Gaia TGAS search for Large Magellanic Cloud runaway supergiant stars

Candidate hypervelocity star discovery and the nature of R 71

¹ ESA, European Space Astronomy Centre, Apdo. de Correos 78, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: danny.lennon@sciops.esa.int
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ VitrocisetBelgium for ESA, European Space Astronomy Centre, Apdo. de Correos 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 16 November 2016
Accepted: 18 March 2017

Abstract

Aims. Our research aims to search for runaway stars in the Large Magellanic Cloud (LMC) among the bright Hipparcos supergiant stars included in the Gaia DR1 Tycho-Gaia astrometric solution (TGAS) catalogue.

Methods. We compute the space velocities of the visually brightest stars in the Large Magellanic Cloud that are included in the TGAS proper motion catalogue. This sample of 31 stars contains a luminous blue variable (LBV), emission line stars, blue and yellow supergiants, and an SgB[e] star. We combine these results with published radial velocities to derive their space velocities, and by comparing with predictions from stellar dynamical models we obtain each star’s (peculiar) velocity relative to its local stellar environment.

Results. Two of the 31 stars have unusually high proper motions. Of the remaining 29 stars we find that most objects in this sample have velocities that are inconsistent with a runaway nature, being in very good agreement with model predictions of a circularly rotating disk model. Indeed the excellent fit to the model implies that the TGAS uncertainty estimates are likely overestimated. The fastest outliers in this subsample contain the LBV R 71 and a few other well known emission line objects though in no case do we derive velocities consistent with fast (~100 km s^-1) runaways. On the contrary our results imply that R 71 in particular has a moderate deviation from the local stellar velocity field (40 km s^-1) lending support to the proposition that this object cannot have evolved as a normal single star since it lies too far from massive star forming complexes to have arrived at its current position during its lifetime. Our findings therefore strengthen the case for this LBV being the result of binary evolution. Of the two stars with unusually high proper motions we find that one, the isolated B1.5 Ia⁺ supergiant Sk-67 2 (HIP 22237), is a candidate hypervelocity star, the TGAS proper motion implying a very large peculiar transverse velocity (~360 km s^-1) directed radially away from the LMC centre. If confirmed, for example by Gaia Data Release 2, it would imply that this massive supergiant, on the periphery of the LMC, is leaving the galaxy where it will explode as a supernova.