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 supernova e in the diffuse interstellar medium might be capable of generating a long- lived hot diffuse bubble with characteristics like those inferred for the L ocal Bubble surrounding the Sun. Several examples of multiple supernova rem nants are explored with a one-dimensional hydrocode able to follow the none quilibrium ionization and dust destruction. The results are generalized via scaling parameters, and a set of input parameters are identified, which sh ould in fact lead to a bubble satisfying all known constraints on size, ext ernal 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 abs orption line characteristics. The exploratory models are a unique example o f the detailed behavior of hot interstellar bubbles, and their characterist ics are presented in considerable detail. Included are the distributions of temperature, density, and pressure, as well as the evolutions of those dis tributions 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 bri ghtness to radius, pressure, and central temperature, the radial dependence of X-ray emissivity, the full X-ray and EUV spectrum, the equivalent width s of potentially observable absorption lines, the specific characteristics of the O VI absorption (column density, centroid velocity, width), the elec tron column density, the emission line intensities for O VI, Fe X, and Fe X IV, the degree to which dust can be expected to have been destroyed, and th e effects of incomplete dust destruction on everything else. Observations h ave 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 o ther hints that much of the iron is in its own population of dust and is ve ry 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 charac teristics. Having a successful model also requires that thermal conduction within the hot bubble occurs at roughly half of its full classical rate. Fo r substantially lower thermal conductivity, the temperature is too high to get the spectral characteristics right, and the density is too low at the h ighest allowed pressure to achieve the observed brightness. Our fiducial mo del assumes that three supernovae took place in the vicinity of the Sun ove r a period of 3 million years, the last having occurred a few million years ago. We show that such accidental occurrences should be sufficiently commo n to provide about 1 chance in 30 (with considerable uncertainty) of being found within one. Certainly supernovae occur outside of OB associations, an d direct evidence for a nearby supernova some 5 million years ago that coul d have been responsible for the latest reheating of the Local Bubble has re cently been found in the excess abundance of Fe-60 in a deep ocean ferroman ganese crust.