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The yellow D lines from neutral sodium are some of the most useful interstellar absorption features for tracing out low density optically thin interstellar material. Hobbs (1978) compared column densities of Nal as determined from the D lines with HI column densities derived from ultraviolet observations and found values of N(NaI)/N(H)<6.3x10-9 in 18 sight-lines with very low reddening, with the exact value of the ratio apparently a function of temperature. If this ratio limit holds for diffuse clouds in nearby space, we expect that observations of the Nal D2 lines at the 3 mA level can be used to trace out nearby clouds with column densities in the range of ≥2.5x1018 cm-2. Thus sensitive measurements of the Nal D lines are a useful way of observing nearby neutral cloud material. With this in mind, we have chosen high-resolution observations of the yellow sodium lines as a means of locating and mapping nearby low density interstellar clouds.
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