title: The double-degenerate, super-Chandrasekhar nucleus of the planetary nebula Henize 2-428.
abstract: The planetary nebula stage is the ultimate fate of stars with masses one to eight times that of the Sun (M(⊙)). The origin of their complex morphologies is poorly understood, although several mechanisms involving binary interaction have been proposed. In close binary systems, the orbital separation is short enough for the primary star to overfill its Roche lobe as the star expands during the asymptotic giant branch phase. The excess gas eventually forms a common envelope surrounding both stars. Drag forces then result in the envelope being ejected into a bipolar planetary nebula whose equator is coincident with the orbital plane of the system. Systems in which both stars have ejected their envelopes and are evolving towards the white dwarf stage are said to be double degenerate. Here we report that Henize 2-428 has a double-degenerate core with a combined mass of ∼1.76M(⊙), which is above the Chandrasekhar limit (the maximum mass of a stable white dwarf) of 1.4M(⊙). This, together with its short orbital period (4.2 hours), suggests that the system should merge in 700 million years, triggering a type Ia supernova event. This supports the hypothesis of the double-degenerate, super-Chandrasekhar evolutionary pathway for the formation of type Ia supernovae.
authors: 1] Observatorio Astronómico Nacional, Apartado de Correos 112, E-28803, Alcalá de Henares, Spain [2] Instituto de Ciencia de Materiales de Madrid (CSIC), Sor Juana Inés de la Cruz, 3, E-28049 Madrid, Spain.
authors: M
authors: M
authors: Santander-García
authors: 1] Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain [2] Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain.
authors: P
authors: P
authors: Rodríguez-Gil
authors: 1] Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain [2] Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain.
authors: R L M
authors: RL
authors: Corradi
authors: 1] Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain [2] Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain.
authors: D
authors: D
authors: Jones
authors: 1] South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa [2] Southern African Large Telescope Foundation, PO Box 9, Observatory 7935, South Africa.
authors: B
authors: B
authors: Miszalski
authors: European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla, Santiago, Chile.
authors: H M J
authors: HM
authors: Boffin
authors: Centro de Astrobiología, CSIC-INTA, Carretera de Torrejón a Ajalvir, km 4, E-28850 Torrejón de Ardoz, Spain.
authors: M M
authors: MM
authors: Rubio-Díez
authors: South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa.
authors: M M
authors: MM
authors: Kotze