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arXiv · 2607.22517

Is S301 the Captured Companion of the Hypervelocity Star S5-HVS1?

Andrea Caputo·Giovanni Maria Tomaselli
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Abstract

Stellar binary disruptions through the Hills mechanism produce two fossils: a hypervelocity star (HVS), and a star tightly bound to the supermassive black hole. Among known galactic HVSs, only S5-HVS1 has unambiguous galactic-centre origin. Its measured mass and velocity determine a relation between the mass and semi-major axis of its captured companion. GRAVITY has now discovered S301, whose orbit and photometrically inferred mass satisfy this relation, making it the only compelling candidate for the captured companion of S5-HVS1. We build a forward model for the Hills origin and compare it to the null hypothesis. This confirms that S301's orbit aligns much more closely with that of S5-HVS1's companion than a typical S-star. However, the catalog-level Bayes factor remains of order unity and dependent on the probabilities of survival and detection. S301 is thus a compelling candidate, but establishing its association with S5-HVS1 will require improved mass measurements, chemical comparisons and GRAVITY-calibrated selection functions.

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Andrea Caputo, Giovanni Maria Tomaselli. 2026-07-24. Is S301 the Captured Companion of the Hypervelocity Star S5-HVS1?. https://arxiv.org/abs/2607.22517

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