LMC origin of the hyper-velocity star HE 0437-5439

Abstract

Context.Hyper-velocity stars move so fast that only a supermassive black hole (SMBH) seems to be capable of accelerating them. Hence the Galactic centre (GC) is the only place suggested as their origin. We discovered earlier that the object HE 0437-5439 is a hyper-velocity star. However, this early B-type star is too short-lived to have reached its current position in the Galactic halo if ejected from the GC, except if it is a blue straggler. Its proximity to the Large Magellanic Cloud (LMC) suggests an origin in this galaxy.Aims.The chemical signatures of stars at the GC are significantly different from those in the LMC. As a result, an accurate determination of the abundance pattern of HE 0437-5439 will yield a new tight constraint on the place of birth of this hyper-velocity star.Methods.High-resolution spectra obtained with UVES on the VLT were analysed using state-of-the-art NLTE modelling techniques.Results.We determined abundances of individual elements to very high accuracy in HE 0437-5439, as well as in two reference stars from the LMC and the solar neighbourhood. The abundance pattern is not consistent at all with that observed in stars near the GC, ruling out an origin from the GC. However, there is a high degree of consistency with the LMC abundance pattern. Our abundance results cannot rule out an origin in the outskirts of the Galactic disk. However, we find the lifetime of HE 0437-5439 to be more than three times shorter than the time of flight to the edge of the disk, rendering a Galactic origin unlikely.Conclusions.Only one SMBH is known to be present in the Galaxy and none in the LMC. Hence the exclusion of an GC origin challenges the SMBH paradigm. We conclude that there must be other mechanism(s) than the SMBH that accelerate stars to hyper-velocity speed. We draw attention to dynamical ejection from dense massive clusters, as recently proposed. © 2008 ESO.