CoBiToM project – I. Contact binaries towards merging

Abstract

Binary and multiple stellar systems are numerous in our solar neighborhood with 80 per cent of the solar-type stars being members of systems with high order multiplicity. The Contact Binaries Towards Merging (CoBiToM) Project is a programme that focuses on contact binaries and multiple stellar systems, as a key for understanding stellar nature. The goal is to investigate stellar coalescence and merging processes, as the final state of stellar evolution of low-mass contact binary systems. Obtaining observational data of approximately 100 eclipsing binaries and multiple systems and more than 400 archival systems, the programme aspires to give insights for their physical and orbital parameters and their temporal variations, e.g. the orbital period modulation, spot activity etc. Gravitational phenomena in multiple-star environments will be linked with stellar evolution. A comprehensive analysis will be conducted, in order to investigate the possibility of contact binaries to host planets, as well as the link between inflated hot Jupiters and stellar mergers. The innovation of CoBiToM Project is based on a multi-method approach and a detailed investigation, that will shed light for the first time on the origin of stellar mergers and rapidly rotating stars. In this work we describe the scientific rationale, the observing facilities to be used and the methods that will be followed to achieve the goals of CoBiToM Project and we present the first results as an example of the current research on evolution of contact binary systems.

BM UMa: a middle shallow contact binary at pre-transition stage of evolution from W-type to A-type

“…After the evolution of BM UMa reaches the stage where mass ratio q close to 1 and goes further the point q < 1.0, the binary will evolve into A-subtype as a deeper normal overcontact binary where the mass transfer takes place from the less massive component (m 2 ) to the more massive one (m 1 ), similar to the present evolutionary stage of V802 Aql (P = 0.2677 d; Yang et al 2008) where its orbital period is increasing and the fill-out factor also gradually grows (f = 35 %) at the beginning stage of A-subtype before becoming a deep low-mass ratio overcontact system. By this way, the evolution of BM UMa will end as a merger or rapid-rotating single star when the mass ratio meet the critical value (q < 0.094, see Rasio 1995;Arbutina 2007;Zhu et al 2016) and the binary will produce a red nova similar to the case of the red outburst V1309 Sco (Tylenda et al 2011) and the recent study of KIC 9832227 (Gazeas et al 2021). The changing of contact type from W-type to A-type has been found in many systems e.g., EM Psc (Yang et al 2005;Qian et al 2008).…”

Space-Based Photometry of Binary Stars: From Voyager to TESS

“…NSVS 5029961 was also observed in the TESS full-frame images and these data were used by Zheng et al [318] to determine the properties and evolutionary status of the components. The CoBiToM project (Gazeas et al [319]) aims to study the merger of contact binaries using data from TESS among other ground-and space-based telescopes.…”

Modeling overcontact binaries I: The effect of tidal deformation

“…Later, over a thousand low-mass, near-contact or overcontact binaries were found in the OGLE-I survey from light curve analysis (Szymanski et al 2001). Extensive studies aim to characterize the population of W UMa binaries and identify systems that will merge (Gazeas et al 2021). Recently, more and more massive overcontact binaries are identified, the most massive of which, VFTS 352 (Almeida et al 2015), has components over 30M .…”