The blue stragglers formed via mass transfer in old open clusters

Abstract

Aims. In this paper, we present the simulations for the primordial blue stragglers in the old open cluster M 67 based on detailed modelling of the evolutionary processes. The principal aim is to discuss the contribution of mass transfer between the components of close binaries to the blue straggler population in M 67. Methods. First, we followed the evolution of a binary of 1.4 M +0.9 M . The synthetic evolutionary track of the binary system revealed that a primordial blue straggler had a long lifetime in the observed blue straggler region of color-magnitude diagram. Second, a grid of models for close binary systems experiencing mass exchange were computed from 1 Gyr to 6 Gyr in order to account for primordial blue-straggler formation in a time sequence. Based on such a grid, Monte-Carlo simulations were applied for the old open cluster M 67. Results. Adopting appropriate orbital parameters, 4 primordial blue stragglers were predicted by our simulations. This was consistent with the observational fact that only a few blue stragglers in M 67 were binaries with short orbital periods. An upper boundary of the primordial blue stragglers in the color-magnitude diagram (CMD) was defined and could be used to distinguish blue stragglers that were not formed via mass exchange. Using the grid of binary models, the orbital periods of the primordial BSs could be predicted. Conclusions. Compared with the observations, it is clear that the mechanism discussed in this work alone cannot fully predict the blue straggler population in M 67. There must be several other processes also involved in the formation of the observed blue stragglers in M 67.

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

“…WOCS 4348, 4540 and 5379 in NGC 188, Gosnell et al 2014). The blue-straggler binaries during the mass-transfer phase have a "low-luminosity boundary" (about 0.75 mag brighter than the ZAMS) given by Tian et al (2006), and can match the observed red-sequence blue stragglers in globular cluster M30 (Ferraro et al 2009;Xin et al 2015). However, for the blue-straggler binaries that have finished mass transfer, including a blue straggler and a white dwarf (the BS-WD binaries), their locations in the color-magnitude diagram of M30 depend on the contribution of white dwarfs to the combined magnitudes of these binaries.…”

“…Hence, the formation of both sequences may be related to the binary evolution. Meanwhile, Lu et al (2010) found that some blue stragglers produced by Case B binary evolution are below the low-luminosity boundary given by Tian et al (2006). Chen & Han (2008a) show that binary merger can produce single blue stragglers very close to or even below the zero-age main sequence (ZAMS), i.e.…”

“…Sills & Bailyn 1999;Sills et al 2000;Xin et al 2007;Tian et al 2006;Chen & Han 2008a,b, 2009Lu et al 2010;Leigh et al 2013). It is generally believed that binary evolution plays an important role in open clusters and in the field, while direct stellar collision are likely important in dense environments such as globular clusters or the core of open clusters (Hills & Day 1976;Sills et al 2002;Glebbeek et al 2008;Mathieu & Geller 2009;Geller & Mathieu 2011).…”

Origin of the early-type R stars: a binary-merger solution to a century-old problem?

“…Recently, models of MS+WD blue straggler systems were constructed by Tian et al (2006) but they cannot take their calculations through the final common envelope phase. Regarding the MS+WD phase, however, they find quite similar results to ours despite their use of a full stellar evolution rather than synthetic code.…”

Close binary evolution and blue straggler formation

“…Blue stragglers formed via mass transfer in old open clusters M67 has been studied (Tian et al 2006). So we focus on case A binaries in young clusters.…”