Wide binary companions to massive stars and their use in constraining natal kicks

The origin of ultrawide massive binaries (orbital separations 10³-2 × 10⁵ au) and their properties are neither well characterized nor understood. Here we use the second Gaia data release to search for wide astrometric companions to Galactic O-B5 stars which share similar parallax and proper motion with the primaries. Using the data we characterize the frequency and properties of such binaries. We find an ultrawide multiplicity fraction of 4.4 ± 0.5 per cent, to our completeness limit (up to ≈17 mag; down to G-stars at distances of 0.3-2 kpc, excluding stars in clusters). The secondary mass-function is generally consistent with a Kroupa initial stellar function; if extrapolated to lower mass companion stars we then might expect a wide-binary fraction of ∼ 27± 5%. In addition we use these data as a verification sample to test the existence of ultrawide binaries among neutron stars (NSs) and black holes (BHs). We propose that the discovery of such binary can provide unique constraints on the weakest natal kicks possible for NSs/BHs. If a compact object is formed in an ultrawide binary and receives a very low natal kick, such a binary should survive as a common proper motion pair. We therefore use Gaia data to search for ultrawide companions to pulsars (normal and millisecond ones) and X-ray binaries. We find no reliable pairs. Future data could potentially provide stringent constraints through this method.