The Sun is thought to lie at the edge (or just within) a warm (T 7000K) low-density ( 0.1 cm-3) interstellar cloud that is itself located within a hot (T 106 K) and rarefied ( 0.005 cm-3) region of space of approximate dimensions 100 pc called the Local Bubble (Frisch 1995). The origin of this rarefied cavity is still debated, but it seems likely that it was caused by a local supernova event and/or the interaction of the stellar winds from OB stars in the nearby Sco-Cen association. Whether the hot Bubble plasma is in collisional ionization equlibrium or not is still uncertain (Breitschwerdt and Schmutzler 1994).
Although most of the volume in the Local Bubble appears to be filled with hot gas, most of the mass is contained within either warm and partially ionized plasma, or several diffuse interstellar clouds ("fluff") of very low neutral gas density (Frisch 1995). The exact size and shape of the tenous Local Bubble cavity have been much debated. Its dimensions can be determined by either tracing the absence of absorption by neutral interstellar gas, or by determining the extent of the hot emitting Bubble gas. Direct absorption measurements of the interstellar neutral hydrogen column density, N(HI), to stars within 100 pc has generally proven to be problematic due to the observational and instrumental difficulties inherent in measuring hydrogen column densities 1018 cm-2. Therefore other indirect means of determining the extent of the low density local cavity have been used. Paresce (1984) has used both direct and indirect estimates of N(HI) absorption towards 82 stars closer than 250 pc to infer the absence of dense neutral gas out to 100 pc in most galactic directions. Observations of the diffuse soft X-ray background radiation have been modelled by Snowden et al. (1990) to reproduce contours of the observed negative correlation between X-ray intensity and neutral interstellar hydrogen column density out to 300 pc. Similarly, Diamond et al. (1995) have used ROSAT Wide-Field Camera observations of extreme ultraviolet (EUV) sources to model the distribution of inferred neutral hydrogen column densities as a function of distance to 150 pc. Welsh et al. (1994), hereafter Paper 1, have used the observed distribution of interstellar sodium (Na I) absorption within 250 pc of the Sun to infer the contours of neutral gas absorption in the local interstellar medium (LISM). The two interstellar Na I D1 and D2 absorption lines at 5890 Å are believed to be a good tracer of relatively cold (T 1000K) neutral gas clouds in the general ISM.
All of the above representations of the Local Bubble cavity are dependent on knowledge of the distances towards the respective line-of-sight stellar targets towards which the level of absorption is measured. For stars closer than 20 pc these distances (mostly derived from ground-based parallax measurements) are generally accurate to within 20%, whereas distance estimates for stars beyond 20 pc are of far greater uncertainty. The new distance determination for stars within 300 pc by the Hipparcos satellite (ESA 1997) now allow us to plot the Local Bubble absorption characteristics with much greater certainty. In this Paper, we re-plot the Na I absorption data originally presented in Paper 1 using the newly available Hipparcos stellar distances to investigate the distribution of neutral gas in the LISM. These plots show that the detailed galactic distribution of Na I absorption has changed significantly from that originally presented in Welsh et al. (1994), and that the Local Bubble cavity is some 50% larger than previously thought.
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998