Structure and evolution of low-mass W Ursae Majoris type systems - II. With angular momentum loss

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 without angular momentum loss (AML). The models exhibit cyclic behaviour about a state of marginal contact on a thermal time‐scale. Part of the time of each cycle is spent in contact and part in a semidetached state. According to observations, W UMa systems suffer AML. We present the models of low‐mass contact binaries with AML due to gravitational wave radiation or magnetic stellar wind (MSW). We find that gravitational radiation cannot prevent the cyclic evolution of W UMa systems, and the effect of gravitational radiation on the cyclic behaviour of contact binary evolution is almost negligible. We also find that the most likely AML mechanism for W UMa systems is magnetic braking, and that magnetic braking effects can increase the period of the cyclic evolution and shorten the fraction of the time spent in the poor thermal contact state exhibiting EB light curve. If W UMa stars do not undergo cyclic evolution, and their AML is caused simultaneously by MSW of both components, we find that the value of the parameter, λ, should be taken as about 3.8 for W UMa systems, which is larger than the largest value of similar single stars derived from observations. This indicates that the AML efficiency in W UMa systems may be lowered in comparison with non‐contact stars because of less mass contained in the convective envelopes of the components in W UMa systems or some feedback mechanism which may have an effect on W UMa systems. If W UMa systems lose their angular momentum at a constant rate, an angular momentum rate of d ln J/dt≈ 1.6 × 10−9 yr−1 can prevent the cyclic behaviour of the model, and the model can keep in good contact with an essentially constant depth of contact.

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

“…The mass contained in the convective envelope of the primary in the good contact phase is slightly more than that in the poor thermal contact phase, and the mass contained in the convective envelope of the secondary in the good contact state is much less than that in the poor thermal contact configuration (see Figs 4c and d). This is caused by the variation of the radiative temperature gradient due to energy transfer between the two components in the common envelope (Li et al 2004b). Because part of the luminosity of the primary is transferred to the secondary in a contact system, the radiative temperature gradient of the primary increases slightly, and the increase of the radiative temperature gradient in the surface layers of the primary drives part of radiative zone to be convective.…”

“…This is caused by the increase of the radiative temperature gradient due to more luminosity loss from the primary. The mass contained in the convective envelope of the secondary in good contact evolution is much less than that in poor thermal contact evolution because of the decrease of the radiative temperature gradient in the secondary owing to more luminosity transfer from the primary to the secondary (Li et al 2004b) in good contact state. Meanwhile, it is seen in Fig.…”

TYC 2675-663-1: a newly discovered W UMa system in an active state

“…In these objects the energy transfer rate appears to be less efficient than in other types of contact binaries. They also show excess angular momentum, which can be understood as resulting from the first stages of the interaction between the components (see Li et al 2004, and references therein). The overall properties of TYC 2675-663-1 suggest a system at or near contact and perhaps in the early stages of evolution toward a state of full geometrical and thermal contact (coalescence).…”

A test of an early-type contact binary model by comparing the relations obtained from theory with those from observations

“…A great part of observed contact binaries are low-mass binaries (W UMa-type systems). The study of the structure and evolution of low-mass contact binary systems has attracted much attention during the last decades [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. However, a number of contact binaries with massive and middle-mass components have been found by astronomical observations in the past years.…”