The A-type star S5-HVS1, ejected from the Galactic Centre roughly 4.8 Myr ago at about 1800 km/s -- nearly twice the ejection speed of every other hypervelocity star consistent with a Galactic-Centre origin -- is the only hypervelocity star unambiguously tied to the supermassive black hole Sgr A*. Its discovery paper asks whether the Galactic-Centre ejection process is steady, as in the classical Hills mechanism of binary disruption by the black hole, or time-variable (a burst of binary supply after the gas-accretion episode that formed the young clockwise stellar disk; slingshot ejections by an intermediate-mass black hole or a black-hole binary), and it flags two linked open objectives: identifying S5-HVS1's captured former binary companion among the S-stars orbiting Sgr A*, and using enlarged hypervelocity-star samples to constrain the Milky Way gravitational potential and its dark-matter substructure. All three threads remain open: steady double-progenitor Hills models, disk-driven burst scenarios, and a supermassive-intermediate-mass black-hole binary proposal all remain in contention with no observational verdict; the GRAVITY-discovered S-star S301 is a compelling but statistically unconfirmed candidate companion (order-unity Bayes factor); and potential inference remains forecast-limited by sample size and by Large-Magellanic-Cloud and Galactic-bar systematics.

Background

A hypervelocity star (HVS) is a star moving faster than the local escape velocity of the Milky Way, so that it will leave the Galaxy. Hills (1988) predicted that when a tight stellar binary passes close to the supermassive black hole at the Galactic Centre (GC), Sgr A* (mass about four million solar masses), the three-body interaction ejects one star at up to a few thousand km/s while the other is captured onto a tight, eccentric orbit around the black hole. Between the first discovery in 2005 and 2019, dedicated surveys (chiefly at the MMT, a 6.5-metre telescope in Arizona) found roughly twenty such stars -- late B-type stars in the Galactic halo with ejection speeds of about 400-750 km/s -- of which about a dozen are statistically consistent with a Galactic-Centre origin (Brown 2015). Astrometry from the European Gaia satellite -- precision measurement of stellar positions, angular motions on the sky (proper motions), and parallax-based distances -- supplies the data needed to reconstruct these stars' orbits backwards in time.

The Southern Stellar Stream Spectroscopic Survey (S5) then serendipitously discovered S5-HVS1: a 2.35-solar-mass A-type star about 9 kpc from the Sun whose backward-integrated orbit unambiguously intersects the Galactic Centre, implying ejection at about 1800 km/s roughly 4.8 Myr ago (Koposov et al. 2019). It is still the only HVS securely tied to Sgr A*. Three properties make it pivotal. First, its ejection speed is nearly twice that of every other Galactic-Centre-consistent HVS, which cluster at about 400-1000 km/s; the published abstract closes by asking whether the two populations have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time. Second, its ejection direction and flight time coincide with the orbital plane and age of the disk of young stars orbiting Sgr A* (the clockwise disk), suggesting a link to the same star-formation episode; if the Hills mechanism operated, the captured former binary companion should still orbit Sgr A* among the S-stars -- the young B-type stars within about 0.01 pc of the black hole whose dynamics are dominated by it. Third, Koposov et al. showed that the star's short flight time makes its trajectory almost insensitive to the Galactic gravitational potential (switching off the entire Milky Way potential shifts its Galactic-plane crossing point by only about 15 pc), so a single fast HVS cannot yet measure the flattening of the dark-matter halo; the same paper asks whether a larger sample could, and whether HVS trajectories could even probe dark-matter substructure, the two forward-looking objectives that motivate this problem.

Problem Statement

Determine whether the hypervelocity-star ejection process at the Galactic Centre is steady -- classical Hills binary disruption with a time-independent ejection rate and velocity spectrum -- or time-variable, and thereby establish the ejection history of the Milky Way's Galactic-Centre hypervelocity stars. The problem has three linked components sharing this single objective: the population model required for the first is the prerequisite for the third, and the companion search anchors the Hills interpretation of the star that motivates all three. The population verdict (1) is the prerequisite for the forecast (3), and the companion identification (2) stands on its own evidence.

(1) Decide, from the observed hypervelocity-star population -- its numbers, sky distribution and anisotropy, distribution of ejection (flight) times, and velocity spectrum, with survey selection functions and contaminants accounted for -- whether S5-HVS1's ejection at about 1800 km/s some 4.8 Myr ago belongs to the same steady process that produced the slower Galactic-Centre-consistent hypervelocity stars (ejection speeds clustered at about 400-1000 km/s), or instead evidences a time-variable process: a burst of binary supply following the gas-accretion and star-formation episode that formed the roughly 5-Myr-old clockwise stellar disk, disk-torque-driven binary disruption, or slingshot ejections by an intermediate-mass black hole (a black hole of roughly one thousand to one hundred thousand solar masses) or a supermassive-black-hole binary companion to Sgr A*. The comparison must allow for trajectory perturbations by the Large Magellanic Cloud (the Milky Way's most massive satellite galaxy) and the Galactic bar (the elongated, bar-shaped concentration of stars at the Milky Way's centre).

(2) Test S5-HVS1's Hills origin directly by identifying, or excluding, its captured former binary companion among the S-stars orbiting Sgr A*: the companion's orbital plane should align with S5-HVS1's ejection direction, and its mass and orbital semimajor axis are fixed by S5-HVS1's measured mass and ejection velocity.

(3) Establish whether an enlarged sample of Galactic-Centre hypervelocity stars can deliver quantitative constraints on the Milky Way gravitational potential -- the flattening of the dark-matter halo (the shape of the Galaxy's extended dark-matter distribution), the disk mass, and dark-matter substructure (the population of small dark-matter clumps predicted by cold-dark-matter structure formation) -- specifying the sample size and the astrometric and spectroscopic precision required, given the perturbing influence of the Large Magellanic Cloud and the Galactic bar.

References

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