Radio observations of HD 93129A: The earliest O star with the highest mass loss?

We present the results of radio continuum observations towards the open cluster Tr 14, where our main targets are the early-type O stars HD 93129A/B and HID 93128. The observations were carried out at 3 cm (8.64 GHz) and 6 cm (4.80 GHz) with the Australia Telescope Compact Array. Only HD93129A (type O2 If*) was detected; we measure flux densities of S-3cm = 2.0 +/- 0.2 mJy and S-6cm = 4.1 +/- 0.4 mJy. The resulting spectral index of alpha = -1.2 +/- 0.3 (S-v proportional to v(alpha)) indicates predominantly non-thermal emission, suggesting HD 93129A may be a binary system. We propose that the observed 3 cm radio emission is mostly coming from the non-thermal wind collision region of a binary, and, to a lesser extent, from the thermal winds of the primary and secondary stars in HD 93129A. At a stellar distance of 2.8 kpc, we derive a mass-loss rate (M) over dot = 5.1 x 10(-5) M-. yr(-1), assuming the thermal fraction of the 3 cm emission is similar to0.5.