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Blue straggler stars lie on or near the main sequences of star clusters (all members of which formed around the same time), but typically are more luminous than the turn-off stars and therefore long ago should have evolved off the main sequence to become giants and white dwarfs. They are thought to derive from normal main-sequence stars that have undergone a recent increase in mass. Statistical evidence indicates that in globular star clusters the blue stragglers probably form from binary stars1. The specific formation processes, such as mass transfer, mergers or stellar collisions during dynamical encounters of binary stars, remain unresolved. Here we report that 16 of the 21 blue stragglers (76 per cent) in the old (7-Gyr; ref. 2) open cluster NGC188 are currently in binary systems, a frequency three times that found among normal solar-type main-sequence stars. These blue straggler binaries have a remarkable period-eccentricity distribution, with all but three having orbital periods of ~1,000 days. Moreover, these stars are rotating faster than normal main-sequence stars of the same surface temperatures. These data show that most, and possibly all, blue stragglers derive from multiple-star systems, and indicate that the several formation processes operate simultaneously. We suggest that rapid rotation of blue stragglers may place upper limits on their ages.
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The colour-brightness diagram of NGC188 shows 21 blue stragglers (Fig. 1). All are considered cluster members, on the basis of having three-dimensional velocities consistent with the distinctive bulk motion of the cluster3,4. As part of the WIYN Open Cluster Study5, we have obtained highly precise (σ=0.4 km s^sup -1^) multiple radial-velocity measurements of all NGC188 proper-motion4 cluster members within a 30-arcmin (17-pc, 13-core-radii) radius of the cluster centre3.
Remarkably, 16 of the 21 blue stragglers show variability in their radial velocities, indicative of them being binary stars. So far, we have obtained orbital solutions for all but one6. These 16 blue straggler binaries represent a short-period-binary (P<3,000 d) frequency of 76±19% (1σ). This frequency is not sensitive to reasonablemodifications of our selection criteria for blue stragglers (Fig. 1); for example, the frequency among bright (V<15mag) blue stragglers is 76±21% and the frequency among those with proper-motion membership probabilities greater than 90%is 81±23%. For comparison, the binary frequency among...