Runaway and hypervelocity stars in the Galactic halo: Binary rejuvenation and triple disruption

Abstract

Young stars observed in the distant Galactic halo are usually thought to have formed elsewhere, either in the Galactic disk or perhaps the Galactic center, and subsequently ejected at high velocities to their current position. However, some of these stars have apparent lifetimes shorter the required flight time from the Galactic disk/center. We suggest that such stars have evolved in close runaway or hypervelocity binaries. Stellar evolution of such binaries can drive them into mass transfer configurations and even mergers. Such evolution could then rejuvenate them (e.g. blue stragglers) and extend their lifetime after their ejection. The extended lifetimes of such stars could then be reconciled with their flight times to the Galactic halo. We study the possibilities of binary runaway and hypervelocity stars and show that such binaries could have been ejected in triple disruptions and other dynamical interactions with stars or with massive black holes. We show that currently observed "too young" star in the halo could have been ejected from the Galactic disk or the Galactic center and be observable in their current position if they were ejected as binaries. Specifically it is shown that the hypervelocity star HE 0437-5439 could be such a rejuvenated star. Other suggestions for its ejection from the LMC are found to be highly unlikely. Moreover, it is shown that its observed metallicity is most consistent with a Galactic origin and a Galactic center origin can not currently rule out. In addition, we suggest that triple disruptions by the massive black hole in the Galactic center could also capture binaries in close orbits near the MBH, some of which may later evolve to become more massive rejuvenated stars.

11 pages, 2 figures, 2 tables. Improved analysis. ApJ, in press

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