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

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2. Data reduction

Using the original list of Na I column density measurements towards 293 stars lying within [FORMULA] 250 pc of the Sun presented in Paper 1, we have assigned each target with an improved distance estimate using data from the on-line Hipparcos catalog.

If [FORMULA] is the Hipparcos parallax in mas (milli-arcsec), [FORMULA] the standard error on [FORMULA], and d the distance in pc ([FORMULA]), the following three criteria were used simultaneously for the selection of stars within 300 pc: [FORMULA] pc; [FORMULA] ; [FORMULA].

For the secondary components of multiple star systems, the parallax of the primary was eventually preferred. The final list thus contains 273 stars with distances [FORMULA] 300 pc.

In Figs. 1a-c we plot the galactic distribution of total integrated line-of-sight sodium column density, [FORMULA], as a function of galactic coordinates for stars in three distance groupings: 0-50 pc, 51-100 pc, and 101-300 pc. To remain consistent with the previous presentation of the data in Paper 1, we have plotted the data at three levels of sodium column density i.e. log [FORMULA] [FORMULA] 10.30 cm-2, 10.30 [FORMULA] [FORMULA] [FORMULA] 11.0 cm-2 and log [FORMULA] [FORMULA] 11.0 cm-2. The astrophysical reasons for the choice of these column density plotting levels are outlined in Paper 1, Sect. 3.1., and are based on the assumption of an empirical relationship between [FORMULA] and that of N(HI).

[FIGURE] Fig. 1a-c. Plots of the galactic spatial distribution of the total Na I column density for stars with three distance ranges (as determined by Hipparcos): a  d [FORMULA] 50 pc, b  d = 51-100 pc, and c  d [FORMULA] 101 pc.

In addition, in Fig. 2 we show the mid-plane extent of Na I absorption to 300 pc using a polar projection plot of stars whose galactic latitude lies between [FORMULA] [FORMULA] [FORMULA]. This plot is directly comparable to that of Fig. 3 in Paper 1, with the distances being actual and not projected values (see Paper 1 for details).

[FIGURE] Fig. 2. A polar projection of the line-of-sight Na I column densities presented in Fig. 1 as a function of distance from the Sun and galactic longitude. Only stars with galactic latitude between [FORMULA] [FORMULA] [FORMULA] are plotted. The dotted line delineates a possible neutral absorption boundary to the Local Bubble cavity.

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© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998