The Complex Magnetic Anatomy of a White Dwarf Chromosphere

Abstract

This study presents phase-resolved spectroscopy and spectropolarimetry of the DAHe white dwarf LP 705-64. Here, two apparently discrete chromospheric regions are observed to alternate in visibility and magnetic polarity with rotation, with field strengths of 9.5 and 22 MG based on the measured Zeeman splitting in Hα. As in the class prototype GD 356, emission line equivalent widths vary in antiphase with the broadband light curve. The 9.5 MG field is visible for 60 per cent of the stellar rotation and stays nearly constant, whereas the 22 MG field increases modestly during the remaining phases. Notably, the time-series transitions between magnetic regions are abrupt and transit-like in morphology, requiring at least one magnetic region to be highly localised and significantly smaller than the areas inferred for the dark surface spots. Both the intensity and the Stokes V/I spectra indicate a field configuration significantly more complex than an offset dipole, but higher-cadence data are necessary to distinguish between a global field and localised magnetic regions. All data are consistent with magnetic, chromospheric, and dark surface regions that are co-located and thus physically related. These findings support a model of chromospheric emission that is intrinsic and localised, consistent with predictions of dynamo-driven electrical currents that heat the upper atmosphere near magnetic poles.