Could the Local Cavity be an Irregularly Shaped Strömgren Sphere?

Extending for 50–200 pc in all directions from the Sun, the Local Cavity has been characterized as an old supernova bubble consisting of low-density million-degree plasma heated by supernova shocks. We summarize the arguments for and against this model and conclude that hydrogen in the Local Cavity is fully ionized, and the plasma near the Galactic plane is mostly warm (10,000–20,000 K) rather than hot (10 6 K). The brightest extreme-ultraviolet source detected in the EUVE all-sky survey is the star ϵ CMa. Its EUV radiation photoionizes the outer layers of the Local Interstellar Cloud and other nearby warm interstellar clouds despite the star’s 124 pc distance. Pulsar dispersion measures indicate an electron density of 0.012 cm −3 in the Local Cavity itself. At this density the Strömgren sphere of ϵ CMa is as large as the Local Cavity. We propose that the Local Cavity is an irregularly shaped Strömgren sphere containing a small percentage of hot gas likely in many filamentary structures. We also propose that shocks from recent supernovae encountered pre-existing Strömgren sphere gas, and that the partially ionized Local Interstellar Cloud and other nearby clouds could have been formed when supernova shocks encountered regions with relatively weak magnetic fields producing compression, higher density, and recombining hydrogen.