The Origins and Physical Properties of the Complex of Local Interstellar Clouds

Abstract

The Complex of Local Interstellar Clouds (CLIC) is a relatively tight grouping of low density, warm, partially ionized clouds within about 15 pc of the Solar System. The Local Interstellar Cloud (LIC) is the cloud observed on most lines of sight and may be the cloud that immediately surrounds our Solar System, the properties of which set the outer boundary conditions of the heliosphere. Using absorption line data toward nearby stars, in situ observations of inflowing interstellar gas from spacecraft in the Solar System, and theoretical modeling of the interstellar radiation field and radiative transfer, we can deduce many characteristics of the LIC. We find that the LIC is partially ionized with modest electron density, n(e) ~ 0.07 cm^-3. The combination of its temperature and ionization favor photoionization/thermal equilibrium over a non-equilibrium cooling cloud picture. The abundances in the LIC suggest moderate dust destruction for silicate dust but complete destruction of carbonaceous grains. An origin for the LIC as a density enhancement in the ambient medium that has been overrun by a shock seems likely, while its velocity away from the Sco-Cen association points to a possible connection to that region and the Loop I bubble.