The orbit of the most massive known astrometric binary

Maíz Apellániz, Jesús
HST Proposal ID #11784. Cycle 17


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. Due the the non-recovery of HRC on SM4, the last visit was changed to STIS.;