HST Astrometry in the 30 Doradus Region. II. Runaway Stars from New Proper Motions in the Large Magellanic Cloud

Platais, Imants; Lennon, Daniel J.; van der Marel, Roeland P.; Bellini, Andrea; Sabbi, Elena; Watkins, Laura L.; Sohn, Sangmo T.; Walborn, Nolan R.; Bedin, Luigi R.; Evans, Christopher J.; de Mink, Selma E.; Sana, Hugues; Herrero, Artemio; Langer, Norbert; Crowther, Paul
The Astronomical Journal, Volume 156, Issue 3, article id. 98, 13 pp. (2018).


We present a catalog of relative proper motions for 368,787 stars in the 30 Doradus region of the Large Magellanic Cloud (LMC), based on a dedicated two-epoch survey with the Hubble Space Telescope and supplemented with proper motions from our pilot archival study. We demonstrate that a relatively short epoch difference of three years is sufficient to reach a level of precision of ∼0.1 mas yr‑1 or better. A number of stars with relative proper motions exceeding a 3σ error threshold represent a mixture of Milky Way denizens and 18 potential LMC runaway stars. Based upon 183 VFTS OB stars with the best proper motions, we conclude that none of them moves faster than ∼0.3 mas yr‑1 in each coordinate—equivalent to ∼70 km s‑1. Among the remaining 351 VFTS stars with less accurate proper motions, only one candidate OB runaway can be identified. We rule out any OB star in our sample moving at a tangential velocity exceeding ∼120 km s‑1. The most significant result of this study is finding 10 stars over a wide range of masses that appear to have been ejected from the massive star cluster R136 in the tangential plane to angular distances from 35″ out to 407″, equivalent to 8–98 pc. The tangential velocities of these runaways appear to be correlated with apparent magnitude, indicating a possible dependence on the stellar mass. Lastly, a comparison to proper motions from Gaia DR 2 shows that for several relatively bright stars the DR 2 has an unexpected scatter that cannot be accounted for by the formal errors.