The Local Interstellar Medium - a Test-Bed for the Galactic Interstellar Medium

Abstract

We outline a method to explore the column density of the Local Interstellar Medium (LISM) using absorptions in the resonance H and K lines of MgII. The intrinsic strengths of these lines in the temperature and density conditions prevailing in warm clouds (T_eff<10⁴ K) in the LISM allows them to be used to explore many lines of sight where lines such a NaD and CaII H and K are too weak, but where Lα is saturated. The number of measurable lines-of-sight is greatly enhanced by using cool stars as the background emitters, but this implies reliable separation of the LISM components from stellar chromospheric selfabsorptions. We explain how to do this, and how to use a combination of column density and radial velocity data to measure the spatial extent and the physical parameters of the single cloud in which the Sun is embedded. This proves to be an oblate spheroid, of characteristic diameter 8 pc, withT_eff ∼ 10⁴ K,n(HI) of 0.1 cm^‑3 and a mass <5M_⊙, streaming in the LSR from a point 1″=4°,B″=+16° with velocity equal to 16 km s^‑1, and is surrounded by the much hotter lower density ionized gas of the local supernova bubble.