A spectroscopic quadruple as a possible progenitor of sub-Chandrasekhar
  Type Ia supernovae
  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.
