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Binaries have received much attention as possible progenitors of Type Ia supernova (SNIa) explosions, but long-term gravitational effects in tight triple or quadruple systems could also play a key role in producing SNIa. Here we report on the properties of a spectroscopic quadruple (SB4) found within a star cluster: the 2+2 hierarchical system HD 74438. Its membership in the open cluster IC 2391 makes it the youngest (43 My) SB4 discovered so far and among the quadruple systems with the shortest outer orbital period. The eccentricity of the 6 y outer period is 0.46 and the two inner orbits, with periods of 20.5 d and 4.4 d, and eccentricities of 0.36 and 0.15, are not coplanar. Using an innovative combination of ground-based high resolution spectroscopy and Gaia/Hipparcos astrometry, we show that this system is undergoing secular interaction that likely 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 SNIa through white dwarf (WD) mergers. Taking into account the contribution of this specific type of SNIa better accounts for the chemical evolution of iron-peak elements in the Galaxy than considering only near Chandrasekhar-mass SNIa., accepted for publication in Nature Astronomy

Neutron stars are possible end products of massive star evolution (M ≥ 8 Msun). They are exotic astronomical objects observed with masses between 1.2 and 2.35 Msun and typically emitting at radio and X-ray wavelengths, especially when being young and rapidly rotating. As they age, their rotation slows down, their emission fades, and they become ‘quiet’, unless they are fuelled by mass transfer from a binary companion. Neutron stars not powered by accretion are difficult to detect and, therefore, very rarely observed. Here, we propose that the giant star in 56 UMa has a neutron star companion in a 46-yr orbit. Combining radial-velocity with HIPPARCOS and Gaia astrometric data, we derived its mass to be 1.31 Msun. Our claim is supported by the discovery of a cavity in the HI interstellar gas around the star, providing evidence of a supernova explosion. The heavy-element abundance profile of the giant bears imprints from the s-process and seemingly as well from the r-process of nucleosynthesis, making it the first known star with a combined rs-pattern at solar metallicity. Our finding opens an opportunity to explore evolutionary scenarios that could lead to such a system and to derive properties related to the progenitor of this supernova that exploded ~100 000 years ago only 170 pc away from Earth.