A cluster of dusty young stars found around our galaxy’s central black hole

Supermassive black holes are insatiable. Clumps of dust and gas are prone to disruption by turbulence and radiation when they are drawn too close. So why do some of them orbit the edge of the Milky Way’s supermassive monster, Sgr A*? Maybe these mysterious spots are hiding something.

After analyzing observations of dusty objects, an international team of researchers, led by astrophysicist Florian Peißker of the University of Cologne, have identified these clusters as potentially hosting young stellar objects (YSOs) shrouded in a cloud of gas and dust. Even more surprising is that these baby stars are younger than an extremely young and bright group of stars already known to orbit Sgr A*, known as S stars.

Finding both of these clusters orbiting so close together is unusual because stars orbiting supermassive black holes are expected to be faint and much more ancient. Peißker and his colleagues “reject the fashionable idea of ​​classifying [these] coreless cloud-like objects in the high-energy radiation field of the supermassive black hole Sgr A*,” they said in a study recently published in Astronomy & Astrophysics.

More than just space dust

To figure out what the objects near Sgr A* might be, researchers had to rule out things that weren’t. Embedded in envelopes of gas and dust, they maintain particularly high temperatures, do not evaporate easily, and each orbits only around the supermassive black hole.

The researchers determined their chemical properties from the photons they emitted, and their mid- and near-infrared emissions were consistent with those of stars. They used one of them, the G2/DSO object, as a case study to test their ideas about what the objects might be. The high brightness and particularly strong emissions of this object make it easier to study. Its mass is also similar to the masses of known low-mass stars.

YSOs are low-mass stars that have passed the protostar stage but have not yet developed into main-sequence stars, with cores fusing hydrogen into helium. These objects like YSO candidates because they cannot be clumps of gas and space dust. Gas clouds without any objects inside to hold them together through gravity could not survive so close to a supermassive black hole for long. Its intense heat causes the gas and dust to evaporate fairly quickly, with the particles excited by the heat colliding with each other and flying off into space.

The team realized that a cloud comparable in size to G2/DSO would evaporate in about seven years. A star orbiting the same distance from the supermassive black hole would not collapse nearly as quickly due to its much higher density and mass.

Another class of object that could hypothetically be dust fringes—but aren’t—is a compact planetary nebula, or CPN. These nebulae are the outer envelopes of expanding gas of small to medium-sized stars in their final death throes. While CPNs have some features in common with stars, the gravitational force of a supermassive black hole would easily tear apart their envelopes of gas and tear them apart.

It is also unlikely that YSOs are binary stars, although most stars form in binary systems. The scorching temperatures and turbulence of SGR A* likely cause the migration of stars that were once part of binaries.

Looking at the stars

Further observations determined that some of the dust-obscured objects are newborn stars, while others are thought to be stars of some type but have not been definitively identified.

The properties that made G2/OSSH an exceptional case study are also why it has been identified as a YSO. D2 is another high-luminosity object, almost as massive as a low-mass star, which is easy to observe in the near- and mid-infrared. D3 and D23 also have similar properties. These are the spots near the black hole that researchers think are most likely to be YSOs.

There are other candidates that need further analysis. These include additional objects that may or may not be YSOs but nevertheless exhibit stellar characteristics: D3.1 and D5, which are difficult to observe. D9’s mid-infrared emission is particularly low when compared to the other candidates, but it is still thought to be some type of star, though probably not a YSO. Objects X7 and X8 both exhibit bow shocks—the shock wave that results from a star’s stellar wind pushing against other stellar winds. Whether one of these objects is actually a YSO remains unknown.

Where they came from and how these dust objects were formed is currently unknown. The researchers suggest that the objects formed together in molecular clouds that were falling towards the center of the galaxy. They also think that, regardless of where they were born, they migrated towards Sgr A*, and whatever was in the binary systems was torn apart by the black hole’s immense gravity.

While it is unlikely that YSOs and potential YSOs originate in the same cluster as the slightly older S stars, they may still be related in some way. They may have experienced similar journeys of formation and migration, and younger stars may eventually reach the same stage.

“Speculatively, dust sources will evolve into low-mass S stars,” Peißker’s team said in the same study.

Even black holes look better with a sparkling diamond necklace.

Astronomy and Astrophysics, 2024. DOI: 10.1051/0004-6361/202449729