High-speed stars: Galactic hitchhikers

Abstract

Context. The search for stars born in the very early stages of the Milky Way star formation history is of paramount importance in the study of the early Universe since their chemistry carries irreplaceable information on the conditions in which early star formation and galaxy buildup took place. The search for these objects has generally taken the form of expensive surveys for faint extremely metal-poor stars, the most obvious but not the only candidates to a very early formation. Aims. Thanks to Gaia DR2 radial velocities and proper motions, we identified 72 bright cool stars displaying heliocentric transverse velocities in excess of 500 km s−1. These objects are most likely members of extreme outer-halo populations, either formed in the early Milky Way build-up or accreted from since-destroyed self-gravitating stellar systems. Methods. We analysed low-resolution FORS spectra of the 72 stars in the sample and derived the abundances of a few elements. Despite the large uncertainties on the radial velocity determination, we derived reliable orbital parameters for these objects. Results. The stars analysed are mainly slightly metal poor, with a few very metal-poor stars. Their chemical composition is much more homogeneous than expected. All the stars have very eccentric halo orbits, some extending well beyond the expected dimension of the Milky Way. Conclusions. These stars can be the result of a disrupted small galaxy or they could have been globular cluster members. Age estimates suggest that some of them are evolved blue stragglers, now on the subgiant or asymptotic giant branches.

Constraints on the abundance of $0.01\,c$ stellar engines in the Milky Way

“…Gaia DR-2 has provided radial velocity information about 7 million stars (Katz et al 2019). A number of studies have already combed through this sample to unearth evidence for hypervelocity stars (Bromley et al 2018;Hattori et al 2018;Du et al 2019;Marchetti et al 2019;Boubert et al 2019;Caffau et al 2020;Li et al 2020). To the best of our knowledge, the fastest hypervelocity star from any survey is S5-HVS1 from the Southern Stellar Stream Spectroscopic Survey with a velocity of ∼ 6 × 10 −3 c (Koposov et al 2020), while HVS 22 from the Multiple Mirror Telescope survey exhibits a similar velocity of ∼ 5×10 −3 c (Kreuzer et al 2020).…”

60 candidate high-velocity stars originating from the Sagittarius dwarf spheroidal galaxy in Gaia EDR3

“…Du et al (2018Du et al ( , 2019 use Gaia and LAMOST data to discover that some high-velocity stars origin from the tidal debris of disrupted dwarf galaxies. Caffau et al (2020) used the Gaia DR2 and VLT data to present the chemodynamical investigation of a 72 high-velocity sample stars and suggested that these stars can be the result of a disrupted small galaxy or globular cluster members. Marchetti (2021) derived that 7 high-velocity stars possibly origin from an extragalactic origin from Gaia EDR 3.…”

Metal poor stars

“…The availability of accurate parallaxes, proper motions, and for the brightest stars also radial velocities has revived this kind of selection. Our group has been using FORS at the VLT to explore the chemical properties of stars with transverse velocities in excess of 500 km s −1 (Caffau et al, 2020). This allowed to select stars with extreme kinematics, including five formally unbound stars, more than half of the sample turned out to be on retrograde orbits.…”