A mass transfer origin for blue stragglers in NGC 188 as revealed by half-solar-mass companions

• Published in: Nature (London) 2011-10, Vol.478 (7369), p.356-359
• Main Authors: Geller, Aaron M., Mathieu, Robert D.
• Format: Article
• Language: English
• Subjects: 639/33/34/863; Acquisitions; Astronomy; Binary stars; Collisions; Earth, ocean, space; Exact sciences and technology; Faint blue stars (including blue stragglers), white dwarfs, degenerate stars, nuclei of planetary nebulae; Humanities and Social Sciences; letter; Mass distribution; Mass transfer; Masses; multidisciplinary; Normal stars (by class): general or individual; Observations; Open clusters; Open clusters in the milky way; Origins; Science; Science (multidisciplinary); Space telescopes; Stars; Stars & galaxies; Stellar characteristics and properties; Stellar clusters and associations; Stellar systems. Galactic and extragalactic objects and systems. The universe; Studies
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature10512

Companions sustained blue stragglers Blue straggler stars challenge the standard theory of stellar evolution, as these main sequence stars are brighter and bluer than others in a cluster thought to have formed at about the same time. In theory, they should have already evolved into giants and stellar remnants. Explanations offered to account for these stragglers include stellar collisions, mass transfer from a companion star or mergers in binaries. Aaron Geller and Robert Mathieu have combined precise observations spanning more than a decade with numerical simulations to show that the binary companions of the majority of the blue stragglers in the old open cluster NGC 188 are consistent with a mass transfer origin, and inconsistent with predictions of the other suggested formation channels. In open star clusters, where all members formed at about the same time, blue straggler stars are typically observed to be brighter and bluer than hydrogen-burning main-sequence stars, and therefore should already have evolved into giant stars and stellar remnants. Correlations between blue straggler frequency and cluster binary star fraction 1 , core mass 2 and radial position 3 suggest that mass transfer or mergers in binary stars dominates the production of blue stragglers in open clusters. Analytic models 4 , 5 , detailed observations 6 and sophisticated N -body simulations 7 , however, argue in favour of stellar collisions. Here we report that the blue stragglers in long-period binaries in the old 8 (7 × 10 9 -year) open cluster NGC 188 have companions with masses of about half a solar mass, with a surprisingly narrow mass distribution. This conclusively rules out a collisional origin, as the collision hypothesis predicts a companion mass distribution with significantly higher masses. Mergers in hierarchical triple stars 9 are marginally permitted by the data, but the observations do not favour this hypothesis. The data are highly consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be white dwarfs of about half a solar mass.