Contact Binaries: The Current State

Abstract

Lucy's model of contact binaries has been serving well since its development in 1968 as an excellent conceptual framework for internal structure and evolution studies of the W UMa type stars. Its over-constrained structural demands were recognized early, but -from the observational point of view -it was particularly appealing in its ability to explain W UMa-type binary light curves. Large numbers of photometric light curves of W UMa type binaries have been solved for geometrical parameters utilizing the clarity and simplicity of Lucy's model to the point of frequent over-interpretation of the meager lightcurve information content. Many, single, photometric solutions do not translate into a satisfactory understanding of these binaries. As the David Dunlap Observatory radial velocity program of bright, close binary stars (conducted in the last 10 years before the observatory closure in 2008) has shown, there are problems when spectroscopic results are confronted with light-curve solutions: Very frequently, the crucial parameter of the mass-ratio determined spectroscopically is entirely different from its photometric analogue (i.e. the one determined assuming a strict adherence to the Lucy's model), the third star incidence is high (which makes light curve solutions difficult, if not entirely wrong, even for totally eclipsing systems) and -last but not least -the model has serious difficulties for the best observed case of the crucially important, very low mass-ratio system AW UMa. One would argue that it is spectroscopy which gives correct results, but it requires access to large telescopes, extensive use of special techniques to analyze strongly blended and rotationally broadened spectra and is simply much harder to do than photometry. If the light curve solutions deceive us into believing in correctness of Lucy's contact-binary model and if only a fraction of (or none among) W UMa systems can be explained by that model, then the subject of contact binaries needs new ideas and new, mostly spectroscopic, high quality observations.

Phenomenological Modeling of Newly Discovered Eclipsing Binary 2MASS J18024395 + 4003309 = VSX J180243.9+400331

“…Although the errors of the phenomenological parameters are not very large (up to few percent), there may be systematic errors up to a dozen percent due to the simplicity of the model. These estimates may be used as preliminary values for further modeling using extended physical models based on the Wilson & Devinney (1971) code and it's extensions (Wilson 1979(Wilson , 1994(Wilson , 2012(Wilson , 2014Zola et al 1997Zola et al , 2010Bradstreet 2005;Kallrath et al, 2009;Linnel 2012;Reed 2012;Rucinski 2010;Prsa et al 2012). Such study of eclipsing binaries with the CBNU observations are reported by Jeong & Kim (2013); Kim & Jeong(2012); Jeong & Kim (2011) and Kim et al (2010a).…”

Mergers of equal-mass binaries with compact object companions from mass transfer in triple star systems

“…Portegies Zwart & Leigh (2019) further argued that the production of compact twin BS binaries (with mass ratios close to unity) could in fact be the dominant channel or evolutionary pathway. We note briefly here a possible connection to contact binaries, called W UMa systems, many of which are know to host BSs (Rucinski 2010). The approximate distribution of predicted orbital periods for this mechanism are shown via the dashed black histograms in Figure 7.…”

Octo-Tiger: A New, 3D Hydrodynamic Code for Stellar Mergers that uses HPX Parallelisation

“…In some cases, the transfer proceeds steadily for a long time; in others mass and energy are exchanged in a common envelope; while in the most dynamic and interesting cases the interaction leads to a merger giving birth to a single star of unusual properties that cannot be produced by the evolution of a single star. We know that all of these processes do occur in nature yielding observable outcomes such as contact binaries (Smith 1984;Rucinski 2010), helium white dwarfs (WD) (Kippenhahn et al 1967(Kippenhahn et al , 1968, R Coronae Borealis stars (Clayton 2012), Type Iax supernovae (Jha 2017;Solheim 2010), and dwarf novae (Warner 2003;Frank et al 2002). Some of the largest explosions in the universe, Type Ia supernovae, are caused by the Roche-Lobe-overflow induced merger of two WD (Hillebrandt & Niemeyer 2000).…”