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This work is based on data obtained with the NASA/ESA Hubble
Space Telescope. We thank O. Fox, W. Zheng, J. Bloom, C. Keeton,
J. Mauerhan, C. Steidel, and A. Strom for helpful discussions,
as well as the Space Telescope Science Institute (STScI) and
Director Matt Mountain for supporting our proposal for follow-up
observations. GLASS is supported by NASA through HST grant
GO-13459. Support for S.A.R. was provided by NASA through
Hubble Fellowship grant HST-HF-51312.01 awarded by STScI, which
is operated by the Association of Universities for Research in
Astronomy for NASA, under contract NAS 5-26555. Follow-up
imaging through the FrontierSN program is supported by NASA
through HST grant GO-13386. A.V.F.’s group at the University of
California Berkeley has received generous financial assistance from
the Christopher R. Redlich Fund, the TABASGO Foundation, Gary
and Cynthia Bengier, and NSF grant AST-1211916. The Dark
Cosmology Centre is funded by the Danish National Research
Foundation. Support for A.Z. was provided by NASA through
Hubble Fellowship grant HF2-51334.001-A awarded by STScI. SN
research at Rutgers University is supported in part by NSF CAREER
award AST-0847157 to S. W.J.. J.C.M. is supported by NSF grant
AST-1313484 and by NASA HST grants GO-13343 and GO-13386;
this research was carried out in part at the Jet Propulsion
Laboratory, California Institute of Technology, under a contract
with NASA. R.G. acknowledges the Centre National d’Etudes
Spatiales for financial support on the GLASS project. Some of the
data presented here were obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership
among the California Institute of Technology, the University of
California, and NASA; the observatory was made possible
by the generous financial support of the W. M. Keck Foundation.
The HST imaging data used in this paper can be obtained
from the Barbara A. Mikulski Archive for Space Telescopes at
https://archive.stsci.edu, and the Keck-I LRIS spectra can be
obtained at http://hercules.berkeley.edu/database.
Materials and Methods
Figs. S1 to S4
Tables S1 to S2
21 November 2014; accepted 10 February 2015
The fastest unbound star in
our Galaxy ejected by a
S. Geier,1,2 F. Fürst,3 E. Ziegerer,2 T. Kupfer,4 U. Heber,2 A. Irrgang,2 B. Wang,5 Z. Liu,5,6
Z. Han,5 B. Sesar,7,8 D. Levitan,7 R. Kotak,9 E. Magnier,10 K. Smith,9 W. S. Burgett,10
K. Chambers,8 H. Flewelling,8 N. Kaiser,8 R. Wainscoat,8 C. Waters10
Hypervelocity stars (HVSs) travel with velocities so high that they exceed the escape
velocity of the Galaxy. Several acceleration mechanisms have been discussed. Only
one HVS (US 708, HVS 2) is a compact helium star. Here we present a spectroscopic
and kinematic analysis of US 708. Traveling with a velocity of ~1200 kilometers per
second, it is the fastest unbound star in our Galaxy. In reconstructing its trajectory, the
Galactic center becomes very unlikely as an origin, which is hardly consistent with the
most favored ejection mechanism for the other HVSs. Furthermore, we detected that US
708 is a fast rotator. According to our binary evolution model, it was spun-up by
tidal interaction in a close binary and is likely to be the ejected donor remnant of a
According to the widely accepted theory for the acceleration of hypervelocity stars (HVSs) (1–3), a close binary is disrupted by the supermassive black hole (SMBH) in the center of our Galaxy, and one component is ejected as a HVS (4). In an alternative
scenario, US 708 was proposed to be ejected from
an ultracompact binary star by a thermonuclear
supernova type Ia (SN Ia) (5). However, previous
observational evidence was insufficient to put
firm constraints on its past evolution. Here we
show that US 708 is the fastest unbound star in
our Galaxy, provide evidence for the SN ejection
scenario, and identify a progenitor population
of SN Ia.
In contrast to all other known HVSs, US 708
has been classified as a hot subdwarf star [sub-
dwarf O- or B-type (sdO/B) star]. Those stars are
evolved, core helium-burning objects with low
masses around 0.5 times the mass of the Sun
ðM⊙Þ. About half of the sdB stars reside in close
binaries with periods ranging from ~0.1 to
~30 days (6, 7). The hot subdwarf is regarded as
the core of a former red giant star that has been
stripped of almost all of its hydrogen envelope
through interaction with a close companion star
(8, 9). However, single hot subdwarf stars like US
708 are known as well. Even in this case, binary
evolution has been proposed, as the merger of
two helium white dwarfs (He-WDs) is a possible
formation channel for those objects (10).
The hot subdwarf nature of US708 poses a
particular challenge for theories that aim to ex-
plain the acceleration of HVSs. Within the sling-
shot scenario proposed by Hills, a binary consisting
of two main-sequence stars is disrupted by the
close encounter with the SMBH in the center of
our Galaxy. While one of the components re-
mains in a bound orbit around the black hole,
the other one is ejected with high velocity (4).
This scenario explains the existence of the so-
called S-stars orbiting the SMBH in the Galactic
center and provides the most convincing evidence
for the existence of this black hole (11). It is also
consistent with the main properties of the known
HVS population consisting of young main-sequence
stars (12, 13). However, more detailed analyses of
some young HVSs challenge the Galactic center
origin (14), and most recently, a new population
of old main-sequence stars likely to be HVSs has
been discovered. Most of those objects are also
unlikely to originate from the Galactic center, but
the acceleration mechanism remains unclear (15).
In the case of the helium-rich sdO (He-sdO)
US 708, the situation is even more complicated.
In contrast to all other known HVSs, which are
normal main-sequence stars of different ages,
this star is in the phase of shell helium burning,
which lasts for only a few tens of millions of
years. More importantly, it has been formed by
close binary interaction. To accelerate a close
binary star to such high velocity, the slingshot
mechanism requires either a binary black hole
(16) or the close encounter of a hierarchical triple
system, where the distant component becomes
bound to the black hole and the two close components are ejected (17). Similar constraints apply
to the dynamical ejection out of a dense cluster,
which is the second main scenario discussed to
explain the HVSs.
Close binarity requires specific modifications
of the canonical HVS scenarios. However, it is a
necessary ingredient for an alternative scenario,
in which US 708 is explained as the ejected donor
1European Southern Observatory, Karl-Schwarzschild-Straße 2,
85748 Garching, Germany. 2Dr. Karl Remeis-Observatory and
Erlangen Centre for Astroparticle Physics, Astronomical
Institute, Friedrich-Alexander University Erlangen-Nuremberg,
Sternwartstraße 7, 96049 Bamberg, Germany. 3Space
Radiation Lab, MC 290-17 Cahill, California Institute of
Technology, 1200 East California Boulevard, Pasadena, CA
91125, USA. 4Department of Astrophysics/Institute for
Mathematics, Astrophysics and Particle Physics, Radboud
University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen,
Netherlands. 5Key Laboratory of the Structure and Evolution of
Celestial Objects, Yunnan Observatories, Chinese Academy of
Sciences, Kunming 650011, China. 6Argelander-Institut für
Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn,
Germany. 7Division of Physics, Mathematics, and Astronomy,
California Institute of Technology, 1200 East California
Boulevard, Pasadena, CA 91125, USA. 8Max-Planck-Institut für
Astronomie, Königstuhl 17, 69117, Heidelberg, Germany.
9Astrophysics Research Center, School of Mathematics and
Physics, Queen’s University Belfast, Belfast BT7 1NN, UK.
10Institute for Astronomy, University of Hawaii at Manoa,
Honolulu, HI 96822, USA.
*Corresponding author. E-mail: email@example.com