7 - Structure and ionisation of the local interstellar medium

Summary

A view of local interstellar space

It is generally accepted that the solar system resides inside a relatively dense local interstellar cloud a few parsecs across with a mean neutral hydrogen density of ≈0.1 cm⁻³ (e.g. Frisch 1994; Gry et al. 1995). This cloud, the so-called local interstellar cloud (LIC) or surrounding interstellar cloud (SIC), lies inside a region of much lower density, often referred to as the local bubble (figure 7.1). The general picture built up is one where this bubble has been created by the shock wave from a past supernova explosion, which would also have ionised the local cloud. The current ionisation state of the local cloud is then expected to depend on the recombination history of the ionised material, i.e. the length of time since the shock wave passed through. However, if the flux of ionising photons from hot stellar sources is significant, the net recombination rate may be reduced (Cheng and Bruhweiler 1990; Lyuand Bruhweiler 1996). The photometric data from the ROSAT WFC survey have been used to map out the general dimensions of the cavity by Warwick et al. (1993) and Diamond et al. (1995), as already discussed in section 3.9.2. However, this relatively crude interpretation of the observations probably hides greater complexity. For example, several studies of the lines-of-sight towards β and ∊ CMa had already demonstrated the existence of a low density tunnel some 200–300 pc in extent, even before any EUV observations were carried out (e.g. Welsh 1991).