THE LOCAL-DISTRIBUTION OF NA-I INTERSTELLAR GAS

We present high-resolution absorption measurements (lambda/Delta lambd a similar to 75,000) of the interstellar Na I D lines at 5890 Angstrom toward 80 southern hemisphere early-type stars located in the local i nterstellar medium (LISM). Combining these results with other sodium m easurements taken from the literature, we produce galactic maps of the distribution of neutral sodium column density for a total of 293 star s generally lying within similar to 250 pc of the Sun. These maps reve al the approximate shape of the mid-plane contours of the rarefied reg ion of interstellar space termed the Local Bubble. Its shape is seen a s highly asymmetric, with a radius ranging from 30 to 300 pc, and with an average radius of 60 pc. Similar plots of the Galactic mid-plane d istribution of sources emitting extreme ultraviolet radiation show tha t they also trace out similar contours of the Local Bubble derived fro m Na I absorption measurements. We conclude that the Local Bubble abso rption interface can be represented by a hydrogen column density, N-H = 2 x 10(19) cm(-2), which explains both the local distribution of Na I absorption and the observed galactic distribution of extreme ultravi olet sources. The derived mid-plane contours of the Bubble generally r eproduce the large-scale features carved out in the interstellar mediu m by several nearby galactic shell structures.