Discovery of a WO star in the Scutum-Crux arm of the inner galaxy

We report the discovery of only the fourth massive WO star to be found in the Milky Way, and only the seventh identified within the Local Group. This has resulted from the first observations made in a programme of follow-up spectroscopy of candidate emission-line stars from the AAO/UKST Southern Galactic Plane Hα Survey. The optical spectrum of this star, to become WR 93b in the Catalogue of Galactic Wolf-Rayet stars, is presented and described. WR 93b is classified as WO3 and is shown to be highly reddened [E(B -V) = 2.1 ±0.1]. A recombination-line analysis of the emission lines yields the abundance ratios C/He = 0.95 and O/He = 0.13 (by number). Comparisons at near-infrared wavelengths of reddening-corrected photometry between WR 93b and both of Sand 2 (WO3, D = 49 kpc) and Sand 5 (WO2, D = 1.75 kpc) yield a consistent distance to WR 93b of 3.4 kpc. Positioned at Galactic coordinates l = 353°.27, b = -0°85, the star is most likely located in the Scutum-Crux arm of the inner Milky Way. We note that none of the four Galactic WO stars lies significantly beyond the solar circle (with two well inside). Estimation of the wind terminal velocity in WR 93b at 5750 km s ^-1 makes this star the current wind-speed record holder among all non-degenerate stars.