In the constellation Taurus there is a cluster of several hundred stars called the Hyades. This star cluster is only 150 light-years away and may contain a stellar-mass black hole. The idea of ​​black holes hidden in star clusters is not new. Clusters often contain large, bright stars that will eventually become neutron stars or black holes. Therefore, it is very possible that previous stars have followed this path. The problem is proving it. Unless a black hole is actively consuming matter nearby, it will be dim and difficult to see amid a bright cluster of stars. Therefore, astronomers must use indirect observations to discover black holes. To search for black holes, the team compared observations of the Hyades cluster by the Gaia spacecraft with N-body computer simulations. The Hyades are an open cluster so they are bound only by weak gravity . Sometimes a close encounter between two stars can knock one of them out of the cluster. Even closer encounters push a star farther toward the center of the cluster, causing it to become more tightly bound to the cluster. All of this contributes to how the density of stars in a cluster changes with distance from its center.

The observed configuration is best matched to models with 2-3 black holes. Credit: Torniamenti, et al. One of the things the team compared was something called half-mass radius. This is the radius that contains half the mass of the cluster. In the presence of a few black holes, the cluster will be a bit denser and thus the half-mass radius will be smaller. Another aspect is the central density, which is expected to increase slightly in the presence of a black hole. Taking all of this into account, a comparison between many-body simulations and Gaia data shows that the best model predicts the presence of 2 or 3 stellar-mass black holes. Unfortunately, the results are not conclusive. Although 2 or 3 black holes are the best solution for the observational data, a model with no black holes or up to 5 black holes is still a reasonable solution. And the cluster’s central density peak is not something Gaia’s observations are sensitive enough to observe. The team even considered the 56 stars in the cluster as candidate binaries, just in case one of them was a star orbiting a black hole, but none of the candidates were suitable for a binary star. black hole’s behavior. So while it’s entirely possible that the Hyades harbors a stellar-mass black hole, more observations are needed to be sure. Only then will we truly know whether there is a monster lurking within us.

source: https://arxiv.org/abs/2303.10188