  In general, HII regions do not show clear signs of self-enrichment in
products from massive stars (M > 8 M_sun). In order to explore why, I modeled
the contamination with Wolf-Rayet star ejecta of metal-poor (Z=0.001) HII
regions, ionised either by a 10^6 M_sun cluster of coeval stars (cluster 1), or
a cluster resulting from continuous star formation at a rate of 1 M_sun yr^-1
(cluster 2). The clusters have Z=0.001 and a Salpeter initial mass function
(IMF) from 0.1 to 120 M_sun. Independent one dimensional constant density
simulations of the emission-line spectra of unenriched HII regions were
computed at the discrete ages 1, 2, 3, 4, and 5 Myr, with the photoionisation
code CLOUDY, using as input, radiative and mechanical stellar feedbacks
predicted by the evolutionary synthesis code STARBURST99. Each HII region was
placed at the outer radius of the adiabatically expanding superbubble of Mac
Low and McCray (1988). For models with thermal and ionisation balance
time-scales of less than 1 Myr, and with oxygen emission-line ratios in
agreement with observations, the interior of the superbubble and the HII region
were uniformly and instantaneously polluted with stellar ejecta predicted by
STARBURST99. I obtained a maximum oxygen abundance enhancement of 0.025 dex,
with cluster 1, at 4 Myr. It would be unobservable.
