# Cas A and the Crab Were Not Stellar Binaries At Death

## TL;DR

This paper investigates the binary status of three recent local supernovae, finding they were not binaries at death, which challenges assumptions about binary prevalence in massive star explosions and implications for stellar evolution models.

## Contribution

It provides the first quantitative limits on the binary status of CasA and the Crab supernovae, and discusses implications for binary fractions and stellar evolution.

## Key findings

- CasA and the Crab were not binaries at death.
- Binary fraction at death is less than 44% with 90% confidence.
- Binary mergers can reduce tension with observed binary statistics.

## Abstract

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.