The extraordinary outflow toward G5.89-0.39

We have observed four rotational transitions of SiO in its ground Vibrational state toward the ultracompact H II region and outflow source G5.89-0.39, using the Pico Veleta 30 m and Effelsberg 100 m radio telescopes and the Very Large Array radio interferometer. This allowed us to probe the low-velocity gas in the flow ''hidden'' from view in CO observations owing to contamination by ambient gas and self-absorption The projected flow diameter is 0.08 pc with a northeast-southwest bipolarity. The thermal pressure (P-therm/k) in the outflowing gas, determined from a statistical equilibrium analysis of the SiO lines, is about 10(8) K cm(-3). Based on NH3 and CO observations, the kinetic temperature of the outflowing gas is around 100 K. The combined results imply densities of order 10(6) cm(-3) in the outflow region. The fractional abundance of SiO relative to H,, based on comparisons of our data with CO spectra of Choi et al., is 1 to 3 x 10(-9) in the outflowing gas. We estimate the total mass in the outflow to be about 77 M(.).