The fastest unbound star in our Galaxy ejected by a thermonuclear supernova

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Science  06 Mar 2015:
Vol. 347, Issue 6226, pp. 1126-1128
DOI: 10.1126/science.1259063

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Stars that blow up and bug out

When stars move at speeds that will launch them out of our Galaxy, eyes often turn to our core supermassive black hole as the slingshot responsible. For at least one hypervelocity star, however, the galactic center remains innocent. Geier et al. traced back the trajectory of a compact helium star, US 708, and deduced a different origin in a binary. In this scenario, US 708 acted as the mass donor in a type Ia supernova pair, which spun US708 to the point of ejection. By knowing this star's exotic past, we learn both about its specific history and about the nature of all type Ia supernovae.

Science, this issue p. 1126


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 thermonuclear supernova.

According to the widely accepted theory for the acceleration of hypervelocity stars (HVSs) (13), 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 [subdwarf 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 Embedded Image. 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 explain the acceleration of HVSs. Within the slingshot 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 remains 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 remnant of a thermonuclear SN Ia (18, 19). Underluminous SN Ia have been proposed to originate from a so-called “double detonation” of a white dwarf (WD) (20, 21). In this scenario, a massive WD is closely orbited by a low-mass helium star. Due to a tightening of the orbit, the helium star will start to transfer mass to its compact companion. After a critical amount of helium is deposited on the surface of the WD through accretion, the helium is ignited, causing a detonation wave that triggers the explosion of the carbon-oxygen WD itself. The ultracompact sdB+WD binary CD –30° 11223 has recently been identified as progenitor candidate for such a scenario and has been linked to the putative ejected donor remnant US 708 (5, 22).

We performed a detailed spectroscopic and kinematic analysis of US 708 based on recently obtained and archival data to trace back its origin and constrain the ejection mechanism. To determine the three-dimensional motion of US 708, both the radial and tangential velocity components have to be determined. We measured the radial velocity from new spectra taken with the Keck and Palomar telescopes and compared it with archival data. We also derived atmospheric parameters and a spectroscopic distance from the new spectra (Fig. 1). In addition, we determined the proper motion by combining archival positions with new measurements from the PanSTARRS survey (fig. S2 and Table 1).

Fig. 1 Fit of model spectrum.

The fit of synthetic models to the helium and nitrogen lines of a Keck/ESI spectrum of US 708 is shown. The normalized fluxes of the single lines are shifted by constant values c for better visualization, and the most prominent lines are labeled. The weaker lines are from N III at 4634 and 4640 Å, He I at 4713 Å (spectral region in the middle), N III at 4379 Å, and He I at 4387 Å (spectral region at the bottom). The shift with respect to the rest wavelengths Δλ (dashed vertical line) caused by the high radial velocity and the substantial broadening of the lines are clearly visible.

Table 1 Parameters of US 708.

The uncertainty of the radial velocity is the 1σ error from a χ2 fit, the uncertainties in the proper motion components have been propagated from the position errors by linear regression, and the uncertainties in the atmospheric parameters are bootstrap errors. The uncertainties of the other parameters have been propagated from the uncertainties of the input parameters. N/A, not applicable.

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With a Galactic rest-frame velocity of 1157 ± 53 km s−1, we find that US 708 is the fastest known unbound star in our Galaxy. Its current distance is 8.5 ± 1.0 kpc, and it is moving away from the Galactic plane into the halo. Tracing back its trajectory and assuming no further deviations, we deduced that it crossed the Galactic disc 14.0 ± 3.1 million years ago. In this way, an origin in the center of our Galaxy can be excluded with high confidence (Fig. 2), but the origin in the Galactic disc is fully consistent with the SN ejection scenario. In contrast to regular SN Ia, double-detonation SN Ia with hot subdwarf donors are predicted to happen in young stellar populations (5).

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 1σ, 3σ, and 5σ 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.

Both the current Galactic rest-frame velocity and the reconstructed ejection velocity from the Galactic disc (998 ± 68 km s−1) are substantially higher than the value published before (~750 km s−1, based on radial velocity alone) (2). This puts new constraints on the possible progenitor system, which can be derived from the observed parameters of US 708. To reach such a high ejection velocity, the progenitor binary must have been very compact and the WD companion rather massive. The likely progenitor system consists of a compact helium star with a mass of ~0.3 Embedded Image and a massive carbon-oxygen WD (1.0 to 1.2 Embedded Image) with an orbital period of ~10 min. We calculated the mass-transfer rate in such a binary and found that the helium is accreted by the WD at a rate suitable for the double-detonation scenario [10−9 to 10−8 Embedded Image year–1 (5)]. Such ultrashort-period systems with compact helium stars have indeed been observed. The eclipsing He-WD+CO-WD binary SDSS J065133+284423 has an orbital period of only 12 min (23). However, the mass of the CO-WD (0.55 Embedded Image) is too low for a double-detonation SN Ia.

The ejection from such a close binary should leave another imprint on the remnant. We know that hot subdwarfs in compact binaries have been spun up by the tidal influence of the close companion (22, 24, 25) to projected rotational velocities (vrotsini) that are substantially higher than the projected rotational velocities of single hot subdwarfs (26, 27). An ejected remnant is predicted to have a high Embedded Image as well (2831), and indeed we measured Embedded Image km s−1, which is substantially higher than expected for a single He-sdO (Fig. 1) (27).

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 solar radii.

In providing evidence that US 708 is probably the donor remnant of a helium double-detonation SN Ia, we have shown an acceleration mechanism 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 expansion (and acceleration) of the universe, their progenitors are still unknown. Our results suggest 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 contribute to the population of typical SNe Ia used as cosmic yardsticks (20).

Supplementary Materials

Materials and Methods

Supplementary Text

Figs. S1 to S7

References (3257)

References and Notes

  1. Acknowledgments: 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.
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