Triple star recipe for supersized stellar-mass black holes

Triple stars could be the origin of the extra-massive stellar black holes discovered by gravitational-wave detectors.
By | Published: March 2, 2021
Artist’s impressions of the HR 6819 triple star system.
ESO/L. Calçada
Pick any star in the sky and there’s a good chance it’s not alone. Stars like company, and many share their little corner of the cosmos with others. It’s not unusual to have two, three, or even more stars dancing around each other in a complex gravitational waltz. Such multiple stars are not easy to spot without practice, but you don’t have to look far to find one: Alpha Centauri, the closest star system to the Sun, is a triple.

Triple stars live in the shadow of their famous binary cousins. But every so often, triples do find themselves in the limelight. For example, triple star systems sometimes harbor exoplanets, and they can serve as unique laboratories for testing the limits of Einstein’s theory of gravity.

And more recently, scientists are starting to suspect that triples may even be linked to gravitational waves from black hole mergers.

Triple stars to explain the black hole mass gap

Imagine two massive stars in a binary system eventually explode as supernovae, leaving behind a system with two black holes. Over eons, these black holes will slowly spiral towards each other until they ultimately collide and merge, producing a short but strong gravitational-wave signal. Since 2015, the LIGO and Virgo gravitational-wave detectors have confirmed numerous “chirps” from such binary black hole mergers.

A selection of binary black hole mergers ordered by mass. Black holes whose mass prior to merging nears or exceeds approximately 50 solar masses cannot be formed from the collapse of a single stars.
LIGO/Caltech/MIT/R. Hurt (IPAC)

This formidable and growing sample contains events in which one or both pre-merger black holes are so massive that they challenge standard theoretical models. A collapsing star should not be able to produce a black hole between about 50 and 130 solar masses, yet several detected black holes come very close or even surpass the lower limit of this so-called mass gap.

And in a paper published January 21 in The Astrophysical Journal Letters, a team of scientists proposed that triple stars could solve this conundrum.

“We know that triples, even quadrupole and higher-order multiples, are common,” says Silvia Toonen, a triple star expert at the University of Amsterdam and co-author of the study. “Their abundance and the progress that is happening in the gravitational-wave field at the moment are the major motivations to really study them.”

The North Star, or Polaris, is also a triple system. The faint Polaris Ab was first discovered spectroscopically in 1929, but it took the superior resolution of the Hubble Space Telescope to directly image it.
NASA/ESA/N. Evans/H. Bond

Now envision a typical triple system, where an inner pair of binary stars is orbited by a third star farther out. Once all three of these stars become black holes — provided they are massive enough — you have triple black hole system.

Over time, the two inner black holes merge together, giving birth to a black hole whose mass can easily exceed the lower limit of the mass gap. After another long wait, the combined central black hole will go on to merge with the third black hole farther out in the system, forming an even more massive final product.

But if astronomers only pick up gravitational waves from this second merger, they will remain oblivious to the system’s turbulent, more complicated history. Therefore, the triple-star merging sequence neatly explains why recent LIGO/Virgo detection have picked up collisions between black holes that seem too massive to have formed from individual stars.

Double down on triple star research

Triple stars are still a young field of study. Despite tremendous progress in the last decade, scientists haven’t yet figured out all aspects of their behavior. How stars and their orbits evolve is heavily dependent on their initial masses and orbital arrangement. The number of possible configurations is overwhelming — and it doesn’t help that detailed observations of triples are still sparse.

“We wanted to understand if we can get triples without a strong dynamical interaction that could form black holes in the mass gap,” says Alejandro Vigna-Gómez, an astrophysicist with the Niels Bohr Institute at the University of Copenhagen and lead author of the study. And working under the assumption of a simple interaction, the team identified several scenarios that could realistically lead to a supersized stellar-mass black hole that’s later involved in another merger.

These scenarios aren’t just untestable hypotheticals, either; they predict characteristic features that astronomers can look for in merger data. “We are searching for LIGO/Virgo mergers with particular mass ratios of the merging black holes and high values of their effective spin,” says Vigna-Gómez.

Indeed, the triple star merger scenario seems to be a favorable model for the merger dubbed GW170729, whose pre-merger black holes weighed in at approximately 51 and 36 solar masses. (However, their model can’t explain the properties of GW190521, the most extreme merger detected so far. This event was spawned by the collision of two black holes weighing roughly 85 and 66 solar masses.)

The proposed triple star model may not be the only — nor the dominant — mechanism for producing high-mass stellar black holes. However, the study does prove the model is a feasible alternative to other possible scenarios, such as a binary system snagging a passing black hole that gets too close. So, the researchers argue, the role triple stars may play in forming extra-large black holes should be taken seriously.

“There is still so much more to explore. We started with simple approximations, and now is the time to add more,” says Toonen.