Reversing the verdict: Cataclysmic variables could be the dominant progenitors of AM CVn binaries after all

Abstract

Context. AM CVn binaries are potential progenitors of thermonuclear supernovae and strong sources of persistent gravitational wave radiation. For a long time, it has been believed that these systems cannot descend from cataclysmic variables (CVs), at least not in large numbers, because the initial conditions need to be fine-tuned and, even worse, the resulting surface hydrogen abundance would be high enough to be detected which contradicts a defining feature of AM CVn binaries. Aims. Here we show that both claimed weaknesses of the CV formation channel for AM CVn binaries are model-dependent and rely on poorly constrained assumptions for magnetic braking. Methods. We performed binary evolution simulations with the MESA code for different combinations of post-common-envelope white dwarf and companion masses as well as orbital periods assuming the CARB model for strong magnetic braking. Results. We found that AM CVn binaries with extremely-low surface hydrogen abundances are one natural outcome of CV evolution if the donor star has developed a non-negligible helium core prior to the onset of mass transfer. In this case, after hydrogen envelope exhaustion during CV evolution, the donor becomes degenerate and its surface hydrogen abundance substantially drops and becomes undetectable. Our simulations also show that the CV formation channel is able to explain the observed AM CVn binaries with very low mass and bloated donor stars (Gaia14aae and ZTF J1637+49). Conclusions. CVs with evolved donors are likely the progenitors of at least a fraction of AM CVn binaries.

The cataclysmic variable orbital period gap: More evident than ever

“…Despite these crucial results on magnetic braking (or actually rather because of them), it might be difficult to soon find a unified magnetic braking prescription. Such a universal magnetic braking prescription would need to convincingly explain observations of single stars, which seem to favour some kind of saturated magnetic braking (Gossage et al 2023), reproduce the donor star radii in cataclysmic variables as well as disrupted magnetic braking (Knigge et al 2011), and provide the strong angular momentum loss rates that are required to explain persistent low-mass X-ray binaries (Van & Ivanova 2019) and AM CVn binaries descending from cataclysmic variables with evolved donors (Belloni & Schreiber 2023b).…”

Helium-deficient ER UMa-type Dwarf Nova below the Period Minimum with a Hot Secondary

“…Their orbital periods are 10 hr (Ritter & Kolb 2003), indicating that they are at a late dynamical evolutionary stage of binary systems and their orbits shrink due to gravitational radiation or magnetic braking as they evolve until the orbital periods become close to the period minimum of about 75 minutes (Paczynski & Sienkiewicz 1981;Rappaport et al 1983;Knigge et al 2011;Goliasch & Nelson 2015). Some dwarf novae are expected to evolve to AM Canum Venaticorum (AM CVn) objects (Kennedy et al 2015;Burdge et al 2022;Belloni & Schreiber 2023), which are WD + WD binaries with an extremely short (40 minutes) orbital period. AM CVn systems are considered to be progenitors of some Type Ia supernovae, as well as sources of substantial gravitational wave radiation (Han & Podsiadlowski 2004;Bildsten et al 2007;El-Badry et al 2021b).…”