Title: Ionization Sources of the Local Interstellar Clouds

Abstract: The structure of the local interstellar clouds is influenced by stellar ionizing sources that shaped it dynamically and thermally over the last 5 million years. The dominant sources include two massive B-type stars, currently at distances of 124 and 151 pc in the Canis Majoris constellation, and three hot white dwarfs (at 52, 60, and 77 pc). Our solar system is surrounded by wispy interstellar gas clouds, 30 lt yr in extent, which shield us from most of this ionizing radiation. However, 4.4 million years ago, those two B-stars stars passed within 30–35 light years of the Sun, leaving behind a trail of ionized and photoelectrically heated gas. Additional ionizing photons come from the Local Hot Bubble, a glow of million-degree plasma generated by shock waves from past supernova explosions in the nearby Scorpius – Centaurus OB association. The unusual level of ionized hydrogen and helium entering the heliosphere likely reflects these sources, altered by the propagation and filtering of EUV and X-ray radiation through the clouds.