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 ≤ 10−5), He-rich (Y ≈ 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 (AMLGR) but also an additional AML owing to α − Ω 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 AMLGR.

Evolved cataclysmic variables as progenitors of AM CVn stars

“…This work does not aim at establishing the Evolved CV channel as a dominant mechanism of AM CVn formation, because even with the increased likelihood of their formation estimated in Section 3, this channel is still sub-dominant compared to the other two channels, as shown by Nelemans et al (2004). In addition, Sarkar et al (2023) show that most of the AM CVn stars discussed in this work can be explained with the He-star formation channel (see their fig. 7).…”

“…Again, the evolution of the system is governed by AML GR (see Yungelson 2008 and references therein). However, the results of Sarkar et al (2023) suggest that other AML mechanisms also contribute to the evolution of systems with 𝑃 orb 20 min. The final channel is known as the Evolved CV channel in which, after a single CE event, a H-rich star commences mass-transfer to a WD accretor while in the Hertzprung gap (between the end of its main sequence and the beginning of its ascent of the red giant branch).…”

“…Its much tighter orbit for its donor mass suggests a degenerate donor (Roelofs et al 2007) and rules out the Evolved CV formation channel. Interestingly, the suggested donor star by Solheim (2010) for many of these systems is either a He-star or a WD with the exception of Gaia14aae, of which the progenitor has been suggested to be an evolved H-star (Campbell et al 2015;Green et al 2018), although recent results of Sarkar et al (2023) show that Gaia14aae and ZTFJ1637+49 likely formed through the He-star channel. The results of Roelofs et al (2007) also suggest a semi-degenerate donor for AM CVn, HP Lib, CR Boo, and V803 Cen.…”

SRGeJ045359.9+622444: A 55 Minute Period Eclipsing AM Canum Venaticorum Star Discovered from a Joint SRG/eROSITA + ZTF Search

“…The N/C ratio may be also informative in distinguishing between formation channels (Nelemans et al 2010). For the He star channel, Nelemans et al (2010), using the "standard" magnetic braking model and Sarkar et al (2023a), using the double-dynamo model, predict N/C ≈5 and 120-200, respectively, while for the He WD and evolved CV channels N/C  100 in both models. However, based on the equivalent widths of the N and C lines, we are able only to estimate for SRGeJ0453 a lower limit of N/C  1, thus allowing for both He WD and He star channels.…”

“…Two CE episodes in the Helium star (He star) channel (Iben & Tutukov 1987) leave behind a carbon-oxygen WD and a low-mass He star, which may be nondegenerate or weakly degenerate (Sarkar et al 2023a). GW radiation brings the helium star to Roche-lobe overflow.…”