The double-degenerate, super-Chandrasekhar nucleus of the planetary nebula Henize 2-428

TL;DR

This paper reports the discovery of a double-degenerate core in the planetary nebula Henize 2-428 with a combined mass exceeding the Chandrasekhar limit, indicating it will likely merge and produce a Type Ia supernova, supporting the double-degenerate super-Chandrasekhar model.

Contribution

It provides observational evidence of a super-Chandrasekhar double-degenerate system in a planetary nebula, supporting a key progenitor channel for Type Ia supernovae.

Findings

The system has a total mass of 1.76 M$_\odot$

Orbital period is 4.2 hours

Expected merger in 700 million years

Abstract

The planetary nebula (PN) stage is the ultimate fate of stars with mass 1 to 8 solar masses (M$_\odot$). 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 it expands during the Asymptotic Giant Branch (AGB) phase. The excess material ends up forming a common-envelope (CE) surrounding both stars. Drag forces would then result in the envelope being ejected into a bipolar PN whose equator is coincident with the orbital plane of the system. Systems in which both stars have ejected their envelopes and evolve towards the white dwarf (WD) stage are called double-degenerates. Here we report that Henize 2-428 has a double-degenerate core with a combined mass unambiguously above the Chandrasekhar limit of 1.4 M$_\odot$. According to its short orbital period (4.2 hours) and total mass (1.76 M$_\odot$), the system should merge in 700 million years, triggering a Type Ia supernova (SN Ia) event. This finding supports the double-degenerate, super-Chandrasekhar evolutionary channel for the formation of SNe Ia.