Model computations of blue stragglers and W UMa-type stars in globular clusters

Abstract

It was recently demonstrated that contact binaries occur in globular clusters (GCs) only immediately below turn-off point and in the region of blue straggler stars (BSs). In addition, observations indicate that at least a significant fraction of BSs in these clusters was formed by the binary mass-transfer mechanism. The aim of our present investigation is to obtain and analyze a set of evolutionary models of cool, close detached binaries with a low metal abundance, which are characteristic of GC. We computed the evolution of 975 models of initially detached, cool close binaries with different initial parameters. The models include mass exchange between components as well as mass and angular momentum loss due to the magnetized winds for very low-metallicity binaries with Z = 0.001. The models are interpreted in the context of existing data on contact binary and blue straggler members of GCs. The model parameters agree well with the observed positions of the GC contact binaries in the Hertzsprung-Russell diagram. Contact binaries in the lower part of the cluster main sequence are absent because there are no binaries with initial orbital periods shorter than 1.5 d. Contact binaries end their evolution as mergers that appear in the BS region. Binary-formed BSs populate the whole observed BS region in a GC, but a gap is visible between low-mass mergers that are concentrated along the zero-age main sequence and binary BSs occupying the red part of the BS region. Very few binary mergers are expected to rotate rapidly and/or possess chemical peculiarities resulting from the exposure of the layers processed by CNO nuclear reactions. All other binary mergers are indistinguishable from the collisionally formed mergers. The results show that binary-formed BSs may constitute at least a substantial fraction of all BSs in a GC.

Accepted for publication in A&A

References in corpus (14)

• Tertiary companions to close spectroscopic binaries
• Mergers and Obliquities in Stellar Triples
• Dynamical age differences among coeval star clusters as revealed by blue stragglers
• Formation Scenario for Wide and Close Binary Systems
• Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process
• Contact binaries with additional components. III. The adaptive optics detections
• RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system
• The short period end of the contact binary period distribution based on the All Sky Automated Survey (ASAS)
• Angular momentum and mass evolution of contact binaries
• The blue stragglers formed via mass transfer in old open clusters
• Ecology of Blue Straggler Stars
• The Clusters AgeS Experiment (CASE). II. The Eclipsing Blue Straggler OGLEGC-228 in the Globular Cluster 47 Tuc
• A model of AW UMa
• Blue Straggler Stars in Globular Clusters: a powerful tool to probe the internal dynamical evolution of stellar systems

Cited by in corpus (5)

• New low mass ratio contact binaries in the Catalina Sky Survey
• Contribution of primordial binary evolution to the two blue-straggler sequences in globular cluster M30
• Zwicky Transient Facility and Globular Clusters: the Period-Luminosity and Period-Luminosity-Color Relations for Late-Type Contact Binaries
• Evolutionary history of four binary blue stragglers from the globular clusters ωCen, M55, 47 Tuc and NGC 6752
• Photometric analysis of three totally eclipsing W UMa stars with increasing periods: TYC 3700-1384-1, V1511 Her and V1179 Her