One Star Was Thrown out of the Galaxy. This May Be the Partner the Black Hole Kept
The Hills mechanism leaves two fossils, and astronomers have never been able to point at both halves of the same pair. A binary drifts too close to Sagittarius A*, the black hole of about four million solar masses at the centre of the galaxy. The pair is torn apart. One star is slingshotted away fast enough to leave the Milky Way altogether; the other is left behind, trapped on a tight and highly eccentric orbit.
S5-HVS1 is an escapee. Among the hypervelocity stars catalogued so far, it is the only one whose origin at the galactic centre is beyond dispute, and its measured mass and velocity pin down a relationship between the mass and the orbital size of whatever partner it left. That prediction has been on the books for years. Finding the partner was the hard part.
In mid-July the GRAVITY Collaboration reported something else entirely. In a paper accepted for publication in Nature, the team described S301, a faint main-sequence star on an 8.7-year orbit around Sgr A* that reaches 25,000 kilometres per second at its closest approach, fast enough for its motion to be sensitive to the black hole's spin. The discovery paper noted in passing that S301's high eccentricity suggests it too was captured through the Hills mechanism.
Andrea Caputo and Giovanni Maria Tomaselli, who are not part of that team, asked the obvious follow-up. Is S301 specifically the star S5-HVS1 left behind? Their answer, in a three-page note posted to arXiv on 24 July, is a heavily qualified maybe. They built a forward model of the Hills origin and compared it with the null hypothesis. The model does confirm that S301's orbit aligns much more closely with the predicted companion orbit than a typical S-star's does. But, in their own words, "the catalog-level Bayes factor remains of order unity and dependent on the probabilities of survival and detection."
A Bayes factor of order unity is a statistician's way of saying the evidence does not choose. Weigh the pairing hypothesis across the catalogue of known stars and it comes out roughly level with coincidence, and how level depends on assumptions about which captured stars survive and which ones we can see at all. The authors call S301 "a compelling candidate" and say in the same breath that establishing the association will need better mass measurements, a chemical comparison between the two stars, and selection functions calibrated to GRAVITY.
That chemical comparison is the cleanest of the three tests. Stars born together out of the same cloud carry the same elemental fingerprint, so a genuine former binary should match.
If the Hills mechanism works the way theorists say, both halves of at least one disruption should already be sitting in stellar catalogues. Whether S5-HVS1 and S301 are that pair will be decided by future mass measurements and spectral analysis.
