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

Abstract

Context. 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. Aims. 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. Methods. 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 models are based on our recently developed code for evolutionary modeling of cool close and contact binaries. Results. The model parameters agree well with the observed positions of the GC contact binaries in the Hertzsprung-Russell diagram, and we achieve detailed parameters of several individual contact and near-contact binaries. 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.

Evolutionary history of four binary blue stragglers from the globular clustersωCen, M 55, 47 Tuc, and NGC 6752

“…Among models calculated in that paper, the one with P init = 1.9 d and component masses (M 1 , M 2 ) = (0.89, 0.80) M ⊙ reproduced the observed parameters of V60 fairly well. However, it could not be regarded as a solution to the present problem, as its metallicity did not conform to the limits given in Table 2, and, moreover, the whole BS population of Stȩpień & Kiraga (2015) was generated without paying attention to the age of the models. Thus, a standard search of the parameter space was necessary.…”

“…The present computations follow those described in Stȩpień & Kiraga (2015), so the reader is referred to that paper for additional details, assumptions etc. However, a couple of modifications were introduced to increase accuracy and computational efficiency of the code.…”

“…The following search for progenitors of the investigated binaries is based on a model developed by one of us (Stȩpień 2006a;Gazeas & Stȩpień 2008;Stȩpień 2009;Stȩpień & Kiraga 2015). It describes the evolution of a cool close binary from the zero-age main sequence (ZAMS) until a stage preceding the merger of the components.…”

“…However, a couple of modifications were introduced to increase accuracy and computational efficiency of the code. In particular, upon approaching the final evolutionary state the time step was substantially shortened to avoid nonphysical period oscillations observed in some cases by Stȩpień & Kiraga (2015). Instead of Padova models (Girardi et al 2000), the newer PARSEC models 2 (Bressan et al 2012) were used, which include up-to-date physics and cover a broader range of metallicities.…”

“…This binary was already used by Stȩpień & Kiraga (2015) to demonstrate the potential of the method outlined in Section 3 in identifying progenitors of close binaries in GCs. Among models calculated in that paper, the one with P init = 1.9 d and component masses (M 1 , M 2 ) = (0.89, 0.80) M ⊙ reproduced the observed parameters of V60 fairly well.…”