The nearest black holes might be hiding just around the corner, 150 light-years away in the Hyades star cluster.
The nearest black holes might be hiding just around the corner, astronomically speaking. New research suggests that two or three stellar-mass black holes lurk in the famous open star cluster the Hyades, a mere 150 light-years away in the constellation Taurus, the Bull.
Two years ago, astronomers presented tantalizing evidence for a black hole in a binary system 1,500 light-years away in Monoceros, the Unicorn. The new result, from a team led by Stefano Torniamenti (University of Padova, Italy), would shatter that record by a factor of 10.
Simon Portegies Zwart (University of Cambridge, UK, and Leiden Observatory, The Netherlands), who studies the dynamics of star clusters and was not involved in the new study, says the claim is “not really surprising. Every open cluster older than some 5 million years and containing over 1,000 stars is expected to host a few black holes.” That’s because the most massive stars in a newborn cluster are expected to go supernova after just a few million years, leaving either a neutron star or a black hole as a remnant.
The surprising thing, according to team member Mark Gieles (University of Barcelona), is that the long-term presence of one or more black holes turns out to have a strong effect on the structure and dynamics of the host cluster, as revealed by the team’s detailed computer simulations. “There was a big difference between clusters with a small number of black holes and clusters without them,” he says.
Throughout the lifetime of a cluster (the Hyades is about 650 million years old), gravitational interactions between stars mean that more massive objects, such as black holes, slowly sink towards the center. Less massive stars gain more kinetic energy and end up in wider orbits, effectively decreasing how compact the cluster is. This compactness is expressed as the half-mass radius: the radius that contains half of the total cluster mass.
Working with the latest astrometry data from the European Space Agency’s Gaia mission, Torniamenti and his colleagues conclude that the observed half-mass radius of the Hyades is about 40% larger than you would expect if the cluster had never possessed black holes. Instead, computer simulations with two or three black holes residing in the cluster for hundreds of millions of years provide a better fit with the observations, as the team describes in a paper in Monthly Notices of the Royal Astronomical Society.
“This is an important result,” says Pavel Kroupa (University of Bonn, Germany), who was not involved in the study, “because it means that when a black hole forms, it can end up with a very small space motion.” Indeed, if a newborn black hole received a large enough “kick,” it would easily escape the cluster altogether. “This provides information on the physical processes acting during a massive star’s death,” Kroupa says.
Kroupa notes that the team’s analysis assumes the validity of our conventional ideas about gravitation. In an alternative theory of gravity, known as Modified Newtonian Dynamics (MOND), the same data might be consistent with no black holes, he says. “Independently confirming the presence of black holes in the Hyades cluster will thus not only test the physics of stellar death, but will also inform us that the Newtonian law of gravitation is probably valid.”
However, convincing evidence that the black holes are really there is very difficult to find, according to team member Zephyr Penoyre (Leiden Observatory, The Netherlands). Three-body interactions of migrating black holes with binary stars in the cluster will most likely result in one star being ejected out of the system and the other orbiting the black hole. But so far, Penoyre’s search for binary systems with one massive companion turned up empty, probably because the expected orbital periods are too long. “We may need to be more inventive to come up with ways to definitely prove the existence of the black holes,” they say.
Incidentally, binary interactions may eventually have flung the black holes out of the cluster after all. If that would have happened in the past 150 million years or so — an astronomical eyeblink — it wouldn’t show up in the present dynamics and density profile of the Hyades, says Gieles. The black holes could have traversed some 200 light-years by now. “In principle, they could have reached our solar system,” he adds, “although the chances are of course extremely low.”