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Abstract: Blue straggler, which are stars that appear to be younger than they shouldbe, are an important population of unusual stars in both stellar clusters andthe halo field of the Galaxy. Most formation scenarios evoke either stellarcollisions or binary stars that transfer mass or merge. We investigatehigh-velocity stars in the Galactic halo and perform a spectral and kinematicalanalysis to shed light on their nature and origin. Here we report thatSDSSJ130005.62+042201.6 J1300+0422 for short is an A-type star of unusuallylarge radial velocity 504.6 $\pm$ 5 \kms. From a quantitative NLTE and LTEspectral analysis of medium-resolution optical spectra, the elementalcomposition is derived. Proper motion measurements combined with aspectroscopic distance estimate allow us to determine its present spacevelocity. Its kinematical properties are derived by integrating the equation ofmotion in the Galactic potential. We find J1300+0422 to be metal poorM-H=$-1.2$ and exhibit an $\alpha$-element enrichment $0.3-0.4$~dex thatis characteristic of the halo population, as confirmed by a kinematicalanalysis of its 3D space motions, which places it on a highly eccentricretrograde Galactic orbit. The mass of J1300+0422 1.15 $\pm$ 0.10 M$ \odot$is higher than the globular cluster turn-off masses indicating that it is ahalo blue straggler star. At a Galactic rest-frame velocity of$\approx$467~\kms, the star travels faster than any known blue straggler but isstill bound to the Galaxy.

Author: A. Tillich, N. Przybilla, R.-D. Scholz, U. Heber

Source: https://arxiv.org/

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