Cas A and the Crab Were Not Stellar Binaries At Death
  The majority of massive stars are in binaries, which implies that many core
collapse supernovae (ccSNe) should be binaries at the time of the explosion.
Here we show that the three most recent, local (visual) SNe (the Crab, CasA and
SN1987A) were not binaries, with limits on the initial mass ratios of
q=M2/M1<0.1. No quantitative limits have previously been set for CasA and the
Crab, while for SN1987A we merely updated existing limits in view of new
estimates of the dust content. The lack of stellar companions to these three
ccSNe implies a 90% confidence upper limit on the q>0.1 binary fraction at
death of fb<44%. In a passively evolving binary model (meaning no binary
interactions), with a flat mass ratio distribution and a Salpeter IMF, the
resulting 90% confidence upper limit on the initial binary fraction of F<63% is
in considerable tension with observed massive binary statistics. Allowing a
significant fraction fM~25% of stellar binaries to merge reduces the tension,
with F<63/(1-fM)~81%, but allowing for the significant fraction in higher order
systems (triples, etc.) reintroduces the tension. That CasA was not a stellar
binary at death also shows that a massive binary companion is not necessary for
producing a Type IIb SNe. Much larger surveys for binary companions to Galactic
SNe will become feasible with the release of the full Gaia proper motion and
parallax catalogs, providing a powerful probe of the statistics of such
binaries and their role in massive star evolution, neutron star velocity
distributions and runaway stars.
