A strange dwarf scenario for the formation of the peculiar double white dwarf binary SDSS J125733.63+542850.5

Abstract

The Hubble Space Telescope observation of the double white dwarf (WD) binary SDSS J125733.63$+$542850.5 reveal that the massive WD has a surface gravity log$g_1\sim 8.7$ (which implies a mass of $M_1\sim 1.06{\mathrm{M}}_{\odot }$) and an effective temperature $T_1\sim 13000$ K, while the effective temperature of the low-mass WD ($M_2<0.24$ {\rm M${}_{\odot }$}) is $T_2\sim 6400K$. Therefore, the massive and the low-mass WDs have a cooling age $\ddot{I}{„}_1\sim 1$ {\rm Gyr} and $\ddot{I}{„}_2\ge 5$ {\rm Gyr}, respectively. This is in contradiction with traditional binary evolution theory. In this Letter, we propose a strange dwarf scenario to explain the formation of this double WD binary. We assume that the massive WD is a strange dwarf originating from a phase transition in a $\sim 1.1$ M${}_{\odot }$ WD, which has experienced accretion and spin-down processes. Its high effective temperature could arise from the heating process during the phase transition. Our simulations suggest that the progenitor of SDSS J125733.63$+$542850.5 can be a binary system consisting of a $0.65{\mathrm{M}}_{\odot }$ WD and a $1.5{\mathrm{M}}_{\odot }$ main sequence star in a 1.492 day orbit. Especially, the secondary star (i.e., the progenitor of the low mass WD) is likely to have an ultra-low metallicity of $Z=0.0001$.