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Astron. Astrophys. 333, 101-105 (1998)

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3. Discussion

3.1. Lines of sight to 50 pc

It is immediately clear from Fig. 1a that in the 43 different galactic directions sampled, the LISM is lacking in any condensations of dense, neutral interstellar gas clouds with most of the lines-of-sight having low values of Na I column density (i.e. log [FORMULA] [FORMULA] 10.30 cm-2). Assuming that the corresponding HI neutral column density to these stars is typically log N(HI) [FORMULA] 18.20 cm-2 (Lallement et al. 1995), then the average density, [FORMULA], is [FORMULA] 0.01 cm-3 throughout the first 50 pc of the Local Bubble. This very low neutral gas density is at least a factor 100 below that observed in the general ISM, and we thus define the Local Bubble as a neutral gas-free region. Our present findings are in contrast to the corresponding plot of Paper 1 in which 5 isolated directions showed appreciable levels of Na I interstellar absorption within 50 pc. The present picture of the LISM in which the first 50 pc is essentially devoid of dense neutral gas in all galactic directions is also in detailed disagreement with the morphology of the LISM presented in both Paresce (1984) and Frisch (1995), who both used pre-Hipparcos stellar distances as the basis of their models. In addition, our current data do not support the presence of any nearby dense interstellar gas clouds in the directions with galactic longitudes, [FORMULA] [FORMULA] [FORMULA], as originally suggested by the optical polarization studies of Tinbergen (1982).

These new plots of the distribution of interstellar Na I absorption are in far better agreement with the findings of both the ROSAT Wide Field Camera (Diamond et al. 1995) and the Extreme Ultraviolet Explorer satellite (Vallerga 1996) in which the galactic distribution of EUV sources is consistent with a neutral gas boundary to the Local Bubble of N(HI) [FORMULA] 1019 cm-2 at a distance of [FORMULA] 70 pc in most directions from the Sun. The EUV data generally infer a mean density in the galactic plane of [FORMULA] 0.1 cm-3 within the first 20 pc, and beyond this distance the integrated density drops to less than 0.04 cm-3 until around 70 pc when the integrated density rises again to [FORMULA] 0.1 cm-3.

Note the presence of at least 7 directions in which tenous (log [FORMULA] [FORMULA] 10.6 cm-2) neutral interstellar clouds have been detected within 50 pc. These "cloudlets" (often refered to as wisps or interstellar fluff) may well be remnants of the proposed supernova explosion that may have created the Local Bubble several million years ago (Frisch 1995). Clearly, the present number of measurements of Na I absorption in all lines-of-sight [FORMULA] 50 pc (and beyond!) is insufficient to comment further on the very detailed morphology of the LISM, and many more observations of Na I are thus required for future analysis.

3.2. Lines of sight 51-100 pc

The new data in Fig. 1b are qualitatively similar to those originally presented in Paper 1. The new Fig. 1b clearly shows the presence of dense neutral gas in [FORMULA] 30% of the 58 directions sampled. However note the lack of detections of any appreciable amounts of neutral gas in the galactic quadrant centered around [FORMULA] = [FORMULA], [FORMULA] = [FORMULA] (see Sect. 3.4). Na I column densities [FORMULA] 1012 cm-2 (corresponding to N(HI) [FORMULA] 1020 cm-2) are not encountered until a distance of 70 pc. This is in excellent agreement with the findings of Diamond et al. (1995), and supports the original postulation of Paresce (1984) in which the Local Bubble cavity is bounded by a dense wall of cold neutral gas (clouds) in many galactic directions.

3.3. Lines of sight 101-300 pc

The new data of Fig. 1c are very similar to those presented in Paper 1, with the majority of the lines-of-sight sampled generally having values of log [FORMULA] [FORMULA] 12.0 cm-2. The main differences between our new data and Paper 1 are best illustrated by the mid-plane absorption characteristics presented in Fig. 2.

3.4. Mid-plane distribution of Na I absorption

The new mid-plane distribution of Na I absorption for stars with galactic latitude between [FORMULA] [FORMULA] [FORMULA] is shown in Fig. 2. The dotted line delineates the limits where Na I absorption has a column density of log [FORMULA] [FORMULA] 11.0 cm-2. This traces the possible contour of the edge of the Local Bubble cavity, which can be defined as a region deficient in neutral gas such that [FORMULA] [FORMULA] 0.01 cm-3. The differences between this plot and the one originally presented in Paper 1 are four-fold. Firstly, the central bubble cavity radius now extends in all galactic directions to at least 50 pc, and (more typically) in most directions to a radius of 70 pc. Thus, the central cavity volume is significantly larger (by more than 50%) than previously thought. Secondly, the well-known interstellar tunnel of low density neutral gas in the direction of galactic longitude, [FORMULA] = [FORMULA] towards the star [FORMULA] CMa is almost twice as wide as that originally determined by Welsh (1991). This interstellar feature of extremely low gas density (n [FORMULA] 0.005 cm-3) has typical dimensions of at least 250 pc long by 90 pc wide. Thirdly, the galactic direction towards Lupus-Norma ([FORMULA] = [FORMULA]) is now confirmed as a narrow ([FORMULA] 15 pc) interstellar tunnel that extends to a distance of at least 170 pc. Based on the upper limit values of Na I absorption derived for both [FORMULA] Lup (d = 156 pc) and [FORMULA] Lup (d = 174 pc) the neutral space density in this interstellar feature is comparable to that of the [FORMULA] CMa tunnel. Further observations of this potentially interesting region are clearly required to define this tunnel's dimensions more accurately. Finally, we note an apparent lack of dense gas in the direction of galactic longitude, [FORMULA] = [FORMULA]. Unfortunately, this direction is not well sampled by the Welsh et al. data, and thus we can presently only tentatively draw a cavity contour that extends to at least 100 pc.

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© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998