Title: Double progenitor origin of the S-star cluster
Authors: Sill Verberne, Elena Maria Rossi, Sergey E. Koposov, Zephyr Penoyre, Manuel Cavieres, Konrad Kuijken
First Author’s Institution: Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, the Netherlands
Status: Accepted in Astronomy & Astrophysics
Look closely at the heart of any galaxy, and more often than not, you will encounter a monstrous supermassive black hole (SMBH), often weighing a whopping million to billions of solar masses. Closer to home, one lies at the center of our galaxy too, called Sagittarius A* (Sag A*), recently imaged by the Event Horizon Telescope. By carefully monitoring over decades the motion of a set of stars orbiting closely around it, collectively called the S-star cluster, astronomers were able to infer the presence and mass of Sag A*. Since the galactic center is enshrouded in dust that scatters light at short wavelengths, the S-stars (some of them shown in Fig 1) are quite difficult to study and are often only observed in the infrared and beyond. We know that they lie primarily on eccentric orbits within 0.04 parsecs of Sag A*, and consist of a mix of mostly young B-type main sequence (MS) stars and some older evolved stars. However, the existence of this star cluster has itself puzzled astronomers, as the intense gravitational environment near an SMBH can rip apart any gas that could form stars in its vicinity. Today’s authors put forward a hypothesis for how the S-star cluster could have formed, strangely by connecting the S-stars to distant stars moving through the galaxy on cruise control.
Hills mechanism to the rescue
Imagine a binary system, that is, two stars merrily orbiting around each other. The binary might not care much about its surroundings, unless it tragically ventures too close to a SMBH. This calls for an interesting three-body interaction called the Hills mechanism. The tidal forces of the SMBH can disrupt the uneventful life of the binary, by capturing one star in a tight eccentric orbit around the SMBH, and yeeting the other one away at speeds approaching or even exceeding thousands of kilometers a second, enough to bid them goodbye out of the galaxy. The ejected stars are called hypervelocity stars (HVS), while the captured stars could be the S-star cluster that we see around Sag A* today. The authors explore this scenario in today’s paper.
Where did the binaries come from?
If the S-stars indeed formed through binary disruption via the Hills mechanism, there are a couple of potential sources for the binaries that met an early demise after facing Sag A*. Surrounding the S-star cluster is a young disk of stars called the clockwise disk (CWD) that formed in the last few million years. A bit further away, the galactic center is also surrounded by a dense nuclear star cluster (NSC), an old stellar population that formed a few billion years ago. To answer the question of where did the binaries that formed the S-star cluster originate, the authors turn to an astronomer’s favourite trick: comparing simulations with observations!
The authors simulate binary populations that originate from either the NSC, CWD or a mixture of both. They take into account their respective star formation histories, the distribution of stellar masses and orbital periods of the binaries and evolve them till the present day. Some of these simulated binaries may venture too close to Sag A*, and get destroyed by the Hills mechanism at an unknown rate. This should leave us with a set of simulated stars in a tight orbit around Sag A*, resembling the S-stars that we see today. The other (better?) half of the broken binaries would appear as HVS. Now, we can finally compare our simulated S-stars and HVS with their observed counterparts.
But wait! Not all of the simulated S-stars would be detectable, as the dense and dusty environment in the galactic center can scatter away a lot of the light directed towards our telescopes. Similarly, the simulated HVS can only be found if they lie within the observed sky patch of a dedicated survey that is actually searching for them! As of now, there is only one HVS that has been confidently traced back to have originated from the galactic center, most likely through the Hills mechanism. The authors take into account such observational biases and selection effects while comparing their simulations with observations. Now we can move on to the key results:
Scenario I: The NSC
Initially, the authors consider a simulated population, where all the binaries came from the old NSC. They find that a single Hills mechanism rate cannot explain all the observables simultaneously (left panel of Fig 2). For example, by fitting for the observed number of MS S-stars, they find that there should be many more evolved stars (because these stars are old and long past the main sequence) and HVS than currently observed.
Scenario II: The CWD
Next, the authors shift gears to understanding binaries that originate from the young CWD. Since the CWD is young, we expect it to predict a sizable number of S-stars on the main sequence, and not many older evolved stars. The authors find exactly this; for binaries originating solely from the CWD, they can simultaneously fit for the observed number of main sequence S-stars and HVS, but not the sizable number of evolved S-stars (right panel of Fig 2). Yet again, the saga of the mysterious origin of the S-stars continues.
Scenario III: Two sources to rule them all
Since it is difficult to explain the ratio of MS stars to evolved stars with just one origin scenario, perhaps there is some middle ground? The authors seek to find this by considering binaries where a fraction of them came from the CWD, while the rest came from the NSC. They find that this scenario works, where the main sequence S stars recently captured by Sag A* originated from the young CWD, while the evolved S-stars originated from the NSC. Not just that, they are also able to reproduce the semi-major axis and apparent Ks magnitude (essentially the star’s brightness in the near-infrared) distributions of the S-stars (Fig 3). Voila, we now have a grand unified mechanism that may explain most observed properties of the S-stars in the galactic center, and the HVS far, far away.
The S-stars near the galactic center were vital to infer the presence of our very own galactic SMBH. Today’s paper shows us that they are also an exciting testbed for the dynamics of three-body interactions, and that the Hills mechanism, originally proposed in the 1980s, is very much in play in this precarious environment. As we better characterize the properties of the S-stars (such as their age and metallicity), and find more HVS cruising through our galaxy with surveys such as Gaia, DESI, WEAVE and 4MOST, we might finally be able to answer the question with certainty: how did the S-star cluster form?
Astrobite edited by Nathalie Korhonen Cuestas
Featured image credit: EHT Collaboration
