29 January 2025: Isobel Romero-Shaw

Speaker: Isobel Romero-Shaw (Cambridge)

Date: Wednesday 29 January 2025

Time: 15:00

Location: 3.21

Chemical evolution and binary stars: Implications for the distant Universe

Before gravitational-wave astronomy, we knew of a handful of stellar-mass black holes: those observable in X-ray emission as they accrete from a companion. Since the first gravitational wave was detected in 2015, the number of known stellar-mass black holes has rapidly increased. Binary black holes, observed via their gravitational wave emission as they inspiral and merge, now dominate the stellar graveyard. Almost 100 detections have been published by the LIGO-Virgo-KAGRA Collaboration, and there have been over 200 confident triggers in the ongoing fourth observing run. 

Despite the wealth of detections, the formation channels of binary black hole mergers remain a mystery: are the components born together as a stellar binary in isolation, evolving slowly over millennia via mass transfer and/or common envelope? Or do they become bound due to dynamical interactions in densely-populated environments, like star clusters and AGN disks?

In this talk, I will unearth the varying emerging biographies of the stellar graveyard. I will explain how the lifetime of a binary black hole can be encoded in its gravitational-wave signal, particularly the signatures of its orbital eccentricity, masses, and spins, and address issues in translating these hidden messages. I will demonstrate that there is only a small overlap between the population of black holes observed in X-ray binaries and those observed with gravitational waves. I will also present new results that strengthen the hypothesis of a dynamically-formed subpopulation, and reveal novel binary and signal characteristics that will help illuminate formation channels for merging binary black holes with upcoming gravitational-wave detectors.