Measuring the spin period of (cool) magnetic white dwarfs to constrain the crystallization-rotation driven dynamo model

Abstract

Context: Understanding the formation of magnetic white dwarfs has been a long-time problem. Recently, it has been proposed that the magnetic elds of some white dwarfs are generated due to the interplay of crystallization and fast rotation in white dwarfs. These magnetic white dwarfs should meet two conditions: (1) they should have a low e ective temperatures such that they have crystallised cores (2) they should have short spin periods gained by accretion of material. Aim: I aim to measure the spin period of cool DZ magnetic white dwarfs to prove the prediction that they posses a shorter spin period than their non magnetic counterparts. Method: I obtained photometric data of ve cool magnetic white dwarf with SPECULOOS, reduced the data with a free python-based software, prose, and searched for periodicities using the Lomb-Scargle periodogram implemented in period04 Results: I measured the rotation period of two out of ve magnetic white dwarfs (6:34 0:10 hr and 4:84 0:02 hr for WD2138-332 and LSPMJ0107+2650, respectively). I found complications for the other three systems: SDSS J003708.42-052532.8 has too few data points for a reliable estimate, WD0322-019 has probably a magnetic eld too low to form the dark spots (B=0.120 MG), and WD0315-293 could either have a spin period shorter than the time resolution of the data or the magnetic eld produces a spot con guration that does not lead to signi cant photometric variability. Conclusions: The short periods measured for WD2138-332 and LSPMJ0107+2650 are consistent with the rotation-crystallization dynamo driven model, which predicts shorter periods for magnetic white dwarfs than for non-magnetic white dwarfs.