HST Proposal ID #10898. Cycle 15
We have recently used FGS and HRC observations to a resolve HD 93129A into two components with very similar optical/UV colors and a magnitude difference of 0.9 and to b detect their relative orbital motion over a span of 8 years. HD 93129Aa is the prototype O2 If* star, with an evolutionary mass near 100 M_Sun, while Ab is likely to be a very early O main-sequence star with a similar or only slightly smaller mass. Our HST astrometric measurements yield a total mass above 100 M_Sun, thus confirming the extremely high mass of the binary, and indicate that the system appears to be approaching periastron. We request new FGS and HRC observations to a calculate the mass ratio of the system by measuring the orbit of each of the components with respect to the nearby stars, b obtain the periastron epoch, and c start measuring the orbit in order to produce an estimate of the total mass. These measurements are crucial to shed light on the value of the stellar upper mass limit.