RESEARCH PATHS: Orbital path shapes of colliding dead stars may indicate origin of binary stellar systems

We measured the shapes of the orbits of dead stars by their *eccentricity*: higher eccentricity means the orbital shape is more squashed, while an eccentricity of 0 means that it is circular. The coloured shapes represent the probability of eccentricity for each event, with the widest point of the shape at the highest point of probability. There are two events with their highest point of probability above the detection threshold for eccentricity, which is indicated with a dotted line.

​The LIGO-Virgo-KAGRA Collaboration recently announced that the number of times we've seen dead stars crashing into each other on the other side of the Universe has grown to 90. It's clearly not uncommon for these dead stars—most of them black holes—to slam together in violent merger events. But one outstanding mystery pervades these detections: how do two compact stellar remnants find each other in the vast emptiness of space, and go on to merge together? In our recent paper, we found clues to solve this mystery from the orbital path shapes formed by the stellar objects before they collided.

Often, stars are born into binary systems containing two stars that orbit each other. If these binary stars undergo specific evolutionary mechanisms, they can remain close when they die, and their corpses—black holes and/or neutron stars—can collide with each other. This kind of binary should trace a circular orbital path before it merges. However, sometimes stellar remnants meet in more exciting environments, like the cores of star clusters. In this kind of environment, binary stellar remnants can trace orbital paths around each other that look like ‘squashed’ circles—more egg-shaped or sausage-shaped.

Dense clusters of stars can produce binaries in circular orbits; however, about 1 in 25 of the mergers that combine in a dense star cluster are expected to have orbital shapes that are visibly squashed. To map the paths taken by cosmic couples in their pre-merger moments, we studied the space-time ripples produced by the collisions of 36 binary black holes. Two of these collisions—one of them being the monster binary black hole GW190521—contained the distinctive signatures of elongated (squashed) orbits. This means that more than a quarter of the observed collisions may be occurring in dense star clusters, because every squashed-orbit system indicates that 24 more mergers may also have happened in this environment.

While this result is exciting, it’s not conclusive: other dense environments, like the centres of galaxies, can also produce merging stellar remnants with squashed orbital shapes. To distinguish the formation habitats of the observed population, we need to scrutinise the orbital shapes of more colliding stellar remnants. Luckily, the number of detected stellar-remnant collisions is growing quickly, so this merger mystery may be solved soon.
 
Written by OzGrav PhD student Isobel Romero-Shaw, Monash University