Emission measure from the gas expected from a bow shock as described in Wilkin (1996), normalized to the expected background emission measure of the entire flow. The computed emission profile at resolution 0.26′′ (the resolution of the KPNO Hα image) has been convolved with a Gaussian of width 1′′ to account for (typical) astronomical seeing at the wavelength of Hα. A cut through the stagnation point and the star is shown, adopting wind parameters from Howarth & Prinja (1989) and ISM parameters calculated in Sect. 5.