  A high quality, medium-resolution K-band spectrum has been obtained of the
ultracompact HII region G29.96-0.02 with the Very Large Telescope (VLT). The
slit was positioned along the symmetry axis of the cometary shaped nebula.
Besides the spectrum of the embedded ionizing O star, the long-slit observation
revealed the rich emission-line spectrum produced by the ionized nebula with
sub-arcsec spatial resolution. The nebular spectrum includes Br$\gamma$,
several helium emission lines and a molecular hydrogen line. A detailed
analysis is presented of the variation in strength, velocity and width of the
nebular emission lines along the slit. The results are consistent with previous
observations, but the much better spatial resolution allows a critical
evaluation of models explaining the cometary shape of the nebula. Our
observations support neither the wind bow shock model nor the champagne flow
model. The measured line ratios of the nebular hydrogen and helium lines are
compared to predictions from case B recombination-line theory. The results
indicate an electron temperature between 6400 and 7500 K, in good agreement
with other determinations and the Galactocentric distance of 4.6 kpc. The
He$^{+}$/H$^{+}$ ratio is practically constant over the slit; we argue that He
is singly ionized throughout the nebula. We review the various observational
constraints on the effective temperature of the ionizing star and show that
these are in agreement with its K-band spectral type of O5-O6V.

Astrophysics