The double white dwarf merger progenitors of SDSS J2211+1136 and ZTF J1901+1458

Abstract

Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, the direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458), might be DWD merger products. J2211+1136 is a $1.27M_{\odot }$ WD with a rotation period of $70.32$ s and a surface magnetic field of $15$ MG. J1901+1458 is a $1.327$--$1.365M_{\odot }$ WD with a rotation period of $416.20$ s, and a surface magnetic field in the range $600$--$900$ MG. With the assumption of single-star evolution, the currently measured WD masses and surface temperatures, the cooling ages of J2211+1136 and J1901+1458 are, respectively, $2.61$--$2.85$ Gyr and $10$--$100$ Myr. We hypothesize that these WDs are DWD merger products and compute the evolution of the post-merged configuration formed by a central WD surrounded by a disk. We show that the post-merger system evolves through three phases depending on whether accretion, mass ejection (propeller), or magnetic braking dominates the torque onto the central WD. We calculate the time the WD spends in each of these phases and obtain the accretion rate and disk mass for which the WD rotational age, i.e., the total time elapsed since the merger to the instant where the WD central remnant reaches the current measured rotation period, agrees with the estimated WD cooling age. We infer the mass values of the primary and secondary WD components of the DWD merger that lead to a post-merger evolution consistent with the observations.

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