vrotsini of the He-sdO is still slower than expected
by about a factor of four. A high loss of mass and
angular momentum caused by the SN impact is
predicted for main-sequence companions in the
standard single-degenerate SN Ia scenario, in
which mass is transferred from such a companion to a WD (29–31). Though the loss of mass and
angular momentum for a more compact companion like US 708 is expected to be smaller, our
results indicate that it is still substantial. This
puts important observational constraints on more
Whereas the observed properties of US 708 are
consistent with the SN ejection scenario, they are
hardly compatible with the slingshot mechanism
because an origin of the star in the center of the
Galaxy is very unlikely (Fig. 2; see also the additional explanation in the supplementary materials). However, it must be stated that the SN
ejection scenario is only applicable to such compact helium stars and cannot be invoked to explain the acceleration of the other HVSs.
Depending on the pollution by SN material,
the effect of the SN impact, and the subsequent
stellar evolution, the surface abundances of US
708 might be substantially affected. Ultraviolet
spectroscopy is necessary to measure the metal
abundances of US 708 and put further constraints on the extreme history of this star, which
witnessed a SN from a distance of less than 0.2
In providing evidence that US 708 is probably
the donor remnant of a helium double-detonation
SN Ia, we have shown an acceleration mecha-
nism for the fastest unbound stars in our Galaxy.
With that, we have also made an important step
forward in understanding SN Ia explosions in
general. Despite the fact that those bright events
are used as standard candles to measure the ex-
pansion (and acceleration) of the universe, their
progenitors are still unknown. Our results sug-
gest that, due to the high WD masses derived for
the progenitor binary, the double-detonation
scenario might not only be applicable to some
underluminous SN Ia (5, 21) but might also con-
tribute to the population of typical SNe Ia used
as cosmic yardsticks (20).
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We thank H. Hirsch for providing us with the Low
Resolution Imaging Spectrometer spectra. This work is based
on observations 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 W. M. Keck Observatory was made possible
by the generous financial support of the W. M. Keck
Foundation. We wish to recognize the important cultural role
and reverence that the summit of Mauna Kea has always had
within the indigenous Hawaiian community. We are most
fortunate to have the opportunity to conduct observations
from this mountain. This work is also based on observations
at the Palomar Observatory. The Pan-STARRS1 Surveys (PS1)
have been made possible through contributions from the
Institute for Astronomy, the University of Hawaii, the
Pan-STARRS Project Office, the Max-Planck Society and its
participating institutes (the Max Planck Institute for
Astronomy, Heidelberg, and the Max Planck Institute for
Extraterrestrial Physics, Garching), The Johns Hopkins University,
Durham University, the University of Edinburgh, Queen’s University
Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las
Cumbres Observatory Global Telescope Network, the National
Central University of Taiwan, the Space Telescope Science
Institute, the NASA under grant no. NNX08AR22G issued through
the Planetary Science Division of the NASA Science Mission
Directorate, the NSF under grant no. AST-1238877, the University
of Maryland, and Eotvos Lorand University (ELTE). Z.H. is
supported by the Natural Science Foundation of China (grant
nos. 11390374 and 11033008). E.Z. and A.I. are supported by the
Deutsche Forschungsgemeinschaft through grant HE1356/45-2.
T.K. acknowledges support from the Netherlands Research School
for Astronomy (NOVA). A.I. acknowledges support from a
research scholarship by the Elite Network of Bavaria. R.K.
acknowledges support from Science and Technologies Council UK
grant no. ST/L000709/1, Queen's University Belfast's contribution
to the PanSTARRS1 science consortium. K.S. acknowledges
support from European Union FP7 Programme ERC grant
no. 291222. F.F. acknowledges NASA contract no. NNG08FD60C
for the NuSTAR mission. The data observed with the SDSS and
Keck telescope are published via the SDSS and Keck data
archive; the PS1 data and catalog are available upon request.
Materials and Methods
Figs. S1 to S7
22 July 2014; accepted 29 January 2015
1128 6 MARCH 2015 • VOL 347 ISSUE 6226 sciencemag.org SCIENCE
Fig. 2. Origin of US 708. A Monte Carlo simulation (108 iterations) of
the past trajectory of US 708 is shown. The color-coded bins mark the
positions where the star crossed the Galactic disc, which is shown pole-on. The contours correspond to the 1s, 3s, and 5s confidence limits. The
position of the Galactic center is denoted by the black dot, the position
of the Sun is given by the star symbol, and the current position of US
708 is marked by a triangle and given in Table 1.
20 10 0 -10 -20
X in kpc