The radiation environment of the local interstellar medium

Abstract

The heliosphere and the very local interstellar medium are closely connected. Understanding the photoionization of the Local Interstellar Cloud (LIC) is essential to both areas of study. The nature of the interstellar radiation field in the LIC is largely determined by the density, temperature and morphology of the local interstellar medium (LISM). The LISM is dominated by the large, low density region known as the Local Cavity or Local Bubble. Despite the low HI density extending 50–250 pc in all directions, no O stars have low enough absorption to contribute significantly to the ionizing radiation field in the LIC. The brightest stellar sources of ionizing radiation are two B stars ( ϵ CMa and β CMa) and several white dwarf stars. Most of the H ionizing photons, however, appear to be contributed by emission from the hot ( T∼10 6 K) gas that fills the Local Bubble. An additional possible source is EUV emission generated at the interface of the hot gas and the LIC. Understanding the ionization of the LIC requires knowledge of the diffuse EUV radiation field, which remains very uncertain. I discuss the nature of all of these contributors to the radiation environment of local interstellar medium.