Gaia DR2 in 6D: searching for the fastest stars in the Galaxy

Abstract

We search for the fastest stars in the subset of stars with radial velocity measurements of the second data release (DR2) of the European Space Agency mission Gaia . Starting from the observed positions, parallaxes, proper motions, and radial velocities, we construct the distance and total velocity distribution of more than 7 million stars in our Milky Way, deriving the full 6D phase space information in Galactocentric coordinates. These information are shared in a catalogue, publicly available at http://home.strw.leidenuniv. nl/~marchetti/research.html. To search for unbound stars, we then focus on stars with a probability greater than 50% of being unbound from the Milky Way. This cut results in a clean sample of 125 sources with reliable astrometric parameters and radial velocities. Of these, 20 stars have probabilities greater than 80 % of being unbound from the Galaxy. On this latter sub-sample, we perform orbit integration to characterize the stars' orbital parameter distributions. As expected given the relatively small sample size of bright stars, we find no hypervelocity star candidates, stars that are moving on orbits consistent with coming from the Galactic Centre. Instead, we find 7 hyper-runaway star candidates, coming from the Galactic disk. Surprisingly, the remaining 13 unbound stars cannot be traced back to the Galaxy, including two of the fastest stars (around 700 km s −1 ). If conformed, these may constitute the tip of the iceberg of a large extragalactic population or the extreme velocity tail of stellar streams.

Runaway and walkaway stars from the ONC with Gaia DR2

“…The astrometry of Gaia DR2 has been instrumental in the continuing search for RW and the even faster hypervelocity stars (e.g. Boubert et al 2018;Bromley et al 2018; De la Fuente Marcos & de la Fuente Marcos 2018; Irrgang et al 2018;Lennon et al 2018;Marchetti et al 2019;Raddi et al 2018;Brown et al 2018;Rate & Crowther 2020).…”

Searching for correlations in Gaia DR2 unbound star trajectories

“…Most of our selection is characterized by negative parallax values, an indication of poor data quality. We recover distances d using the same Bayesian framework as in Marchetti et al (2018), adopting an exponential prior P(d) ∝ d 2 exp(−d/L) with L = 2600 kpc, best suited for stars with distances d 2 kpc from the Sun (Astraatmadja & Bailer-Jones 2016), as we expect given the negative parallaxes. Figure 1 shows that most of our HVS selection is characterized by total Galactocentric velocities V tot ∼ O(10 3 ) -O(10 4 ) km/s.…”

“…We use the same parameters as in Marchetti et al (2018), summarized in table 1. In the PBH CDM scenario the inner part of the halo is better described by an Einasto profile (Einasto 1965), but this only affects the dynamics close to the Galactic center, where a few of our HVS candidates lie.…”

Solar System. Angular Momentum. New Physics

“…In frames of the developed Rotational Fission model it is easy to explain hyper-runaway stars unbound from the Milky Way with speeds of up to ~700 km/s [18]: they were launched by overspinning Core of the Large Magellanic Cloud with the speed higher than the escape velocity.…”