Blue extreme disk-runaway stars with Gaia EDR3

Abstract

Since the discovery of hypervelocity stars in 2005, it has been widely believed that only the disruption of a binary system by a supermassive black hole at the Galactic center (GC), that is, the so-called Hills mechanism, is capable of accelerating stars to beyond the Galactic escape velocity. In the meantime, however, driven by the Gaia space mission, there is mounting evidence that many of the most extreme high-velocity early-type stars at high Galactic latitudes do originate in the Galactic disk and not in the GC. Moreover, the ejection velocities of these extreme disk-runaway stars exceed the predicted limits of the classical scenarios for the production of runaway stars. Based on proper motions from the Gaia early data release 3 and on recent and new spectrophotometric distances, we studied the kinematics of 30 such extreme disk-runaway stars, allowing us to deduce their spatial origins in and their ejection velocities from the Galactic disk with unprecedented precision. Only three stars in the sample have past trajectories that are consistent with an origin in the GC, most notably S5-HVS 1, which is the most extreme object in the sample by far. All other program stars are shown to be disk runaways with ejection velocities that sharply contrast at least with classical ejection scenarios. They include HVS 5 and HVS 6, which are both gravitationally unbound to the Milky Way. While most stars originate from within a galactocentric radius of 15 kpc, which corresponds to the observed extent of the spiral arms, a group of five stars stems from radii of about 21−29 kpc. This indicates a possible link to outer Galactic rings and a potential origin from infalling satellite galaxies.

The Chemical Composition of Extreme-Velocity Stars

“…Its orbit is indicative of interaction with Sag. A* (Koposov et al 2020;Irrgang et al 2021). Though the Hills mechanism does accelerate stars at low velocities as well, the stars in our sample are not compatible with the Hills mechanism.…”

“…Opportunities to discover and characterize HVSs have considerably increased since the initial discoveries, specially owing to the Gaia data releases. For example, Shen et al (2018) reported the discovery of three white dwarfs with velocities exceeding 1000 km s −1 , or the discovery of S5-HVS 1, an unbound A type star located at 9 kpc from the Sun travelling at a staggering 1700 km s −1 (Koposov et al 2020;Irrgang et al 2021). Bromley et al (2018), Marchetti et al (2018b,a), Boubert et al (2018), and Hattori et al (2018a) have all searched for HVSs candidates, with a focus on late-type stars, and found over 30 stars.…”

The Chemical Composition of Extreme-velocity Stars* †

“…Opportunities to discover and characterize HVSs have considerably increased since the initial discoveries, especially owing to the Gaia data releases. For example, Shen et al (2018) reported the discovery of three white dwarfs with velocities exceeding 1000 km s −1 , and the discovery of S5-HVS 1, an unbound A-type star located at 9 kpc from the Sun and traveling at a staggering 1700 km s −1 (Koposov et al 2020;Irrgang et al 2021). Bromley et al (2018), Marchetti et al (2018aMarchetti et al ( , 2018b, Boubert et al (2018), and Hattori et al (2018a) have all searched for HVS candidates, with a focus on late-type stars, and found over 30.…”

Constraints on the Galactic Centre environment from Gaia hypervelocity stars

“…With the notable exception of HVS candidate S5-HVS1 (Koposov et al 2020), it is notoriously difficult to indisputably associate known HVS candidates with an origin in the GC. For many candidates, tracing their trajectories backwards in time leads to a wide range of possible ejection locations (see Irrgang et al 2019;Kreuzer et al 2020;Irrgang et al 2021) due to their imprecise astrometry and large helicentric distances. Additionally, it is well-known that the GC is not the only potential birthplace of extreme-velocity stars.…”