The Formation of EL CVn-type Binaries

Abstract

EL CVn-type binaries are eclipsing binaries that contain an A-or F-type dwarf star and a very low mass (∼ 0.2M ⊙ ) helium white dwarf precursor (proto-He WD). A number of such objects have been discovered in the WASP and Kepler photometric surveys. Here we have studied the formation of EL CVn-type binaries and give the properties and the space density of this population of stars in the Galaxy. We show that EL CVn binaries cannot be produced by common envelope evolution as previously believed because this process leads to merging of the components when giants have such low-mass cores. Stable mass transfer in low-mass binaries, from more than 65,000 runs of Population I binary evolution, may well reproduce the properties of EL CVn stars such as the evolutionary phase, mass ratios and the WD mass-period (M WD − P ) relation. The study shows that the most common donor mass range for producing the observed EL CVn-type stars is 1.15 − 1.20M ⊙ and that the lifetime of such objects increases dramatically with decreasing proto-He WD mass. This leads to an intrinsic peak mass around the minimum mass of proto-He WDs of ∼ 0.16M ⊙ . The most probable proto-He WD mass is 0.17 − 0.21M ⊙ after selection effects included. We estimated the number of EL CVn stars with P ≤ 2.2 d to be 2 − 5 × 10 6 in the Galaxy, indicating a local density of 4 − 10 × 10 −6 pc −3 . We conclude that many more EL CVntype binaries remain to be discovered and that these binaries will be predominantly low-mass systems in old stellar populations.

Formation of Extremely Low-mass White Dwarfs in Double Degenerates

“…(2) The CO WD + MS system evolves into a DD binary with an ELM WD through the RL channel or CE channel. RL channel : The secondary (the MS star in the system) fills its Roche lobe in the late MS or during the Hertzsprung gap (HG; beyond but very close to the bifurcation point in low-mass binary evolution; see Chen et al 2017), starting to transfer mass to the CO WD. The MT process is stable (depending on binary parameters; see Section 5.1), and a proto-He WD is formed after the termination of MT.…”

“…For systems with initial orbital periods P orb,i < P b , the donors evolve to smaller masses and luminosities and cannot develop a compact core. So the ELM WDs can be formed from the RL channel only when the initial period is longer than P b (see Chen et al 2017 for more details). Meanwhile, the exact value of P b changes with the assumptions of binary evolution.…”

“…For each observed ELM WD in DD, we then 5 According to Figure 7 (Section 5.1.3) shown below, for donor mass less than 1.5 M , the envelope mass changes little with He WD mass, which indicates that the choice of donor mass has little effect on our results. 6 The contribution of internal energy is small when the giant is near the base of giant branch (Han et al 1994;Chen et al 2017), and is not considered here. The values of α0 are obtained by assuming that all of the observed systems are produced from the CE channel; see the text for the detailed calculation methods.…”

Pulsation in faint blue stars

“…Both groups may have some connection to BLAPs and both contain pulsating variables; Jeffery & Saio (2013) argue that reduced hydrogen abundance in the envelope led to helium being the principle opacity driver. The evolution of EL CVn systems was examined by Chen et al (2017).…”

Gravitational-wave Radiation of Double Degenerates with Extremely Low-mass White Dwarf Companions

“…In comparison to relatively massive WDs, ELM WDs have much thicker H-rich envelopes and longer timescales in proto stage 3 (after the stripped of envelope and before the extinct of shell burning). This stage is the most luminous stage except for flashes and the timescales for this phase is proportional to M −8 WD , where M WD is the mass of proto-He WD (Chen et al 2017). In addition, when the WDs enter into the cooling stage, the radii of massive WDs are smaller than that of low-mass WDs with a same effective temperature (Romero et al 2019).…”