Letter to the Editor

Extreme adaptive optics astrometry of R136

Searching for high proper motion stars^⋆,⋆⋆

¹ School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
e-mail: KhorramiZ@cardiff.ac.uk
² Universite de Lyon, Universite Lyon 1, CNRS, CRAL UMR5574, Saint-Genis Laval, France
³ Universite Cote d’Azur, OCA, CNRS, Lagrange, France
⁴ School of Physics and Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁵ Universite Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁶ Department of Physics and Astronomy, Hounsfield Road, University of Sheffield, Sheffield S3 7RH, UK
⁷ Astronomical Institute of the Czech Academy of Sciences, BočníII 1401/1a, 141 00 Praha 4, Czech Republic
⁸ LESIA, Observatoire de Paris, CNRS, Universite Paris 7, Universite Paris 6, 5 place Jules Janssen, 92190 Meudon, France
⁹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 26 February 2021
Accepted: 22 April 2021

Abstract

We compared high-contrast near-infrared images of the core of R136 taken by VLT/SPHERE, in two epochs separated by 3.06 years. For the first time we monitored the dynamics of the detected sources in the core of R136 from a ground-based telescope with adaptive optics. The aim of these observations was to search for High prOper Motion cAndidates (HOMAs) in the central region of R136 (r < 6″) where it has been challenging for other instruments. Two bright sources (K < 15 mag and V < 16 mag) are located near R136a1 and R136c (massive WR stars) and have been identified as potential HOMAs. These sources have significantly shifted in the images with respect to the mean shift of all reliable detected sources and their neighbours, and six times their own astrometric errors. We calculate their proper motions to be 1.36 ± 0.22 mas yr⁻² (321 ± 52 km s⁻¹) and 1.15 ± 0.11 mas yr⁻² (273 ± 26 km s⁻¹). We discuss different possible scenarios to explain the magnitude of such extreme proper motions, and argue for the necessity to conduct future observations to conclude on the nature of HOMAs in the core of R136.