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

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: sgeier@eso.org
² Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander University Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ Department of Physics, University of Warwick, Conventry CV4 7AL, UK
⁴ Key Laboratory of the Structure and Evolution of Celestial Objects, Yunnan Observatory, Chinese Academy of Sciences, 650011 Kunming, PR China
⁵ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA
⁶ The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
⁷ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁸ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁰ South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa
¹¹ Southern African Large Telescope Foundation, PO Box 9, Observatory, 7935 Cape Town, South Africa
¹² Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, Suite 102, Santa Barbara, CA 93117, USA
¹³ Department of Physics, Broida Hall, University of California, Santa Barbara, CA 93106, USA
¹⁴ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
¹⁵ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
¹⁶ Institute of Astronomy, K.U. Leuven, Celestijnenlaan 200D, 3001 Heverlee, Belgium
¹⁷ Australian Astronomical Observatory, PO Box 915, North Ryde NSW 1670, Australia
¹⁸ Centre of Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK

Received: 3 March 2013
Accepted: 16 April 2013

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.