Formation of merging black holes and neutron stars

6 problems

• What fraction of merging black holes comes from each channel?

  Open

  Black-hole pairs could form from two stars born together, from encounters in crowded star clusters, or inside gas disks around supermassive black holes.

• Can common-envelope ejection produce tight merging black-hole binaries?

  Open

  Many merging pairs are thought to have shrunk when one star swelled and engulfed its companion, which spiraled in and blew off the shared envelope; whether the envelope can actually be thrown off and leave a tight enough orbit has not been shown.

• Is there a gap between neutron star and black hole masses?

  Contested

  Few compact objects have been found between about 2.5 and 5 solar masses, possibly because supernovae rarely make them.

• What produces the excess of merging black holes near 35 solar masses?

  Contested

  Merging black holes pile up near 35 solar masses, more than a smooth distribution would give; this could mark the pile-up just below the pair-instability gap or a feature of one formation route.

• Where is the lower edge of the pair-instability mass gap?

  Contested

  Very massive stars should explode completely, because light in their cores turns into electron-positron pairs, leaving no black holes between roughly 50 and 130 solar masses; yet black holes in that range are seen merging.

• Why do X-ray binary black holes spin fast, merging ones slowly?

  Contested

  Black holes feeding on companion stars appear to spin near the maximum rate, while most black holes seen merging barely spin.