Mapping the contours of the Local bubble: preliminary results

We present preliminary results from a long-term program of mapping the neut ral absorption characteristics of the local interstellar medium, taking adv antage of Hipparcos stellar distances. Equivalent widths of the NaI D-line doubler at 5890 Angstrom are presented for the lines-of-sight towards some 143 new target stars lying within 300 pc of the Sun. Using these data which were obtained at the Observatoire de Haute Provence, together with previou sly published NaI absorption measurements towards a further 313 nearby targ ets, we present absorption maps of the distribution of neutral gas in the l ocal interstellar medium as viewed from 3 different galactic projections. I n particular, these maps reveal the Local Bubble region as a low neutral de nsity interstellar cavity in the galactic plane with radii between 65-250 p c that is surrounded by a (dense) neutral gas boundary (or "wall"). We have compared our iso-column contours with the contours derived by Snowden et a l. (1998) using ROSAT soft X-ray emission data. Consistency in the global d imensions derived for both sets of contours is found for the case of a mill ion degree hot LB plasma of emissivity 0.0023 cm(-6) pc with an electron de nsity of 0.005 cm(-2). We have detected only one relatively dense accumulat ion of cold, neutral gas within 60 pc of the Sun that surrounds the star de lta Cyg, and note that the nearest molecular cloud complex of MBM 12 probab ly resides at the very edge of the Local Bubble at a distance of similar to 90 pc. Our observations may also explain the very different physical prope rties of the columns of interstellar gas in the line-of-sight to the two ho t stars epsilon CMa and beta CMa as being due to their locations with respe ct to the Bubble contours. Finally, in the meridian plane the LB cavity is found to be elongated perpendicularly to the Could's Belt plane, possibly b eing "squeezed" by the expanding shells of the Sco-Cen and Perseus-Taurus O B associations.