A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar type la supernovae

Binaries have received much attention as possible progenitors of type la supernova explosions(1), but long-term gravitational effects(2,3) in tight triple or quadruple systems could also play a key role in producing type la supernovae. Here we report on the properties of a spectroscopic quadruple found within a star cluster: the 2 + 2 hierarchical system HD 74438(4). Its membership in the open cluster IC 2391 makes it the youngest (43 Myr) spectroscopic quadruple known to us, and among the quadruple systems with the shortest outer orbital period. The eccentricity of the 6yr outer period is 0.46, and the two inner orbits, with periods of 20.6d and 4.4d and eccentricities of 0.36 and 0.15, are not coplanar. Using an innovative combination of ground-based high-resolution spectroscopy(5-7) and Gaia/Hipparcos astrometrys (8-11), we show that this system is undergoing secular interaction, which probably pumped the eccentricity of one of the inner orbits higher than expected for the spectral types of its components. We compute the future evolution of HD 74438 and show that this system is an excellent candidate progenitor of sub-Chandrasekhar type la supernovae through white dwarf mergers. Taking into account the contribution of this specific type of type la supernova accounts for the chemical evolution of iron-peak elements in the Galaxy better than considering only near Chandrasekhar-mass type la supernovae(12).