The presence of the star X3a near the supermassive black hole at the center of the Milky Way suggests a new star formation model

The star X3a in its envelope of gas and dust (Image courtesy Florian Peißker)
The star X3a in its envelope of gas and dust (Image courtesy Florian Peißker)

An article published in “The Astrophysical Journal” reports the discovery of a very young and massive star, cataloged as X3a, in an environment in which it shouldn’t exist, as it orbits Sagittarius A*, or simply Sgr A*, the supermassive black hole at the center of the Milky Way. A team of researchers led by Florian Peißker of the Institute of Astrophysics at the University of Cologne, Germany, used several instruments to locate X3a.

According to the researchers, the star must have formed in a cloud farther from Sagittarius A* and then been attracted by it. This suggests a model of star formation near an environment where conditions seem impossible.

The extremely powerful gravity of a supermassive black hole is only the most obvious of the reasons why star formation seems impossible in its proximity. Actually, further interference comes from electromagnetic emissions from that area. The ongoing processes generate X-rays and ultraviolet rays that prevent clouds of gas orbiting a supermassive black hole from clumping together and starting the formation of stars. For this reason, the discovery of the star X3a, using infrared observations conducted with various instruments mounted on ESO’s VLT, the New Technology Telescope (NTT), NIRCAM2 and OSIRIS at the Keck Observatory, and radio waves with the ALMA radio telescope, was surprising.

Actually, young stars close to Sagittarius A* were already discovered in the past and astronomers haven’t found an explanation for their formation. They called it the paradox of youth because old stars can form elsewhere and then approach the supermassive black hole while young stars shouldn’t be in that area. X3a has a mass about ten times the Sun’s and its discovery could lead to the solution to the mystery.

The study by Florian Peißker’s team shows that a few light-years from Sagittarius A* there’s an area where the conditions for star formation exist. A ring of gas and dust around the supermassive black hole is cool enough and protected from the most energetic emissions that would interfere with star formation. Clouds containing enough gas to form many massive stars can form in that area and move towards Sagittarius A* due to collisions between them and scattering, which remove the angular momentum.

According to the researchers, the high mass of the star X3a may also be due to the formation of very hot lumps in its vicinity that were attracted during its formation. However, those lumps could only have been swallowed by an object that was already massive, not cause its birth.

In the researchers’ scenario, a cloud with a mass about 100 times the Sun’s formed in the ring around Sagittarius A* and then collapsed under its star’s gravity into one or more protostars. The so-called fall time is more or less equal to the age of the star X3a.

This scenario needs to be verified, so new observations are needed, possibly with instruments such as the James Webb Space Telescope and in the future, with the ELT. If the hypothesis offered by the researchers is correct, rings of gas and dust are also present around supermassive black holes at the center of other galaxies. This means that the attempts to find young, massive stars in those areas will have different targets to understand if this star formation model is valid.

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