Testing the cataclysmic variable formation channel for SDSS J1257+5428 with saturated magnetic braking

Abstract

Binary systems containing two white dwarfs (WDs) are objects of great interest because they are involved in extreme physical phenomena and because they are possible progenitors of Type Ia supernovae (SN Ia). Despite significant recent progress, however, we do not properly understand how close WD binaries form and evolve. In this context, observationally well-characterized systems that allow testing evolutionary scenarios are of utmost importance. Among such systems, SDSS J1257+5428 stands out, consisting of an extremely low-mass WD (≲ 0.2 M⊙) with an effective temperature of ∼ 6 400 K, and a massive WD with a mass of ∼ 1 M⊙ and an effective temperature of ∼ 13 000 K. Recently, the presence of a third WD has been inferred, which provides a lower limit on the total age of the system. The properties of this system appear to be at odds with theoretical expectations, giving rise to its characterization as a paradoxical system. We investigate whether it is possible to reproduce SDSS J1257+5428 through a previously ignored evolutionary channel, the cataclysmic variable (CV) formation channel, considering a new prescription for magnetic braking (MB). We use the MESA code to perform detailed binary evolution models and to explore whether a system evolving from a post–common-envelope configuration can reproduce the observed properties of SDSS J1257+5428. In our setup, we adopt a saturated and enhanced MB prescription. We find that it is indeed possible to explain the existence of the paradoxical system. Our best model predicts that the companion star reached the end of the MS, forming a Helium core before the onset of mass transfer. Following the mass transfer episode, the companion underwent hydrogen flashes, ending as an extremely low-mass WD similar to the observed one. The WD accretor becomes more massive, consistent with the observed mass. We conclude that SDSS J1257+5428 can be explained as a descendant of a CV, using the same MB prescription used to model CVs.