A Nearby Polluted White Dwarf with a 6.2 h Spin Period

This letter reports the first detection of a periodic light curve whose modulation is unambiguously due to rotation in a polluted white dwarf. TESS observations of WD 2138-332, at a distance of 16.1 pc, reveal a 0.39 per cent amplitude modulation with a 6.19 h period. While this rotation is relatively rapid for isolated white dwarfs, it falls within the range of spin periods common to those with detectable magnetic fields, where WD 2138-332 is notably both metal-rich and weakly magnetic. Within the local 20 pc volume of white dwarfs, multi-sector TESS data find no significant periodicities among the remaining 16 polluted objects (five of which are also magnetic), whereas six of 23 magnetic and metal-free targets have light curves consistent with rotation periods between 0.7 and 35 h (three of which are new discoveries). This indicates the variable light curve of WD 2138-332 is primarily a result of magnetism, as opposed to an inhomogeneous distribution of metals. From 13 magnetic and metallic degenerates with acceptable TESS data, a single detection of periodicity suggests that polluted white dwarfs are not rotating as rapidly as their magnetic counterparts, and planet ingestion is thus unlikely to be a significant channel for rapid rotation.