New insights into the helium star formation channel of AM CVn systems with explanations of Gaia14aae and ZTFJ1637+49

Abstract

We model helium-rich stars with solar metallicity ($X=0.7,Z=0.02$) progenitors that evolve to form AM Canum Venaticorum systems through a helium-star formation channel, with the aim to explain the observed properties of Gaia14aae and ZTFJ1637+49. We show that semi-degenerate, H-exhausted ($X\le 10^{-5}$), He-rich ($Y\approx 0.98$) donors can be formed after a common envelope evolution (CEE) phase if either additional sources of energy are used to eject the common envelope, or a different formalism of CEE is implemented. We follow the evolution of such binary systems after the CEE phase using the Cambridge stellar evolution code, when they consist of a He-star and a white dwarf accretor, and report that the mass, radius, and mass-transfer rate of the donor, the orbital period of the system, and the lack of hydrogen in the spectrum of Gaia14aae and ZTFJ1637+49 match well with our modelled trajectories wherein, after the CEE phase Roche lobe overflow is governed not only by the angular momentum loss (AML) owing to gravitational wave radiation ($\mathrm{AM}{\mathrm{L}}_{\mathrm{GR}}$) but also an additional AML owing to $\alpha -\Omega$ dynamos in the donor. This additional AML is modelled with our double-dynamo (DD) model of magnetic braking in the donor star. We explain that this additional AML is just a consequence of extending the DD model from canonical cataclysmic variable donors to evolved donors. We show that none of our modelled trajectories match with Gaia14aae or ZTFJ1637+49 if the systems are modelled only with $\mathrm{AM}{\mathrm{L}}_{\mathrm{GR}}$.

arXiv comment11 pages, 11 figures. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

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• Reversing the verdict: Cataclysmic variables could be the dominant progenitors of AM CVn binaries after all
• Evolved cataclysmic variables as progenitors of AM CVn stars
• A Catalogue of Ultracompact Mass-Transferring White Dwarf Binaries
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