Contribution of Primordial Binary Evolution to the Two Blue-straggler Sequences in Globular Cluster M30

Abstract

Two blue-straggler sequences discovered in globular cluster M30 provide a strong constraint on the formation mechanisms of blue stragglers. We study the formation of the blue-straggler binaries through binary evolution, and find that binary evolution can contribute to the blue stragglers in both of the sequences. Whether a blue straggler is located in the blue sequence or red sequence depends on the contribution of the mass donor to the total luminosity of the binary, which is generally observed as a single star in globular clusters. The blue stragglers in the blue sequence have a cool white-dwarf companion, while the majority (∼ 60%) of the objects in the red sequence are the binaries that are still experiencing mass transfer, but there are also some objects that the donors have just finished the mass transfer (the stripped-core stars, ∼ 10%) or the blue stragglers (the accretors) have evolved away from the blue sequence (∼ 30%). Meanwhile, W UMa contact binaries found in both sequences may be explained by various mass ratios, that is, W UMa contact binaries in the red sequence have two components with comparable masses (e.g. mass ratio q ∼ 0.3-1.0), while those in the blue sequence have low mass ratio (e.g. q < 0.3). However, the fraction of blue sequence in M30 cannot be reproduced by binary population synthesis if we assumed the initial parameters of a binary sample to be the same as that of the field. This possible indicates that dynamical effects on binary systems is very important in globular clusters.

Discovery of a Double Blue Straggler Sequence in M15: New Insight into the Core-collapse Process

“…Indeed a few W Uma Variables (which are contact binaries) have been found along both the BSS sequences in M30 (Ferraro et al 2009) and NGC 362 (Dalessandro et al 2013). Using dedicated theoretical models, Jiang et al (2017) found that the contribution of the MT-BSSs to the blue or red sequence strongly depends on the contribution of the donor to the total luminosity of the unresolved binary system. Hence our observations further support the possibility that both collisional and MT-BSSs can populate the blue sequence.…”

“…Recent Monte-Carlo simulations of synthetic MT-BSSs (Jiang et al 2017) showed that the blue-BSS sequence can be also populated by MT-BSSs. This finding by itself does not invalidate the possibility that the blue BSS sequence might be mainly populated by collisional BSSs, since the easiest way to reproduce the observed narrowness of the blue sequence and the adjacent gap is still the hypothesis that the vast majority of BSSs along this sequence formed almost simultaneously, instead as a result of a continuous process extending over a long time interval.…”

Detection of a White Dwarf Companion to a Blue Straggler Star in the Outskirts of Globular Cluster NGC 5466 with the Ultraviolet Imaging Telescope (UVIT)

“…Ferraro et al (2009) discovered two BSS sequences in the optical CMD of GC M30, suggesting that the redder ones arise from the evolution of close binaries that are still experiencing MT which was in agreement with binary evolution models. Another explanation for the two BSS sequences in M30 was given by Jiang et al (2017) where they showed that binary evolution contributes to the formation of BSS in both sequences. Thus, identification of BSSs with hot companions using UV ob-servations are crucial to understanding their formation mechanism in binary systems.…”

An empirical clock to measure the dynamical age of star clusters

“…In fact, the comparison with evolutionary models of BSSs formed by direct collisions of two MS stars (Sills et al (2009)) showed that the blue-BSS sequence is well fit by collisional isochrones with ages of ∼2 Gyr. Instead, the red-BSS population is far too red to be reproduced by collisional isochrones of any age, and it turned out to be compatible with the portion of the CMD where MT binary models are expected (Xin et al (2015), see also recent models by Jiang et al (2017) showing that MT-BSS might also "contaminate" the blue-BSS sequence). However the impressive agreement of the blue-BSS with the COLL isochrones suggests that the vast majority of these stars have a collisional origin and they are nearly co-eval possibly generated by short-lived event (the Core Collapse -CC).…”