Structure and evolution of low-mass W Ursae Majoris type systems - III. The effects of the spins of the stars

Abstract

In a previous paper, using Eggleton's stellar evolution code, we have discussed the structure and evolution of low‐mass W Ursae Majoris (W UMa) type contact binaries with angular momentum loss owing to gravitational radiation or magnetic braking. We find that gravitational radiation is almost insignificant for cyclic evolution of low‐mass W UMa type systems, and it is possible for angular momentum to be lost from W UMa systems in a magnetic stellar wind. The weaker magnetic activity shown by observations in W UMa systems is likely caused by the lower mass of the convective envelopes in these systems than in similar but non‐contact binaries. The spin angular momentum cannot be neglected at any time for W UMa type systems, especially for those with extreme mass ratios. The spin angular momenta of both components are included in this paper and they are found to have a significant influence on the cyclic evolution of W UMa systems. We investigate the influence of the energy transfer on the common convective envelopes of both components in detail. We find that the mass of the convective envelope of the primary in contact evolution is slightly more than that in poor thermal contact evolution, and that the mass of the convective envelope of the secondary in contact evolution is much less than that in poor thermal contact evolution. Meanwhile, the rate of angular momentum loss of W UMa type systems is much lower than that of poor thermal contact systems. This is indeed caused by the lower masses of the convective envelopes of the components in W UMa type systems. Although the models with angular momentum loss for W UMa systems exhibit cyclic evolution, they seem to show that a W UMa system cannot continue this type of cyclic evolution indefinitely, and it might coalesce into a fast‐rotating star after about 1200 cycles of evolution (about 7.0 × 109 yr).

Energy transfer and its effects on the secondaries in W Ursae Majoris type contact binaries

“…Although it seems probable that energy transfer occurs in the common envelope of W UMa systems, it is not clear at present where and how the transfer takes place in the common envelope. The structure and evolution of W UMa contact binaries have been investigated by several authors in recent years (Kähler 2002a,b; Li, Han & Zhang 2004, 2005; Yakut & Eggleton 2005). It is found that loss of contact is avoided if the energy transfer is assumed to be sufficiently effective (Kähler 2002a,b).…”

Structure and Evolution of W UMa-type Systems

“…Based on a rate of angular momentum loss (AML) owing to magnetic stellar winds of Hurley et al [4], we have investigated the non-conservative evolution of W UMa systems [8], and a evolutionary model is shown in Figure 2. It is seen in Figure 2 that the rate of AML in contact evolution is much lower than that in non-contact stage, which is consistent with the results predicted by observations and a polytropic model developed by Rucinski [12].…”

“…We also investigated the merger of W UMa systems with an extreme mass ratio and well-determined physical parameters recently, it is found that a large amount of mass and angular momentum should be lost from binary systems, otherwise, the rotation velocities of merged single stars would exceed their breakup velocities [6]. The time dependence of contact degree F, effective temperature T eff of the primary (solid) and the secondary (dashed), together with the ratio of spin angular momentum to orbital angular momentum J rot /J orb , the luminosity transfer rate ∆L, the total mass, M tenv , contained in convective envelopes, and rate of AML [8]. The time dependence of contact degree F, effective temperature T eff of the primary (solid) and the secondary (dashed), together with the ratio of spin angular momentum to orbital angular momentum J rot /J orb , the luminosity transfer rate ∆L, the total mass, M tenv , contained in convective envelopes, and rate of AML [8].…”

Primordial binary evolution and blue stragglers

“…This contact binary eventually coalesces into a single star (Webbink 1976;Eggleton 2000;Li et al 2005). If both components are on the main sequence, their remnant is also a main-sequence star and evolves similarly to a normal star with that mass.…”