Diffuse x-ray emission from the quiescent superbubble M17, the omega nebula

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 × 10³³ ergs s^-1 and the hot gas temperature of ∼8.5 × 10 ⁶ K and mass of ∼1 M⊙ to model predictions. We find that bubble models with heat conduction overpredict the X-ray luminosity by 2 orders of magnitude; the strong magnetic fields in M17, as measured from H I 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 dumpiness, and strong magnetic field in the ambient interstellar medium.