Multiple Supernova Remnant Models of the Local Bubble and the Soft X-Ray Background

This paper examines the possibility that a series of two or three supernovae in the diffuse interstellar medium might be capable of generating a long-lived hot diffuse bubble with characteristics like those inferred for the Local Bubble surrounding the Sun. Several examples of multiple supernova remnants are explored with a one-dimensional hydrocode able to follow the nonequilibrium ionization and dust destruction. The results are generalized via scaling parameters, and a set of input parameters are identified, which should in fact lead to a bubble satisfying all known constraints on size, external pressure, external density, X-ray surface brightness and band ratios, the upper limit to X-ray emission in the 0.5-1 keV range, and the O VI absorption line characteristics. The exploratory models are a unique example of the detailed behavior of hot interstellar bubbles, and their characteristics are presented in considerable detail. Included are the distributions of temperature, density, and pressure, as well as the evolutions of those distributions with time, the evolution of bubble radius during both expansion and contraction, the evolution of the X-ray emission in observed bands and specific observed lines, a diagnostic for normalizing the X-ray surface brightness to radius, pressure, and central temperature, the radial dependence of X-ray emissivity, the full X-ray and EUV spectrum, the equivalent widths of potentially observable absorption lines, the specific characteristics of the O VI absorption (column density, centroid velocity, width), the electron column density, the emission line intensities for O VI, Fe X, and Fe XIV, the degree to which dust can be expected to have been destroyed, and the effects of incomplete dust destruction on everything else. Observations have shown that the emission lines of iron from the Local Bubble are weak or absent. This is somewhat surprising, since considerable grain destruction is expected, even if only by thermal sputtering. The result is similar to other hints that much of the iron is in its own population of dust and is very resistant to destruction. In our models, having iron alone depleted by a factor of 10 makes it much easier to accommodate the X-ray emission characteristics. Having a successful model also requires that thermal conduction within the hot bubble occurs at roughly half of its full classical rate. For substantially lower thermal conductivity, the temperature is too high to get the spectral characteristics right, and the density is too low at the highest allowed pressure to achieve the observed brightness. Our fiducial model assumes that three supernovae took place in the vicinity of the Sun over a period of 3 million years, the last having occurred a few million years ago. We show that such accidental occurrences should be sufficiently common to provide about 1 chance in 30 (with considerable uncertainty) of being found within one. Certainly supernovae occur outside of OB associations, and direct evidence for a nearby supernova some 5 million years ago that could have been responsible for the latest reheating of the Local Bubble has recently been found in the excess abundance of ⁶⁰Fe in a deep ocean ferromanganese crust.