Observational Constraints on Superbubble X-ray Energy Budgets
  The hot, X-ray-emitting gas in superbubbles imparts energy and enriched
material to the interstellar medium (ISM) and generates the hot ionized medium,
the ISM's high-temperature component. The evolution of superbubble energy
budgets is not well understood, however, and the processes responsible for
enhanced X-ray emission in superbubbles remain a matter of debate. We present
Chandra ACIS-S observations of two X-ray-bright superbubbles in the Large
Magellanic Cloud (LMC), DEM L50 (N186) and DEM L152 (N44), with an emphasis on
disentangling the true superbubble X-ray emission from non-related diffuse
emission and determining the spatial origin and spectral variation of the X-ray
emission. An examination of the superbubble energy budgets shows that on the
order of 50% of the X-ray emission comes from regions associated with supernova
remnant (SNR) impacts. We find some evidence of mass-loading due to swept-up
clouds and metallicity enrichment, but neither mechanism provides a significant
contribution to the X-ray luminosities. We also find that one of the
superbubbles, DEM L50, is likely not in collisional ionization equilibrium. We
compare our observations to the predictions of the standard Weaver et al. model
and to 1-D hydrodynamic simulations including cavity supernova impacts on the
shell walls. Our observations show that mass-loading due to thermal evaporation
from the shell walls and SNR impacts are the dominant source of enhanced X-ray
luminosities in superbubbles. These two processes should affect most
superbubbles, and their contribution to the X-ray luminosity must be considered
when determining the energy available for transport to the ISM.
