Spin evolution modelling for a newly formed white dwarf resulting from binary white dwarf merger

ABSTRACT Merger of two white dwarfs (WDs) has been proposed to form an isolated WD having high magnetization and rapid rotation. We study the influence of the magnetohydrodynamic (MHD) wind on spin evolution of the newly formed merger product. We consider the scenario that the merger product appears as a giant-star-like object with a radius of ${>}10^{10}\,{\rm cm}$ and a luminosity of the order of an Eddington value. We solve a structure of the merger product under the hydrostatic equilibrium and identify the position of the slow-point in the hot envelope. It is found that if such a giant-star-like object is spinning with an angular speed of the order of the Keplerian value, the MHD wind can be produced. The mass-loss rate is estimated to be of the order of ${\sim} 10^{20-21}\,{\rm g\,s^{-1}}$, and the time-scale of the spin down is ${\sim} 10\rm{{\!-\!}}10^{3}$ yr, which depends on stellar magnetic field. We discuss that the final angular momentum when the MHD wind is terminated is related to the magnetic flux and initial radiation luminosity of the merger product. We apply our model to three specific magnetic WD sources ZTF J190132.9+145808.7, SDSS J221141.8+113604.4, and PG 1031+234 by assuming that those WDs were as a result of the merger product. We argue that the current periods of ZTF J190132.9+145808.7 and PG 1031+234 that are strongly magnetized WDs are related to the initial luminosity at the giant phase. For SDSS J221141.8+113604.4, which is mildly magnetized WD, its angular momentum was almost determined when the spin-down time-scale due to MHD wind is comparable to the cooling time-scale in the giant phase.