9 Blue stragglers

One of the characteristics of runaway stars produced according to the binary-supernova scenario is that these stars are expected to be blue stragglers (Sect. 1). The mass transfer in close binary systems from the primary to the secondary prior to the supernova explosion deposits a large amount of hydrogen onto the future runaway. This new supply of fuel makes the runaway appear younger than the association/cluster in which it was born, i.e., it appears rejuvenated.

Figure 17 shows the colour vs. absolute magnitude diagrams of the parent clusters discussed in this paper. The association members (dots) have been de-reddened following the Q-method; only the early-type members (A0 and earlier) are shown. The solid lines denote the Schaller et al. (1992) isochrones for Solar metallicity and a standard mass loss rate for the ages of the associations. The runaway stars are denoted by starred symbols. A runaway can appear in more than one panel if two or more possible parents have been identified. The B-V colour and absolute magnitude have been determined using the spectral type of the runaways (Table 3; Schmidt-Kaler 1982). Three stars in Fig. 17 clearly are blue stragglers: HIP 38518, $\xi$ Per, and $\lambda$ Cep; and three others could be blue stragglers depending on the correct identification of the parent: $\zeta$ Pup, HIP 49934, and HIP 91599. The latter three stars have uncertain parent identifications (see Sects. 5 and 6). The blue straggler nature of the former three stars confirms their identification as BSS runaway (see Table 5). The star $\zeta$ Oph has also been claimed to be a blue straggler (e.g., Blaauw 1993). However, the Upper Scorpius panel shows that $\zeta$ Oph is the bluest star of the group, but it lies on the main sequence, as also found by de Geus et al. (1989) on the basis of $uvby\beta$ photometry.

By contrast to the BSS runaways, those produced by the DES are expected to follow the main sequence of the parent group. These runaways most likely did not experience a period of binary evolution in which mass transfer was important. The runaway stars which we identified securely as DES runaways (AE Aur, $\mu$ Col, HIP 22061, and HIP 29678) indeed fall on the main sequence of their parents (the Trapezium and the $\lambda$ Ori cluster).