HE 0437-5439: An Unbound Hypervelocity Main-Sequence B-Type Star

Abstract

We report the discovery of a 16 th magnitude star, HE 0437−5439, with a heliocentric radial velocity of +723 ± 3 km s −1 . A quantitative spectral analysis of high-resolution optical spectra obtained with the VLT and the UVES * Based on observations collected at the European Southern Observatory, La Silla and Paranal, Chile (Proposal No. 68.D-0192 and 70.D-0334). spectrograph shows that HE 0437−5439 is a main sequence B-type star with T eff =20 350 K, log(g) = 3.77, solar within a factor of a few helium abundance and metal content, rotating at v sin(i) = 54 km s −1 . Using appropriate evolutionary tracks we derive a mass of 8 M ⊙ and a corresponding distance of 61 kpc. Its galactic rest frame velocity is at least 563 km s −1 , almost twice the local Galactic escape velocity, indicating that the star is unbound to the Galaxy. Numerical kinematical experiments are carried out to constrain its place of birth. It has been suggested that such hyper-velocity stars can be formed by the tidal disruption of a binary through interaction with the super-massive black hole at the Galactic center (GC). HE 0437−5439 needs about 100 Myrs to travel from the GC to its present position, much longer than its main sequence lifetime of 25 Myrs. This can only be reconciled if HE 0437−5439 is a blue straggler star. In this case, the predicted proper motion is so small that it can only be measured by future space missions. Since the star is much closer to the Large Magellanic Cloud (LMC, 18 kpc) than to the GC, it can reach its position from the center of the LMC. The proper motion predicted in this case is about 2 mas yr −1 (relative to the LMC), large enough to be measurable with conventional techniques from the ground. The LMC origin could also be tested by a high-precision abundance analysis.

Kinematics in galactic tidal tails

“…If we select only stars within the overdensity region defined by Abadi et al (2009) the spread reduces to ∼250 km s −1 resulting in a minimum progenitor mass still higher than 10 10 M . Concerning the known satellite galaxies the respective authors of the HVS discovery papers have already excluded a kinematical connection to them (Brown et al 2005(Brown et al , 2006a(Brown et al , 2009aEdelmann et al 2005;Hirsch et al 2005). Since we do not expect such a system to have escaped observations to date, we conclude that a satellite origin for the subsample to be unlikely.…”

“…Recent observations of 17 stars in the Galactic halo with unusually high velocities (Brown et al 2005;Hirsch et al 2005;Edelmann et al 2005;Brown et al 2006aBrown et al ,b, 2007aBrown et al ,b, 2009aTillich et al 2009) have raised new interest in the topic. By the design of the search strategy, these stars typically have blue colors.…”

Milky Way mass models for orbit calculations

“…Various trial trajectories, including those of the LSR, were successfully calculated in order to test the self-written ISIS-implementation of the adopted integrator. The hypervelocity star HE 0437-5439 (Edelmann et al 2005) is analyzed as a first and very interesting application because the spatial origin of this star is still under debate. When hypervelocity stars were first discovered in 2005 (Brown et al 2005), dynamical ejection from the supermassive black hole at the GC (Hills 1988) was supposed to be their only origin because these stars move so fast that they are unbound to the Galaxy.…”

“…When hypervelocity stars were first discovered in 2005 (Brown et al 2005), dynamical ejection from the supermassive black hole at the GC (Hills 1988) was supposed to be their only origin because these stars move so fast that they are unbound to the Galaxy. This scenario was challenged by the discovery of the third hypervelocity star HE 0437-5439 (Edelmann et al 2005), a massive B star, because the travel time from the GC to its present position would exceed its lifetime by a factor of 4. Edelmann et al (2005) notice that the star is much closer to the Large Magellanic Cloud (LMC) than to the Galaxy and might therefore originate in the LMC.…”

“…This scenario was challenged by the discovery of the third hypervelocity star HE 0437-5439 (Edelmann et al 2005), a massive B star, because the travel time from the GC to its present position would exceed its lifetime by a factor of 4. Edelmann et al (2005) notice that the star is much closer to the Large Magellanic Cloud (LMC) than to the Galaxy and might therefore originate in the LMC. This was corroborated by a sophisticated differential abundance analysis of high-quality, high-resolution spectra by Przybilla et al (2008), who shows that the abundance pattern is inconsistent with that of GC stars but consistent with that of LMC B stars to within error limits.…”