HUBBLE-SPACE-TELESCOPE FINE GUIDANCE SENSOR INTERFEROMETRIC OBSERVATIONS OF THE CORE OF 30-DORADUS

We present the results of the first high angular resolution observatio ns taken with a Fine Guidance Sensor (FGS) aboard the Hubble Space Tel escope (HST) of a star cluster embedded in very bright background. The strong and complex background around the R136 cluster in the 30 Dor n ebula does not prevent the FGS from achieving performance close to its angular resolution limit of approximately 0.015'' per axis with relia ble photometry. These FGS observations establish that the central obje ct in R136a is a triple star with the third component DELTAV = 1.1 mag fainter than the primary star a1 approximately 0.08'' away. We estima te from the grid of models of Maeder (1990) that the present mass of a 1 is between 30 and 80 solar masses, with the main-sequence progenitor between 60 and 120 solar masses.