Type Ia supernovae are the thermonuclear explosions of white dwarfs in binary star systems. They are one of the most common types of supernovae and have a fundamental importance as cosmological distance indicators. Despite this, the identity of the progenitors (or original form) of Type Ia supernovae has remained a mystery. Many theoretical models have arisen over the past few decades to explain how these stars explode, but until now there has been no direct confirmation that any of these scenarios actually succeeds in nature.

One model, dubbed the "dynamically driven double-degenerate double-detonation" (D6) scenario, predicts the possibility that the other star in the binary system, which is another white dwarf, might survive the explosion of its companion. Such a surviving star would be flung away from the system when the gravitational pull of its companion disappeared and would continue zipping away at speeds between 1000 - 2500 km/s.

Shen et al. (2018) searched for such hypervelocity survivors in Gaia's second data release in April 2018 and discovered three likely candidates. These stars were followed up with ground-based telescopes, including the Southern African Large Telescope, and found to possess many of the predicted features for survivors of D6 Type Ia supernovae: a lack of hydrogen and strong signatures of carbon, oxygen, and magnesium, as well as luminosities and temperatures unlike almost all other stars.  Furthermore, the past location of one of the stars is spatially coincident with a known supernova remnant, making it highly probable that it was ejected from a system that underwent a supernova.

The combination of the precise high speed motion of these hypervelocity stars (measured by Gaia) with the measurement of the radial component of the stars' motion obtained through ground-based spectroscopic observations shows that these stars are among the fastest freely moving stars in our Milky Way.

Much follow-up work remains to be done to ascertain precise characteristics of these stars and the explosions that gave birth to their hypervelocity natures. However, it is very likely that these stars represent the first discoveries of surviving companions to Type Ia supernovae, and that they confirm the success of the D6 "dynamically driven double-degenerate double-detonation" model.

Figure 2: The three hypervelocity candidates are shown here in the colour-magnitude diagram with the green, blue and orange circles. Some other white dwarfs are indicated in this diagram as well. The black circles and colored regions show the reliably measured stars from Gaia. Image credit: Shen et al. (2018)

The team undertaking the observations and data analysis was led by Ken Shen, a researcher at the University of Califonia at Berkely (USA). The SALT observations were taken by Marissa Kotze at the Southern African Astronomical Observatory as part of a program led by Saurabh Jha from Rutgers, the State University of New Jersey (USA).

The article by Shen et al. was published in the Astrophysical Journal on 20 September 2018.

The news release by SALT can be found here.

Video animation produced using Gaia Sky showing the path of the hypervelocity star candidate by Shen et al (2018), with its movement exaggerated by a factor of 300 billion times. The path of the candidate is first shown moving forward in time and subsequently going into the past, back to the supernova remnant. The circle indicates the location of the remnant of the supernova G70.0-21.5. Credits: S. Jordan, T. Sagrista

Credits: ESA/Gaia/DPAC, Ken Shen, Saurabh Jha, Stefan Jordan, Toni Sagrista

[Published: 19/11/2018]