Ambient Interstellar Pressure and Superbubble Evolution

High ambient interstellar pressure is suggested as a possible factor to explain the ubiquitous observed growth-rate discrepancy for supernova-driven superbubbles and stellar wind bubbles. Pressures of P/k ~ 1e5 cm-3 K are plausible for regions with high star formation rates, and these values are intermediate between the estimated Galactic mid-plane pressure and those observed in starburst galaxies. High-pressure components also are commonly seen in Galactic ISM localizations. We demonstrate the sensitivity of shell growth to the ambient pressure, and suggest that superbubbles ultimately might serve as ISM barometers.