Simulations of supernova remnants in diffuse media and their application to the lower halo of the Milky Way. I. The high-stage ions

This paper reports on detailed, nonequilibrium hydrodynamic simulations of supernova remnants (SNRs) evolving in a warm, low-density, nonthermal pressure-dominated ambient medium (T = 10(4) K, n = 0.01 cm(-3), P-nt = 1.8 x 10(3) K cm(-3)), with the goals of characterizing their structure and C+3, N+4 and O+5 content, emission, and line profiles and investigating the effects of supernova remnants in the lower Galactic halo. If undisturbed by external objects, these remnants have great longevity, surviving for similar to 1.7 x 10(7) yr. During the adiabatic phase, they contain large quantities of C+3, N+4, and O+5 in the hot gas behind their shock fronts. They emit brightly in the ultraviolet resonance lines and would appear edge brightened to observations of column density and emission. At the end of the adiabatic phase, each SNR develops a zone of cooling and recombining C+3, N+4, and O+5 in th, transition region between the hot bubble and the cool shell. The resonance line luminosities plummet, and the edge brightening diminishes. As the remnants evolve, the interiors cool faster than the ions can recombine to their equilibrium levels. Thus, during most of the remnants' lifetimes the C+3 line widths are smaller than expected from collisional equilibrium, and after the remnants have completely cooled, some C+3 remains. The O+5, N+4, and C+3 distributions overlap incompletely. The O+5 ions are more plentiful in the warmer gas at smaller radii than are the N+4 or C+3 ions. As a result, after the shell forms the thermal pressure in the O+5-rich gas is at least twice as large as that in the C+3-rich gas. During most of its lifetime, the remnant's interior is less than 10(6) K. Therefore, the fraction of area covered or volume filled by very hot SNR gas is much smaller than that filled by warm SNR gas. These simulations have been combined with the statistical distribution of isolated supernova progenitors in order to derive rough estimates of the appearance of the ensemble of isolated supernova remnants in the lower halo. The agreement between the simulation results and observational results in terms of average column density and spatial patchiness shows that much, if not all, of the high-latitude O+5, N+4, and C+3 between the local bubble and roughly a kiloparsec can be attributed to isolated SNRs in the lower halo. The simulations may also be of interest to studies of the external galaxies and the hypothesis that the Local Bubble is a single supernova remnant evolving in a low-density ambient medium.