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Abstract: Young stars observed in the distant Galactic halo are usually thought to haveformed 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 timefrom the Galactic disk-center. We suggest that such stars have evolved in closerunaway or hypervelocity binaries. Stellar evolution of such binaries can drivethem into mass transfer configurations and even mergers. Such evolution couldthen rejuvenate them e.g. blue stragglers and extend their lifetime aftertheir ejection. The extended lifetimes of such stars could then be reconciledwith their flight times to the Galactic halo. We study the possibilities ofbinary runaway and hypervelocity stars and show that such binaries could havebeen ejected in triple disruptions and other dynamical interactions with starsor with massive black holes. We show that currently observed -too young- starin the halo could have been ejected from the Galactic disk or the Galacticcenter and be observable in their current position if they were ejected asbinaries. Specifically it is shown that the hypervelocity star HE 0437-5439could be such a rejuvenated star. Other suggestions for its ejection from theLMC are found to be highly unlikely. Moreover, it is shown that its observedmetallicity is most consistent with a Galactic origin and a Galactic centerorigin can not currently rule out. In addition, we suggest that tripledisruptions by the massive black hole in the Galactic center could also capturebinaries in close orbits near the MBH, some of which may later evolve to becomemore massive rejuvenated stars.

Author: Hagai B. Perets



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