We report on the highest angular resolution observation to date of the bright core, R136a, of the massive star cluster R136 within the 30 Doradus complex in the LMC. This visual observation was obtained with the interferometric fringe mode of operation of Fine Guidance Sensor No. 3 (FGS3) on board HST. Crowding and strong diffuse background from nebular emission make this a challenging observation.
The giant HII region 30 Dor has provided some of the best candidates for the most massive stars, like R136al. We provide evidence for a new component, R136a1B, within the previously known R136a1-a2 system with a separation of 80 mas (or ≈ 4000 au from a1 at the distance of the LMC), and δV = 1.1 mag fainter than the brightest component a1. Estimates from current evolutionary models of massive stars based on the new FGS photometry predict, after subtraction of a1B, that the present mass of R136a1 is 30 M⊙ with a main sequence progenitor of 60 M⊙. To date, this is the lowest direct estimate of the mass of R136a1.
The success of this difficult observation adds a new, unique feature to FGS3 and gives a much expanded, astrophysically very rewarding, role to the interferometer.