Title: On the formation of a 33 solar-mass black hole in a low-metallicity binary

Authors: Kareem El-Badry

First Author’s Institution: Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA

Status: Published open access in The Open Journal of Astrophysics.

Black holes are ubiquitous throughout astronomy, from the supermassive giants lying in the center of most galaxies, to their stellar mass counterparts formed when massive stars die. For decades, we’ve found the latter in X-ray binaries, and more recently through gravitational wave signals. However, the last two years have seen a new black hole discovery machine: everyone’s favourite space astrometry satellite, Gaia. Since 2013, Gaia has been observing the motions of over a billion stars in our galaxy! A significant fraction of them live in binary systems, which causes tiny wobbles in their motion that Gaia can measure. Their shape, size, and duration can reveal key information about the binary, such as the orbital period and component masses. Sometimes we are lucky, and the companion to the visible star turns out to be an otherwise invisible black hole. We have already found three black holes this way in our (galactic)-backyard, aptly named Gaia BH1, Gaia BH2, and most recently -wait for it- Gaia BH3. Today’s bite focuses on the origin of Gaia BH3, or rather, how it could not have formed.

Firstly, let’s note down everything that we know about this binary 590 parsecs from us. The black hole in this system is the most massive stellar mass black hole found so far in our galaxy, a whopping 33 solar masses! The visible companion, a rather metal-poor giant of around 0.8 solar masses, is interesting in its own right. Gaia has also constrained the orbital period to be around 11.6 years, with an eccentricity of about 0.73.

There are many roads to forming black hole binaries. Popular amongst them (illustrated in Fig 1) are isolated binary evolution, where two stars are simply born together, and the dynamical channel, where interactions in dense environments like star clusters may pair up black holes with stars. In fact, for Gaia BH3, we know that it is bound to a disrupted cluster, ED-2, hinting towards a dynamical origin. But can we decisively show that isolated binary evolution cannot explain its current orbit? The authors attempt to tackle this question.

Since black holes like the one in Gaia BH3 form when massive stars die, the authors try to figure out the possible progenitor for the black hole in this system. A star exploding in a binary is a spectacular but uncertain moment for the fate of the system. Depending on how much mass is blown away and whether the remnant compact object receives a kick in some direction, the binary orbit can dramatically change, often even breaking the system!

To decipher the pre-explosion orbit, the authors run Monte Carlo simulations. They simulate millions of binaries, starting with circular or eccentric orbits, to explore how different combinations of mass loss, kick speed, and direction, shape the orbit right after the explosion. Then, among the simulated post-explosion orbital parameters, they filter for systems matching Gaia BH3’s observed period and eccentricity. They find a wide range of pre-explosion orbital configurations and kick strengths explaining the current orbit! However, there is another observational constraint we need to account for. Remember that Gaia BH3 is bound to ED-2, which has an escape velocity of about 10 km/s. The simulated binaries cannot have post-explosion speeds exceeding this number! Applying this constraint, they find that only relatively compact pre-explosion orbits with small kicks can explain the observations (Fig 2).

Too big to fit?

Although they have found a small set of orbital configurations that explain all observed features about Gaia BH3, there’s another catch! Stars expand during their lifetimes during the main sequence and even more so after exhausting their central hydrogen. In a binary, once a star’s radius crosses a size called the Roche Lobe radius, matter beyond this point is no longer bound to it and can transfer to the companion. In highly asymmetric systems like Gaia BH3, this arrangement is unstable and the binary gets engulfed in the overflowed matter, called the “common envelope phase.” The envelope creates friction, which rapidly shrinks and circularizes the binary orbit, often leading to a merger. However, Gaia BH3’s wide and eccentric orbit today shows no signs of any past interaction. Thus, to test whether the black hole’s progenitor can stay inside its Roche lobe pre-explosion, the authors use the stellar evolution code, MESA. They simulate a small set of potential progenitors, exploring how different masses, metallicities, rotation rates and convection efficiencies affect its stellar radius just before explosion. They find that for most plausible models (except those with inefficient convection or rapid rotation), the simulated stars overflow their Roche lobes in the tight pre-explosion orbits required for Gaia BH3 to remain bound to ED-2 (Fig 3). Thus the authors conclude that formation through isolated evolution is difficult given the observational constraints, and a dynamical origin is more likely.

Black holes don’t like metals?

Are systems like Gaia BH3 picky about their metallicity? The authors show that in our galactic backyard (within a kiloparsec), only 1 in 10000 giants have metallicities similar to or lower than Gaia BH3. Yet, the only system that we have found one turned out to be around a nearby metal poor star (which are not that common). Perhaps such systems are indeed more prevalent at low metallicity, or we just got really lucky?

Exciting times ahead?

Gaia BH3 was easy to spot as it was nearby and the visible companion was a bright red giant. However, the initial-mass-function predicts many more nearby stars at lower masses as well. This could mean numerous more black holes lurking around those stars, hidden in the haystack of the much awaited Gaia DR4 (coming to you in 2026!), and waiting to be discovered! Gaia has truly ushered in a renaissance in studying binary systems. Discoveries like Gaia BH1, Gaia BH2 and Gaia BH3 are crucial for probing uncertainties in binary evolution, explosions in massive stars to form compact objects, and in understanding pathways to forming the gravitational wave sources that we are constantly discovering in the distant universe.

Astrobite edited by Sarah Stevenson and Cole Meldorf

Featured image credit: ESO/L. Calçada via Wikimedia Commons