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

2. Data reduction

Using the original list of Na I column density measurements towards 293 stars lying within 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 is the Hipparcos parallax in mas (milli-arcsec), the standard error on , and d the distance in pc (), the following three criteria were used simultaneously for the selection of stars within 300 pc: pc; ; .

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 300 pc.

In Figs. 1a-c we plot the galactic distribution of total integrated line-of-sight sodium column density, , 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 10.30 cm^-2, 10.30 11.0 cm^-2 and log 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 and that of N(HI).

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 50 pc, b  d = 51-100 pc, and c  d 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 . 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).

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 are plotted. The dotted line delineates a possible neutral absorption boundary to the Local Bubble cavity.

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
helpdesk.link@springer.de