A nearby polluted white dwarf with a 6.2 h spin period

Abstract

ABSTRACT 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, multisector 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.