The Origin of the Cluster of Local Interstellar Clouds

Abstract

The interstellar medium within $\rm\approx 15 \; pc$ of the Sun consists of a complex of fifteen diffuse, partially ionized clouds. Located within the Local Bubble, these clouds, known as the Cluster of Local Interstellar Clouds (CLIC), constitute the interstellar environment impinging upon our heliosphere. While each individual cloud can be modeled with a distinct velocity vector, the complex demonstrates a coherent bulk motion suggestive of a common origin. Here we examine two theories for the origin of the CLIC: that it formed due to an ionization front associated with nearby Str\"{o}mgren spheres and/or due to a nearby supernova explosion that occurred within the pre-evacuated cavity of the Local Bubble. Tracing back the trajectory of the clouds, we disfavor a purely Str\"{o}mgren sphere origin, given the CLIC's position interior to the surface of the most significant nearby Stromgren sphere and its motion transverse to the sphere's trajectory. Turning to a supernova origin, we model the formation of the CLIC assuming individual clouds have been swept up over time due to the expansion of a supernova remnant in its pressure-driven snowplow phase. We find that the 3D spatial-dynamical properties of the CLIC can be explained by the most recent supernova that exploded in the nearby Upper Centaurus Lupus cluster $\approx \rm 1.2 \; Myr$ ago and propagated into an ambient density of $n \approx 0.04 \;\rm cm^{-3}$. Our model predicts that the formation of the individual CLIC clouds occurred progressively over the past $1 \; \rm Myr$ and offers a natural explanation for the observed distribution, column density, temperature, and magnetic field structure of the complex.