A progenitor binary and an ejected mass donor remnant of faint type Ia supernovae

Abstract

Type Ia supernovae (SN Ia) are the most important standard candles for measuring the expansion history of the universe. The thermonuclear explosion of a white dwarf can explain their observed properties, but neither the progenitor systems nor any stellar remnants have been conclusively identified. Underluminous SN Ia have been proposed to originate from a so-called double-detonation of a white dwarf. After a critical amount of helium is deposited on the surface through accretion from a close companion, the helium is ignited causing a detonation wave that triggers the explosion of the white dwarf itself. We have discovered both shallow transits and eclipses in the tight binary system CD-30 • 11223 composed of a carbon/oxygen white dwarf and a hot helium star, allowing us to determine its component masses and fundamental parameters. In the future the system will transfer mass from the helium star to the white dwarf. Modelling this process we find that the detonation in the accreted helium layer is sufficiently strong to trigger the explosion of the core. The helium star will then be ejected at such high velocity that it will escape the Galaxy. The predicted properties of this remnant are an excellent match to the so-called hypervelocity star US 708, a hot, helium-rich star moving at more than 750 km s −1 , sufficient for it to leave the Galaxy. The identification of both progenitor and remnant provides a consistent picture of the formation and evolution of underluminous SNIa.

The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project

“…However, only two known sdB binaries with periods shorter than 0.1 d and ΔRV max ∼ 700 km s −1 have WD companions with masses exceeding 0.7 M (Geier et al 2007(Geier et al , 2013b. Hot subdwarf binaries with NS/BH companions and similar periods would have ΔRV max > 1000 km s −1 .…”

“…The photometric followup campaign of the MUCHFUSS project will be described in detail in Schaffenroth et al (in prep.). During dedicated spectroscopic MUCHFUSS follow-up runs bright sdB binary candidates were observed in a supplementary programme (Geier et al 2013b(Geier et al , 2014a. Hot subdwarfs with a high but constant RV were studied in the Hyper-MUCHFUSS project (Tillich et al 2011).…”

“…A significant shortening of the orbital period due to angular momentum lost by gravitational wave emission is only predicted for the most compact binaries with the most massive companions, which turned out to be quite rare (e.g. Geier et al 2007Geier et al , 2013b. Furthermore, the orbital evolution will always lead to shorter periods and therefore higher RV-amplitudes.…”

“…To allow the massive companion to spiral in deep enough to form such compact binaries during the common envelope phase, the red-giant progenitors of the sdB stars are predicted to have tightly bound envelopes and to be rather massive (2−3 M , Geier et al 2013b). Such stars are only found in young field populations like the Galactic thin disk and the two sdB binaries with the most massive WD companions known so far indeed belong to this population (Maxted et al 2000;Geier et al 2007Geier et al , 2013b. Figure 5 shows the distribution of spectroscopic distances for the sample.…”

“…Yoon & Langer 2004, and references therein) or a subsequent merger of the system (Tutukov et al 1981;Webbink 1984). Two sdBs with massive WD companions have been identified to be good candidates for being SN Ia progenitors (Maxted et al 2000;Geier et al 2007Geier et al , 2013b; Vennes et al 2012). Neutron star (NS) or even black hole (BH) companions are predicted by theory as well (Podsiadlowski et al 2002;Pfahl et al 2003).…”