Hyper-velocity star challenges the Galactic Centre scenario

A team of astronomers led by Ulrich Heber, Norbert Przybilla (University of Erlangen-Nuremberg) and Fernanda Nieva (Max Planck Institute for Astrophysics), has discovered the first hyper-velocity star ejected from the outer rim of the Milky Way's disk with signatures of chemical enrichment from a close supernova explosion: the first hyper-runaway star?

Fig. 1: The young star, now in the Galactic halo, was ejected from the outer rim of the Galactic disk (Heber et al., 2008, A&A, 483, L21), instead of the Galactic Centre, as expected. The image represents the Milky Way.
Original photo in Fig. 1 courtesy of ESO.

Fig. 2: Chemical abundance pattern of HD 271791 shows pollution by a supernova/hypernova explosion (filled circles; abundance relative to solar as a function of atomic number Z, normalized to Fe; open squares yields of Nomoto et al., 2006, NuPhA, 777, 424).

Fig. 3: Proposed binary ejection scenario.
a) Initial system: wide binary, M1 > 55 solar masses,
M2 = 11 solar masses (HD 271791)
b) Fast expansion of the primary near end of H-burning: common-envelope phase with spiral-in of the secondary; dissipation of the envelope.
c) Short-period binary, M1 = 20 solar masses (Wolf-Rayet WC star with strong stellar wind), M2 = 11 solar masses
d) Asymetric hypernova/supernova explosion of primary, formation of a black hole
e) Disruption of the system: HD 271791 is accelerated to beyond the Galactic escape velocity.

We acknowledge Mrs. Mayr-Ihbe (MPE/MPA) for modifications of original figures 1 and 3.

Using the ESO 2.2m telescope, the team has recorded a massive star (11 times as massive as the Sun) moving at 2.2 million kilometres per hour. The gravitational pull of the Milky Way is too small to keep the star bound to it. Hence the star will escape into intergalactic space. Such objects have first been discovered in 2005 and systematic surveys have identified another 16 such so-called hyper-velocity stars in the meantime. It is general consensus that it requires a super-massive black hole to accelerate stars to such an extraordinary speed. As the Galactic centre is known to host such a black hole, it is generally believed that the hyper-velocity stars come from the very centre of the Milky Way. HD 271791, as the new hyper-velocity star is called, is different.

It was recently shown from kinematic measurements that HD 271791 can not originate from the Galactic centre. Quite to the opposite it comes from the outer rim of the Galaxy where no massive black hole is known to lurk. Hence, there must be another mechanism that is powerful enough to eject a star into intergalactic space. The team proceeded with a meticulous study of the data following their methodology developed for normal young early-B stars that allow very precise chemical abundances to be derived linkPfeil.gif[1]. The hypervelocity star's chemical composition show small but significant deviations from normal. The fingerprints point to a violent death: elements that are formed when very massive stars die in a supernova explosion are overabundant in HD 271791 linkPfeil.gif[2].

The authors investigated a scenario in which the hyper-velocity star originally orbited an even more massive star linkPfeil.gif[2]. This primary's life was very short and ended with a supernova explosion. The binary was disrupted and the companion (HD 271791) released at its orbital speed. Supernova ejecta fell onto the companion and polluted its atmosphere with freshly produced elements.

Up to now such a scenario has been dismissed for hyper-velocity stars, because the supernova precursor usually is a super-giant star and any companion has to be at large distance in order to orbit the star. Hence the orbital velocities are fairly modest. The most massive stars in the Galaxy, however, end their lives as quite compact so-called Wolf-Rayet stars rather than as super-giants. The compactness of the primary leaves room for a companion to move rapidly on a close orbit of about 1 day-period. When the Wolf-Rayet-star exploded its companion HD 271791 was released at very high speed. In addition, HD 271791 made use of the Milky Way rotation to finally achieve escape velocity.

Hence HD 271791 is witness for the death of a very massive star (more than 55 times that of the Sun) that collapsed about 20 million years ago into a black hole. This may have been a truly spectacular event, as the collapse of a Wolf-Rayet star is not necessarily accompanied by an ordinary supernova explosion - it is also suggested as the origin of the most violent explosion in the universe, a Gamma-ray burst.

Maria Fernanda Nieva


[1] Maria Fernanda Nieva, "Quantitative Spectroscopy of OB stars.", 2007, cotutelle PhD thesis at University of Erlangen-Nuremberg (Germany) and Observatorio Nacional (Brazil), Verkannten Verlag (Berlin), ISBN: 978-3-00-023064-6

[2] Norbert Przybilla, Maria Fernanda Nieva, Ulrich Heber and Keith Butler, "HD 271791: An Extreme Supernova Runaway B Star Escaping from the Galaxy", 2008, Astrophysical Journal Letters, 684, L103