Supermassive black holes are hungry. 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 own supermassive monster, Sgr A*? Maybe these mysterious drops are hiding something.
After analyzing observations of dusty objects, an international team of researchers led by astrophysicist Florian Peißker from the University of Cologne identified these clusters as potentially harboring young stellar objects (YSOs) shrouded in a haze of gas and dust. Even stranger, these small stars are younger than the unusually young and bright cluster of stars already known to orbit Sgr A*, known as S-stars.
Finding both of these groups orbiting so close together is unusual because stars that orbit supermassive black holes are expected to be faint and much older. Peißker and his colleagues “rejected the fashionable idea of classifying [these] objects like coreless clouds 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, scientists 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 the supermassive black hole alone.
Scientists determined their chemical properties from the photons they emitted, and their mid- and near-infrared radiation was consistent with the stars’ emissions. They used one of them, the G2/DSO object, as a case study to test their ideas about what these objects might be. The high brightness and particularly strong emissions of this object make it easy to study. Its mass is also similar to that of known low-mass stars.
YSOs are low-mass stars that have outgrown the protostar stage but not yet evolved into main-sequence stars, with cores that fuse hydrogen into helium. These objects like YSO candidates because there is no way they can be clumps of gas and cosmic dust. Clouds of gas 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 relatively quickly, with the thermally excited particles colliding and flying off into space.
The team found that a cloud comparable in size to G2/DSO will evaporate in about seven years. A star orbiting at the same distance from a supermassive black hole would not be destroyed nearly as quickly because it has a much higher density and mass.
Another class of objects that dust droplets could hypothetically be—but aren’t—is the Compact Planetary Nebula, or CPN. These nebulae are the expanding outer gas envelopes of small to medium-sized stars in their final death throes. While CPNs have some features in common with stars, the gravitational pull of a supermassive black hole would easily separate their gaseous envelopes and tear them apart.
YSOs are also unlikely to be binaries, even though most stars form in binaries. SGR A*’s searing temperatures and turbulence would likely cause the migration of stars that were once part of binaries.
See the stars
Further observations have determined that some of the dust-obscured objects are nascent stars, while others are thought to be stars of some kind but have not been definitively identified.
The properties that made G2/DSO an exceptional case study are also why it was identified as a YSO. D2 is another high-luminosity object about the same mass as a low-mass star that is easily observed in the near- and mid-infrared. D3 and D23 also have similar properties. These are the blobs near the black hole that scientists think are most likely YSOs.
There are other candidates that need further analysis. These include other objects that may or may not be YSOs but still show stellar characteristics: D3.1 and D5, which are difficult to observe. The mid-infrared emission of D9 is particularly low compared to the other candidates, but it is still thought to be some type of star, although perhaps not a YSO. Objects X7 and X8 exhibit a shock wave—a shock wave that results from a star’s stellar wind pushing against other stellar winds. Whether any of these objects are actually YSOs remains unknown.
Where these dusty objects come from and how they came to be is not yet known. Scientists suggest that the objects formed together in molecular clouds that fell toward the center of the galaxy. They also think that regardless of where they were born, they migrated towards Sgr A* and all that were in binaries were separated by the black hole’s massive gravity.
Although YSOs and potential YSOs are unlikely to form in the same cluster as slightly older S stars, they may still be related in some way. They may have experienced similar formation and migration paths, and younger stars may have eventually reached the same stage.
“Speculatively, the dust sources evolve into low-mass 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