Limiting the population of collisional triples using Gaia DR2

Gaia CMD for resolved double WDs
Gaia colour-magnitude diagram for resolved double WDs within 120 pc, with projected separations <300 au. Pairs are connected by black solid lines, where the photometric primary (secondary) is marked by a blue triangle (red circle). None of the pairs have a tertiary companion with a projected separation <9000 au. The number density in this parameter space of the full 120 pc WD sample from Gaia (17,395 sources) is shown in grayscale for reference (Hallakoun and Maoz 2019).

The collisional-triple SN Ia progenitor model posits that SNe Ia result from head-on collisions of double WDs, driven by dynamical perturbations by the tertiary stars in mild-hierarchical triple systems. To reproduce the Galactic SN Ia rate, at least ∼30−55% of all WDs would need to be in triple systems of a specific architecture. We tested this scenario by searching the Gaia DR2 database for the postulated progenitor triples. Within a volume out to 120 pc, we searched around Gaia-resolved double WDs with projected separations up to 300 au, for physical tertiary companions at projected separations out to 9000 au. At 120 pc, Gaia can detect faint low-mass tertiaries down to the bottom of the main sequence and to the coolest WDs. Around 27 double WDs, we identified zero tertiaries at such separations, setting a 95% confidence upper limit of 11% on the fraction of binary WDs that are part of mild hierarchical triples of the kind required by the model. As only a fraction (likely ∼10%) of all WDs are in <300 au WD binaries, the potential collisional-triple progenitor population appears to be at least an order of magnitude (and likely several) smaller than required by the model.

For further reading: