CoBiToM Project

Publications

CoBiToM paper I

Contact binaries towards merging

K. Gazeas, G. Loukaidou, P. Niarchos, S. Palafouta, D. Athanasopoulos, A. Liakos, S. Zola, A. Essam and P. Hakala

ABSTRACT

Binary and multiple stellar systems are numerous in our solar neighbourhood 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 multimethod 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.

Reference: Gazeas et al., 2021, MNRAS, 502, 2879

CoBiToM paper II

Evolution of contact binary systems close to the orbital period cut-off

G. Loukaidou, K. Gazeas, S. Palafouta, D. Athanasopoulos, S. Zola, A. Liakos, P. Niarchos, P. Hakala, A. Essam and D. Hatzidimitriou

ABSTRACT

Ultra-short orbital period contact binaries (P < 0.26 d) host some of the smallest and less massive stars. These systems are faint and rare, and it is believed that they have reached a contact configuration after several Gyrs of evolution via angular momentum loss, mass transfer and mass loss through stellar wind processes. This study is conducted in the frame of Contact Binaries Towards Merging (CoBiToM) Project and presents the results from light curve and orbital analysis of 30 ultra-short orbital period contact binaries with the aim to investigate the possibility of them being red nova progenitors eventually producing merger events. Approximately half of the systems exhibit orbital period modulations, as a result of mass transfer or mass loss processes. Although in contact, their fill-out factor is low (less than 30 per cent), while their mass ratio is larger than longer period contact binaries. The present study investigates the orbital stability of these systems, with the aid of correlation diagrams and examines their physical and orbital parameters in comparison to those of the entire sample of known and well-studied contact binaries, based on a combined spectroscopic and photometric analysis. It is found that ultra-short orbital period contact binaries have very stable orbits, while very often additional components move in wide orbits around the central binary system. We confirmed that the evolution of such systems appears to be very slow, which explains why the components of ultra-short orbital period systems still exist as main sequence stars after several Gyrs of evolution.

Reference: Loukaidou et al., 2022, MNRAS, 514, 5528

CoBiToM paper III

Physical characterisation of the potential merger candidates AL Lep, ASAS J082243+1927.0, and ZZ PsA

S. Palafouta, K. D. Gazeas, P. Hakala, S. Zola, A. Kafkas, and M. Garro

ABSTRACT

Context. Contact binaries (CBs) are key systems for investigating the late evolutionary stages that may lead to stellar coalescence. Systems with extremely low mass ratio and high fill-out factor are of particular interest as potential merger candidates and possible progenitors of luminous red novae.
Aims. A combined photometric and spectroscopic study of the contact binary systems AL Lep, ASAS J082243+1927.0, and ZZ PsA is presented. The aim was to derive self-consistent orbital and physical parameters, assess their evolutionary state, and examine whether they have reached the the terminal and unstable phase of their evolution.
Methods. High-resolution VLT/UVES spectra were analysed to derive radial velocity values, spectroscopic mass ratio, and metallicity. The results were combined with multi-band light-curve modelling within a Monte Carlo (MC) framework to determine the system geometries and absolute parameters. Orbital period variations were investigated through O − C analysis, while the evolutionary status of the systems was assessed using the Darwin instability criterion, critical mass ratio estimates, fill-out factor, and orbital period variation. The derived parameters were also compared with PARSEC zero-age Main Sequence (ZAMS) and terminal-age Main Sequence (TAMS) reference curves, selected according to the metallicity of each system.
Results. Mass ratio values of q = 0.1332, 0.1041, and 0.0789 were derived for AL Lep, ASAS J082243+1927.0, and ZZ PsA, respectively. The corresponding primary mass values are M1 = 1.656 ± 0.081 M⊙, 1.275 ± 0.078 M⊙, and 1.795 ± 0.111 M⊙. The location of their components show significant deviations from single-star ZAMS and TAMS reference curves, indicating strong binary interaction and energy redistribution through the common envelope. The spin-to-orbital angular momentum (AM) ratio for AL Lep and ASAS J082243+1927.0 remains below the standard Darwin instability threshold, while ZZ PsA lies close to the instability regime and may be consistent with an unstable configuration within the derived uncertainties.

Reference: Palafouta et al., 2026, A&A, ###

Data used in this work are available here.

Further dissemination

EWASS Conference (2019)

Evolutionary paths in stellar merging candidates

G. Loukaidou and K. Gazeas

ABSTRACT

This poster presents the photometric results on 10 low temperature contact binary systems. The aim of this research is to determine the orbital and physical parameters of these systems and investigate the progenitors of stellar mergers. Evolutionary status of these components is studied with the aid of correlation diagrams and by comparing their parameters with those of a uniform sample of contact binaries, obtained accurately by a combined spectroscopic and photometric method. The extracted physical parameters (mass, radius and luminosity) describe the environmental circumstances, in which stellar components exist and evolve through angular momentum loss or mass transfer processes.

Telć Conference (2019)

The population of W Ursae Majoris-type binaries in the solar neighborhood

K. Gazeas

ABSTRACT

W Ursae Majoris-type binaries belong to the old population of our Galaxy, while their metallicity is close to solar. Their physical properties, kinematics and spatial distribution re ect the properties of their stellar progenitors. This study focuses on the spatial distribution of W UMa's in our solar neighborhood within a 500 pc radius, with a combined astrometric, photometric and spectroscopic determination of their stellar parameters. The sample is carefully selected, in order to fulll certain criteria, and has well dened metallicity and distance parameters. H-R diagram, as well as similar correlation plots (mass-radius and mass-luminosity), show that the primary (more massive) components in such systems are located close or below the ZAMS region, while secondary components seem to be evolved, as a result of their common envelope geometry. Some prominent outliers are carefully examined in order to judge the environmental properties and evolution in certain locations of the Milky Way. It is found that metallicity is not correlated with distance, but there is a weak correlation between metallicity [M/H] and evolution state, as it is expressed by the location of the systems in the H-R diagram, the type of binary (A or W), and temperature.

MW-Gaia Workshop (2021)

CoBiToM Project - Contact Binaries Towards Merging

K. Gazeas, G. Loukaidou, P. Niarchos, S. Palafouta, D. Athanasopoulos, A. Liakos S. Zola A. Essam and P. Hakala

ABSTRACT

Contact Binaries Towards Merging (CoBiToM) Project 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. Observational data of approximately 100 eclipsing binaries and multiple systems and more than 400 archival systems will be obtained. Additional photometric, spectroscopic and astrometric information will be provided by Gaia mission, which run for a certain period of time or they are currently operational. 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. Further investigation will be performed upon the possibility of contact binaries to host planets, as well as the link between inflated hot Jupiters and stellar mergers. 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.