Gaia EDR3 in 6D: searching for unbound stars in the galaxy

Abstract

The early third data release (EDR3) of the European Space Agency satellite Gaia provides coordinates, parallaxes, and proper motions for ∼1.47 billion sources in our Milky Way, based on 34 months of observations. The combination of Gaia DR2 radial velocities with the more precise and accurate astrometry provided by Gaia EDR3 makes the best dataset available to search for the fastest nearby stars in our Galaxy. We compute the velocity distribution of ∼7 million stars with precise parallaxes, to investigate the high-velocity tail of the velocity distribution of stars in the Milky Way. We release a catalogue with distances, total velocities, and corresponding uncertainties for all the stars considered in our analysis†. By applying quality cuts on the Gaia astrometry and radial velocities, we identify a clean subset of 94 stars with a probability $P_\mathrm{ub}> 50 \%$ to be unbound from our Galaxy. 17 of these have $P_\mathrm{ub}> 80\%$ and are our best candidates. We propagate these stars in the Galactic potential to characterize their orbits. We find that 11 stars are consistent with being ejected from the Galactic disk, and are possible hyper-runaway star candidates. The other 6 stars are not consistent with coming from a known star-forming region. We investigate the effect of adopting a parallax zero point correction, which strongly impacts our results: when applying this correction, we identify only 12 stars with $P_\mathrm{ub}> 50\%$, 3 of these having $P_\mathrm{ub}> 80\%$. Spectroscopic follow-ups with ground-based telescopes are needed to confirm the candidates identified in this work.

Gaia Early Data Release 3

“…The aim of the processing improvements described in this paper is to clean EDR3 radial velocities to avoid false-positive hypervelocity candidates, as occurred with DR2 (Boubert et al 2019). EDR3 has already been used to search for hypervelocity stars (Marchetti 2021). As the number of sources with radial velocities increases from EDR3 (∼7 million) to DR3 (∼30 million) to DR4 (∼150 million 10 ), the challenge of excluding spurious radial velocities to produce reliable hypervelocity candidates increases.…”

Properties and applications of a predicted population of runaway He-sdO/B stars ejected from single degenerate He-donor SNe

“…As such, we deem it useful to discuss the entirety of the current sample of candidate objects in detail. In the following, we follow up some astrometric properties of the objects under discussion using the Gaia DR2 and EDR3 data sets (Gaia Collaboration et al 2016, 2018, 2021.…”

“…We mention that origins as former companion stars in core collapse SNe have also been considered (Renzo et al 2019;Evans et al 2020). In any case, HVS stars of all masses and configurations are a topic of active research (Raddi et al 2020;Li et al 2020) and observational campaigns focused on their detection, especially in the era of the Gaia Instrument (Gaia Collaboration et al 2016), are ongoing (Irrgang et al 2018;Bromley et al 2018;Raddi et al 2020;Marchetti 2021;Irrgang et al 2021) and not limited to the Galaxy (Evans et al 2021b). Most recently, ejection of high velocity stars from dynamical disruption of triple systems has also been considered (Hamers et al 2021).…”

Constraints on the origins of hypervelocity stars: velocity distribution, mergers and star-formation history

“…Using Gaia DR2 data Boubert et al (2018) found most of the tangential velocity stars are actually bound to the Galaxy, and consequently ruled out all but one of the historical late-type hypervelocity candidates. More recently, ★ E-mail: al.generozov@campus.technion.ac.il Marchetti (2021) identified 17 late-type stars in Gaia EDR3 that are likely unbound.…”

“…The origins of some candidates can be constrained by integrating their orbits backwards in time in a model Galactic potential. In particular, a Galactic Centre origin has been ruled out for all existing late-type candidates (Marchetti 2021). However, due to astrometric uncertainties, the origin of many candidates cannot be directly constrained.…”