Why are surviving companion stars absent near Type Ia remnants?
In plain words
If a white dwarf explodes while pulling gas from a normal star, that companion should survive and be seen near the center of the debris. Deep searches of young remnants have found no such star.
Precise statement
Single-degenerate models predict a surviving donor at the center of young SN Ia remnants (for example Tycho, Kepler, SN 1006, SNR 0509-67.5), inflated and overluminous for $1e3 \text{ to } 1e4\,\mathrm{yr}$ and moving at a few hundred km/s; deep HST searches exclude such stars in several remnants. Explain the nondetections (double-degenerate origin, spin-up/spin-down delay that lets the donor shrink, or misidentified models), and reconcile the observed hypervelocity white dwarfs moving above $1000\,\mathrm{km/s}$ with the number expected if double detonations eject surviving companions.
What would settle it
A complete census of hypervelocity runaway white dwarfs from Gaia compared with the predicted ejection rate, together with deep companion limits in a larger set of Ia remnants.
Status in the literature
Hypervelocity runaway white dwarfs found in Gaia data since 2018 are candidate survivors of double detonations; whether their numbers match the SN Ia rate is debated.