Local clouds: Ionization, temperatures, electron densities and interfaces, from GHRS and IMAPS spectra of $\mathsf{\epsilon}$ Canis Majoris

Abstract

The composition and physical properties of several local clouds, including the Local Interstellar Cloud (LIC) in which the Sun is embedded, are derived from ab- sorption features in the UV spectrum of the star CMa. We derive temperatures and densities for three compo- nents by combining our interpretations of the ionization balance of magnesium and the relative population of Cii in an excited ne-structure level. We nd that for the LIC ne ~ =0 :12 0:05 cm 3 and T = 70001200 K. We derive the ionization fractions of hydrogen and discuss the ionizing processes. In particular the hydrogen and he- lium ionizations in the LIC are compatible with photoion- ization by the local EUV radiation elds from the hot stars and the cloud interface with the hot gas. We conrm the detection of high ionization species : Siiii is detected in all clouds and Civ in two of them, includ- ing the LIC, suggesting the presence of ionized interfaces around the local clouds.