RADIATIVE MAGNETIZED THERMAL CONDUCTION FRONTS

We study the evolution of plane-parallel magnetized thermal conduction fronts in the interstellar medium (ISM). Separating the coronal (∼ 106 K) ISM phase and interstellar clouds, these fronts have been thought to be the site of the intermediate-temperature regions whose presence was inferred from O VI absorption-line studies. We follow the front evolution numerically, starting from the initial discontinuous temperature distribution between the hot and cold medium, and ending in the final cooling stage of the hot medium. Initially, electron thermal conduction transports energy from the hot to the cold medium, evaporating and heating the cold gas to temperatures slightly over half of the initial hot medium temperature. The front broadens continuously until radiative losses balance conductive heating in the evaporated gas. Subsequently, the evaporated gas cools down and a condensing front propagates into the hot medium. For the typical ISM pressure of 4 × 103 K cm-3 and the hot medium temperature of 106 K, the transition from evaporation to condensation in a nonmagnetized front occurs when the front thickness is 15 pc. This thickness is a factor of 5 smaller than previously estimated. The O Vi column densities in both evaporative and condensation stages agree with observations if the initial hot medium temperature Th exceeds 7.5 × 105 k. Condensing conduction fronts give better agreement with observed O VI line profiles because of lower gas temperatures. Collisionally excited C and O lines in both evaporative and condensing fronts, as well as the line He II λ304 in the evaporative front, could dominate the diffuse ultraviolet and extreme ultraviolet line emission. A fraction of the observed soft X-ray background may also be produced in a conduction interface. An ordered magnetic field reduces conductive energy transport, but the reduction is substantial only for large inclinations of magnetic field lines in the hot gas to the front normal.