Session R04: Gravitational Wave Sources: Compact Binary Formation Scenarios

What do LIGO’s Black Holes Remember?

Black-hole merger observations are now becoming routine. As the available statistics increases, pressing questions on the astrophysical formation of such systems will soon be within our reach. Do LIGO's black holes remember how they formed? What are the key mechanisms to bring the binary to the critical separation where gravitational waves can efficiently drive the merger? And last but not least: how do we find it out? While mass distributions and rates tend to overlap in many formation models, our best hopes lie with eccentricity and spin measurements. Spins, in particular, are arguably the cleanest indicators of some precise formation mechanisms, such as natal kicks, tidal interactions and the occurrence of multiple merger generations.   

Uncovering the Formation of Compact Objects Through Gravitational Wave Observations

The gravitational waves emitted by coalescing compact-object binaries carry information about the formation and evolution of the binary. With the recent explosion of binary black hole gravitational wave detections, we have tantalising hints that either (a) nearly all the angular momentum in the stellar cores that will become the black holes is removed during the lifetime of the star or (b) we are seeing a population of binaries that is dynamically formed \emph{after} the constituent stars collapse to black holes. I will describe the measurements that lead us to this conclusion, speculate about the future of such measurements, and highlight other ways we might try to discover the formation processes underlying the observed population of merging binary black holes.   

Dynamics versus isolated binary evolution: place your bets

Understanding the formation channels of compact-object binaries is one of the main challenges of astrophysics in the upcoming era of gravitational-wave astronomy. In this talk, I will discuss the importance of progenitor's metallicity, mass-loss rate and Eddington factor to explain the mass of black hole binaries, and I will show that core-collapse and electron-capture supernovae are a key ingredient to investigate the rate of neutron star mergers. Dynamical processes also affect the formation of black hole binaries: dynamical exchanges in star clusters or Kozai-Lidov effects in hierarchical triples systems can lead to the formation of more massive binaries with (relatively) high eccentricity and misaligned spins. The runaway collision scenario can even lead to the formation of intermediate-mass black holes. Finally, I will discuss new models of the the merger rate evolution across cosmic time, which are crucial to distinguish between various formation channels.