A post-common-envelope binary with double-peaked Balmer emission lines from TMTS

Context. The dynamical method provides an efficient way to discover post-common-envelope binaries (PCEBs) with faint white dwarfs (WDs), thanks to the development of time-domain survey projects. As close binary systems undergo a common-envelope phase, they offer unique opportunities to study the astrophysical processes associated with binary evolution. Aims. We perform a comprehensive analysis of the PCEB system TMTS J15530469+4457458 (J1553), discovered by the Tsinghua University-Ma Huateng Telescopes for Survey, to explore its physical origin and evolutionary fate. Methods. This system is characterized by double-peaked Balmer emission lines, and we applied a cross-correlation function to derive its radial velocity (RV) from a series of phase-resolved Keck spectra. The physical parameters of this binary were obtained by fitting the light curves and RV simultaneously. The locations of the Balmer lines were inferred from Doppler tomography, and a MESA simulation was performed to explore the evolution of this system. Results. Analyses using the cross-correlation function suggest that this system is a single-lined spectroscopic binary and only one star is optically visible. Further analysis through Doppler tomography indicates that J1553 is a detached binary without an accretion disk. Under such a configuration, the simultaneous light-curve and RV fitting reveal that this system contains an unseen WD with mass M A = 0.56±0.09 M ⊙ , and an M4 dwarf with mass M B = 0.37±0.02 M ⊙ and radius R B = 0.403 +0.014 -0.015 . The extra prominent Balmer emission lines seen in the spectra can trace the motion of the WD; these lines are likely formed near the WD surface as a result of wind accretion. According to the MESA simulation, J1553 could have evolved from a binary consisting of a 2.0–4.0 M ⊙ zero-age-main-sequence star and an M dwarf with an initial orbital period P i ≈201−476 d, and the system has undergone a common-envelope (CE) phase. After about 3.3×10 6 yr, J1553 should evolve into a cataclysmic variable, with a transient state as a supersoft X-ray source at the beginning. J1553 is an excellent system for studying wind accretion, CE ejection physics, and binary evolution theory.