Origin of the optical–infrared stellar magnetic field discrepancy

Determine the physical origin of the systematic discrepancy between average unsigned magnetic field strengths inferred from optical Stokes I spectra via Zeeman intensification of saturated lines and those inferred from infrared spectra via Zeeman broadening of magnetically sensitive lines in solar-type stars, including cases such as WASP-85 A where disc-averaged fields agree with infrared-based scaling relations but not with optical intensification results.

Background

This paper measures local magnetic fields in star spots on the G5 dwarf WASP-85 A by modeling Zeeman broadening in high-resolution transit spectroscopy, finding spot field strengths of 2.7–4.4 kG and disc-averaged unsigned fields Bf of 16–61 G. When comparing these disc-averaged values against literature relations between magnetic flux and Rossby number, the authors find agreement with infrared (IR) Zeeman broadening–based results but inconsistency with optical Zeeman intensification–based results.

The broader literature reports that optical Stokes I measurements using Zeeman intensification of saturated lines and infrared measurements using Zeeman broadening can yield systematically different magnetic field strengths for similar stars. Although the discrepancy is well documented, its underlying cause has not been established, motivating a clear open question about reconciling these measurement techniques or identifying the physical and methodological factors that produce divergent results.