Mapping the contours of the Local bubble: preliminary results

We present preliminary results from a long-term program of mapping the neutral absorption characteristics of the local interstellar medium, taking advantage of Hipparcos stellar distances. Equivalent widths of the NaI D-line doublet at 5890 Å 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 previously published NaI absorption measurements towards a further 313 nearby targets, we present absorption maps of the distribution of neutral gas in the local interstellar medium as viewed from 3 different galactic projections. In particular, these maps reveal the Local Bubble region as a low neutral density interstellar cavity in the galactic plane with radii between 65-250 pc 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 al. (\cite{snowden98}) using ROSAT soft X-ray emission data. Consistency in the global dimensions derived for both sets of contours is found for the case of a million degree hot LB plasma of emissivity 0.0023 cm(-6) pc with an electron density of 0.005 cm(-2) . We have detected only one relatively dense accumulation of cold, neutral gas within 60 pc of the Sun that surrounds the star delta Cyg, and note that the nearest molecular cloud complex of MBM 12 probably resides at the very edge of the Local Bubble at a distance of ~ 90 pc. Our observations may also explain the very different physical properties of the columns of interstellar gas in the line-of-sight to the two hot stars epsilon CMa and beta CMa as being due to their locations with respect to the Bubble contours. Finally, in the meridian plane the LB cavity is found to be elongated perpendicularly to the Gould's Belt plane, possibly being ``squeezed'' by the expanding shells of the Sco-Cen and Perseus-Taurus OB associations. Tables 1 and 2 are also available in electronic form at the CDS (Strasbourg) via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html