Comparing hypervelocity star populations from the Large Magellanic Cloud and the Milky Way

Abstract

We predict and compare the distributions and properties of hyper-velocity stars (HVSs) ejected from the centres of the Milky Way (MW) and the Large Magellanic Cloud (LMC). In our model, HVSs are ejected at a constant rate – equal in both galaxies – via the Hills mechanism and are propagated in a combined potential, where the LMC orbits the MW on its first infall. By selecting m > 2 M⊙ HVSs well-separated from the Magellanic Clouds and Galactic midplane, we identify mock HVSs which would stand out from ordinary stars in the stellar halo in future data releases from the Gaia satellite and the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). We find that in these deep surveys, LMC HVSs will outnumber MW ones by a factor ∼2.5, as HVSs can more easily escape from the shallower potential of the LMC. At an assumed HVS ejection rate of 10−4 yr−1, HVSs detectable in the final Gaia data release and LSST from the LMC (MW) will number $125_{-12}^{+11}$ ($50_{-8}^{+7}$) and $140_{-11}^{+10}$ ($42_{-7}^{+6}$), respectively. The MW and LMC HVS populations show different kinematics and spatial distributions. While LMC HVSs have more modest total velocities and larger Galactocentric distances clustered around those of the LMC itself, HVSs from the MW show broader distributions, including a prominent high-velocity tail above 500 km s−1 that contains at least half of the stars. These predictions are robust against reasonable variation of the Galactic potential and of the LMC central black hole mass.

The Chemical Composition of Extreme-velocity Stars* †

“…We also remark that our calculations did not include the Magellanic Clouds' potential and we are unable to infer a probability of one of the stars having originated from the LMC. According to Evans et al (2021) the LMC can be an important source of hypervelocity stars, particularly for "lower" velocity stars, and could contribute more than the Galactic center to the number of observed HVSs, but so far only one HVS has been confirmed to have originated in the LMC (Erkal et al 2019). Evans et al (2021) also points out that the high-velocity tail, above 500 km s −1 , should be mostly composed of stars accelerated by the center of the Milky Way.…”

“…According to Evans et al (2021) the LMC can be an important source of hypervelocity stars, particularly for "lower" velocity stars, and could contribute more than the Galactic center to the number of observed HVSs, but so far only one HVS has been confirmed to have originated in the LMC (Erkal et al 2019). Evans et al (2021) also points out that the high-velocity tail, above 500 km s −1 , should be mostly composed of stars accelerated by the center of the Milky Way. As most of our stars have total velocities below 500 km s −1 it is possible that orbital calculations including the Magellanic Clouds' potential could provide valuable additional information.…”

Constraints on the Galactic Centre environment from Gaia hypervelocity stars

“…We generate populations of Hills mechanism-ejected HVSs from the GC following a Monte Carlo (MC) ejection model similar to that implemented in Evans et al (2021) (see also Marchetti et al 2018). We describe the model as follows.…”

“…Others HVS candidates have trajectories which suggest an origin outside the Milky Way (Marchetti et al 2019;Marchetti 2021). Additionally, while only one HVS can be conclusively associated with an ejection from the Large Magellanic Cloud (Przybilla et al 2008;Erkal et al 2019), it is a potentially fruitful font of HVSs in the future (Boubert & Evans 2016;Boubert et al 2017;Evans et al 2021).…”

Constraints on the Galactic Centre environment from Gaia hypervelocity stars II: The evolved population

“…Our model for the generation, ejection, propagation and observation of our mock HVS populations is similar to the Monte Carlo (MC) model used in E22. We briefly describe the model and its updates here and refer the reader to E22 (see also Marchetti et al 2018;Evans et al 2021) for more detailed information.…”