynamo, evolving on the timescale of mass loss rather than the dynamo itself.