RSAA Colloquia / Seminars / Feast-of-Facts: Tuesday, 21 August 2018, 11:00-12:00; Duffield Lecture Theatre

Rosemary Wyse

"Fast Stars and the Mass of the Milky Way"

The fastest moving stars provide insight into several fundamental properties of the Galaxy, including the escape velocity as a function of Galactocentric radius, the total mass, and the nature and frequency of stellar encounters with the central supermassive black hole. The recent second data release from the Gaia satellite has allowed the identification of new samples of stars with extreme velocities. Discrimination among the possible origins of these stars is facilitated by chemical abundance information. I will present the results from our high-resolution spectroscopic follow-up of late-type `hypervelocity’ star candidates, estimated (by others) to have a probability larger than 50% to be unbound from the Milky Way. We find that all stars observed are metal-poor giants and are chemically indistinguishable from typical halo stars. We conclude that these stars are simply the high-velocity tail of the stellar halo and that previous models used to predict the probability that these stars were unbound had too low an escape velocity. This conclusion is supported by our independent determination of the escape velocity curve based on kinematics derived from Gaia DR2 data. The inferred total mass of the Galaxy is also higher than the previous consensus value, lowering the baryon fraction and increasing the expected level of dark-matter substructure.