  The massive red supergiant (RSG) W26 in Westerlund 1 is one of a growing
number of RSGs shown to have winds that are ionized from the outside in. The
fate of this dense wind material is important for models of second generation
star formation in massive star clusters. Mackey et al. (2014) showed that
external photoionization can stall the wind of RSGs and accumulate mass in a
dense static shell. We use 1D R-HD simulations of an externally photoionized
wind to predict the Halpha and [NII] emission arising from photoionized winds
both with and without a dense shell. We analyse spectra of the Halpha and [NII]
emission in the environment around W26 and compare them with predicted
synthetic emission. Simulations of slow winds that are decelerated into a dense
shell show strongly limb-brightened line emission, with line radial velocities
that are independent of the wind speed. Faster winds (>22 km/s) do not form a
dense shell, have less limb-brightening, and the line radial velocity is a good
tracer of the wind speed. The brightness of the [NII] and Halpha lines as a
function of distance from W26 agrees reasonably well with observations when
only the line flux is considered. The radial velocity disagrees, however: the
brightest observed emission is blueshifted by ~25 km/s relative to the radial
velocity of the star, whereas a spherically symmetric wind has the brightest
emission at zero radial velocity. Our results show that the bright nebula
surrounding W26 must be asymmetric; we suggest it is confined by external ram
pressure from the wind of the nearby supergiant W9. We obtain a lower limit on
the nitrogen abundance within the nebula of 2.35 times solar. The line ratio
strongly favours photoionization over shock ionization, and so even if the
observed nebula is pressure confined there should still be an ionization front
and a photoionization-confined shell closer to the star.
