SS 433: a phenomenon imitating a Wolf-Rayet star

We present mid-infrared (2–12 μm) spectra of the microquasar SS 433 obtained with the Infrared Space Observatory (spectroscopic mode of ISOPHOT and ISOCAM). We compare them to the spectra of four Wolf-Rayet stars: WR 78, WR 134, WR 136, and WR 147 in the same wavelength range. The mid-infrared spectrum of SS 433 mainly shows $\ion{H}{i}$ and $\ion{He}{i}$ emission lines and is very similar to the spectrum of WR 147, a WN8(h)+B0.5V binary. The 2–12 μm continuum emission of SS 433 corresponds to optically thin and partially optically thick free-free emission, from which we calculate a mass loss rate of $2{-}3\times 10^{-4} ~M_\odot ~\mathrm{yr}^{-1}$ if the wind is homogeneous and a third of these values if it is clumped. This is consistent with a strong stellar wind from a WN star. However, following recent studies concluding that the mass donor star of SS 433 is not a Wolf-Rayet star, we propose that this strong wind out flows from a geometrically thick envelope of material that surrounds the compact object like a stellar atmosphere, imitating the Wolf-Rayet phenomenon. This wind could also wrap the mass donor star, and at larger distances (~40 AU), it might form a dust envelope from which the thermal emission, detected with ISOPHOT at 25 μm and 60 μm, would originate. This wind also probably feeds the material that is ejected in the orbital plane of the binary system and that forms the equatorial outflow detected in radio at distances $>100$ AU.