W UMa type binaries have two defining characteristics. These are (i) the effective temperatures of both components are very similar, and (ii) the secondary (currently less massive) component is overluminous for its current mass. We consider the latter to be an indication of its mass before the mass-transfer event. For these stars, we define a mass difference (δM ) between the mass determined from its luminosity and the present mass determined from fitting the binary orbit. We compare the observed values of the mass difference to stellar models with mass-loss. The range of initial secondary masses that we find for observed W UMa type binaries is 1.3 2.6M⊙. We discover that the A- and the W-subtype contact binaries have different ranges of initial secondary masses. Binary syste ms with an initial mass higher than 1.8 ± 0.1M⊙ become A-subtype while systems with initial masses lower than this become W-subtype. Only 6 per cent of systems violate this behavior. We also obtain the initial masses of the primaries using the following constraint for the reci procal of the initial mass ratio: 0 < 1/qi < 1. The range of initial masses we find for the primaries is 0.2 1.5M⊙, except for two systems. Finally in comparing our models to observed systems, we find evidence that the mass transfer process is not conservative. We find that on ly 34 per cent of the mass from the secondary is transferred to the primary. The remainder is lost from the system.

On the origin of W UMa type contact binaries – a new method for computation of initial masses

We present a detailed light-curve analysis of publicly available V-band observations of 62 binary stars, mostly contact binaries, obtained by the All Sky Automated Survey (ASAS)-3 project between 2000 and 2009. Eclipsing binaries are important astronomical targets for determining the physical parameters of component stars from the geometry of their orbits. They provide an independent direct method of measuring the radii of stars. We improved the ASAS determined periods and ephemerides, and obtained the Fourier parameters from the phased light curves of these 62 stars. These Fourier parameters were used for preliminary classification of the stars in our sample. The phased light curves were then analysed with the aid of the Wilson-Devinney light-curve modelling technique in order to obtain various geometrical and physical parameters of these binaries. The spectroscopic mass ratios as determined from the radial velocity measurements available in the literature were used as one of the inputs to the light-curve modelling. Thus reliable estimations of parameters of these binaries were obtained with combined photometric and spectroscopic data, and error estimates were made using the heuristic scan method. For several systems in the sample, the solutions were obtained for the first time and would serve as a good source in the future for light-curve analysis based on more precise follow-up CCD photometric observations. Out of 62 stars in the sample, photometric analysis of 39 stars is presented here for the first time using the ASAS photometry and precise spectroscopic mass ratios. From the analysis, we found 54 contact binaries, six semidetached binaries and two detached binaries. The Fourier parameters in the a 2 ―a 4 plane were used for preliminary classification, and the final classification was done based on the Roche lobe geometry obtained from the light-curve modelling.

Physical parameters of 62 eclipsing binary stars using the All Sky Automated Survey‐3 data − I

We present results of the modelling of multicolour light curves of 10 contact binary systems: V376 And, V523 Cas, CC Com, BX Dra, FG Hya, UZ Leo, XY Leo, AM Leo, EX Leo and RT LMi. The solutions resulted in a contact configuration for all systems. We found only FG Hya and UZ Leo to be in deep contact, the latter almost filling the outer critical lobe. The absolute parameters of the components have been determined with an accuracy of about a few per cent based on combined photometric and radial velocity curves, enlarging the sample of systems to 58 for which the physical parameters have been obtained in a uniform way. All but three systems (BX Dra, AM Leo and RT LMi) show asymmetries and peculiarities in the observed light curves, interpreted as resulting from their magnetic activity.

Physical parameters of components in close binary systems – VII

Various scenarios of contact binary evolution have been proposed in the past, giving hints of (sometimes contradictory) evolutionary sequences connecting A- and W-type systems. As the components of close detached binaries approach each other and contact binaries are formed, following evolutionary paths transforms them into systems of two categories: A-type and W-type. The systems evolve in a similar way but under slightly different circumstances. The mass/energy transfer rate is different, leading to quite different evolutionary results. An alternative scenario of evolution in contact is presented and discussed, based on the observational data of over one hundred low-temperature contact binaries. It results from the observed correlations among contact binary physical and orbital parameters. Theoretical tracks are computed assuming angular momentum loss from a system via stellar wind, accompanied by mass transfer from an advanced evolutionary secondary to the main-sequence primary. A good agreement is seen between the tracks and the observed graphs. Independently of details of the evolution in contact and a relation between A- and W-type systems, the ultimate fate of contact binaries involves the coalescence of both components into a single fast rotating star.

/pdf/angular-momentum-and-mass-evolution-of-contact-binaries-11x6utr5j1.pdf

Angular momentum and mass evolution of contact binaries

Well-determined physical parameters of 130 W Ursae Majoris (W UMa) systems were collected from the literature. Based on these data, the evolutionary status and dynamical evolution of W UMa systems are investigated. It is found that there is no evolutionary difference between W- and A-type systems in the M-J diagram, which is consistent with the results derived from the analysis of observed spectral type and of M-R and M-L diagrams of W UMa systems. M-R and M-L diagrams of W- and A-type systems indicate that a large amount of energy should be transferred from the more massive to the less massive component, so that they are not in thermal equilibrium and undergo thermal relaxation oscillation. Moreover, the distribution of angular momentum, together with the distribution of the mass ratio, suggests that the mass ratio of the observed W UMa systems decreases with decreasing total mass. This could be the result of the dynamical evolution of W UMa systems, which suffer angular momentum loss and mass loss as a result of the magnetic stellar wind. Consequently, the tidal instability forces these systems towards lower q values and finally to rapidly rotating single stars.

/pdf/the-evolutionary-status-of-w-ursae-majoris-type-systems-5ambsjf7pk.pdf

The evolutionary status of W Ursae Majoris-type systems

We present results of modelling of multicolour light curves of ten contact binary systems: V376 And, V523 Cas, CC Com, BX Dra, FG Hya, UZ Leo, XY Leo, AM Leo, EX Leo, and RT LMi. The solutions resulted in a contact configuration for all systems. We found only FG Hya and UZ Leo to be in deep contact, the latter almost filling the outer critical lobe. The absolute parameters of the components have been determined with an accuracy of about w few percent based on combined photometric and radial velocity curves, enlarging to 58 the sample of systems for which the physical parameters have been obtained in a uniform way. All but three systems (BX Dra, AM Leo and RT LMi) show asymmetries and peculiarities in the observed light curves, interpreted as resulting from their magnetic activity.

http://inspirehep.net/record/814966/files/arXiv:0903.1364.pdf

Physical parameters of close binary systems: VI

We present the results of a statistical investigation of the period-color and period-bolometric magnitude relations using a carefully selected sample of 120 contact systems with known physical parameters

S. Zola

Papers

Physical parameters of close binary systems: VI

The W UMa-type Stars Program: First Results, Current Status and Perspectives

Monitoring spots via the TESS data strengthened by the ETV behaviour: An investigation on the contact binary PP Lac