A growing family of two-faced stars

Roughly one in five white dwarfs (WDs) within 100 pc undergo spectral transitions from one type to another. These changes are driven by internal convective processes that mix surface materials, leaving surface compositions homogeneous. However, a few very rare cases — so-called double-faced WDs — show spectral variability over just a few hours, a timescale too rapid and surface composition too inhomogeneous for homogeneous atmospheric models to explain. Adam Moss and colleagues discovered one such double-faced WD, SDSS J0847+4842, and revisited another known WD, LB 8915, confirming its double-faced nature. By investigating these two, as well as five previously known double-faced WDs, the authors propose that magnetism drives the observed frequent spectral-type changes.

Through a time-series spectroscopic survey of six unresolved WD binary candidates, the authors found that one of them (SDSS J0847+4842) is not a binary but a massive single WD showing spectroscopic variations over 6.5 or 8.9 hours. They also revisited LB 8915 — a known DBA-type WD — whose time-series spectra show hydrogen-line variability every 5.7 hours, while helium lines remain consistently strong. The time-resolved spectra of both objects are well explained by a model with hydrogen polar caps and a helium equatorial belt. These discoveries increase the number of known double-faced WDs to seven.