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

Abstract

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 HD 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 HD 93129A (type O2 If*) was detected; we measure flux densities of S<SUB>3cm</SUB> = 2.0 ± 0.2 mJy and S<SUB>6cm</SUB> = 4.1 ± 0.4 mJy. The resulting spectral index of α = -1.2 ± 0.3 (S<SUB>v</SUB> ∝ v<SUP>α</SUP>) 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 = 5.1 × 10<SUP>-5</SUP> M⊙ yr<SUP>-1</SUP>, assuming the thermal fraction of the 3 cm emission is ∼0.5.