A new mechanism of long-term period variations for W UMa-type contact binaries

Abstract

W UMa-type contact binaries belong to close binary systems whose components exactly overflow their Roche lobes and share a common convective envelope (CCE). In the last twenty years, the long-term variations of their orbital periods have been thought to depend on several mechanisms. Now, we suggest a new mechanism: CCEdominated mechanism. The CCE-dominated mechanism is found based on our numerical result, especially at high mass ratios, that the orbital periods (P ) of contact binaries change very much with their fill-out factors (f ). Because f is taken as a measurement of the thickness of CCE, the physical cause for the variation of P is a mass transfer between CCE and components. Further, an f -dominated simplification model for this mechanism is introduced. According to it, P may change in a long-term oscillation way with a similar time scale of the thermal modulation, meanwhile q is decreasing slowly till the two components merge. It could be also applied to explain the presence of extremely short period, high mass ratio and deep contact binaries. Moreover, the CCE-dominated mechanism should always work due to mass transfer and mass loss both occurring via CCE. Therefor, the effect of CCE on the variations of orbital periods may have been underestimated before.

Timing of AR CrB eclipses

“…We conducted a timing of its eclipses using times of minima obtained from our observations (see Table 1) combined with times of minima from the B.R.N.O. database 1 , we also calculated (see Tables 2 and 3) and used times of minima from Super-WASP light curves 2,3 (Butters et al, 2010, Paunzen et al, 2014. Our calculations show that the AR CrB orbital period can possess periodical variations that can be explained by the gravitational influence of a third companion in a highly eccentric orbit around the central binary.…”

“…For the SuperWASP data we used the "Mag2" column (this column had 1-4 points with the same HJD, so we averaged their magnitude with the same HJD), only symmetric light curves around minima were used to find times of minima. To estimate parameters of the binary a computer code by Kozyreva & Zakharov (2001) was used. Due to the code features we cannot estimate precisely all geometrical quantities of an EW type system, so we present here only several of them: the orbital eccentricity of the central binary is e = 0.0014 ± 0.0005 (the existence of such eccentricity also can be confirmed by the differences in the initial epochs of primary and secondary minima of 0.0003 days), the inclination of this orbit is i = 82.0 ± 4…”

“…Less favourable explanations (according to θ values) of the orbital period change (in case of the parabolic curve) in AR CrB are the mass transfer between components (that Applegate's (1992) magnetic mechanism that changes the quadrupole gravitational momentum of one of the components (or both of them).…”

The first photometric and spectroscopic analysis of the extremely low mass ratio contact binary NSVS 5029961

“…Recently, Liu et al (2018) tried to explain the relationship between q and f based on the contact binary samples from Yakut & Eggleton (2005). Combining their explanation, we assume that the extremely low-mass-ratio contact binaries with deep and shallow degree of contact have different origins.…”

Photometric study of contact binary star MW And

“…MW And contains star spot on each star so this further supports mass exchange or loss. According to the popular view of the stellar evolution and structure of contact binary stars contact system evolve from detached systems to semi-detached and then finally to contact state (de Loore & Doom 1992;Sahade et al 1993;Liu et al 2018;Goderya et al 1996). Mass transfer can occur during the core H-burning phase (Case A type mass transfer) or during the shell H-burning phase (Case B type mass transfer).…”