  The emission nebula M17 contains a young ~1 Myr-old open cluster; the winds
from the OB stars of this cluster have blown a superbubble around the cluster.
ROSAT observations of M17 detected diffuse X-ray emission peaking at the
cluster and filling the superbubble interior. The young age of the cluster
suggests that no supernovae have yet occurred in M17; therefore, it provides a
rare opportunity to study hot gas energized solely by shocked stellar winds in
a quiescent superbubble. We have analyzed the diffuse X-ray emission from M17,
and compared the observed X-ray luminosity of ~2.5*10^33 ergs/s and the hot gas
temperature of ~8.5*10^6 K and mass of ~1 M_Sun to model predictions. We find
that bubble models with heat conduction overpredict the X-ray luminosity by two
orders of magnitude; the strong magnetic fields in M17, as measured from HI
Zeeman observations, have most likely inhibited heat conduction and associated
mass evaporation. Bubble models without heat conduction can explain the X-ray
properties of M17, but only if cold nebular gas can be dynamically mixed into
the hot bubble interior and the stellar winds are clumpy with mass-loss rates
reduced by a factor of >=3. Future models of the M17 superbubble must take into
account the large-scale density gradient, small-scale clumpiness, and strong
magnetic field in the ambient interstellar medium.
