Small-scale interstellar medium structure: The remarkable sight line toward μ crucis

We present very high resolution (Δv ≈ 0.4 km s^-1) observations of the interstellar Na I D and Ca II K lines toward the binary μ Cru, which consists of two B stars separated by 38.8″ at a distance of 170 pc. These observations reveal line strength variations in four velocity components, indicative of interstellar medium structure on scales less than the projected binary separation of 6600 AU (0.03 pc). The components exhibiting the greatest Na I column density variations have the narrowest line widths and the largest N(Na I)/N(Ca II) ratios. The most visually striking of these variations involves a Na I component seen at a heliocentric velocity of -8.6 km s^-1 toward μ¹ Cru but not at all toward μ² Cru. This component has an appreciable column density [N(Na I) = 7 X 10¹⁰ cm^-2], and it clearly exhibits hyperfine splitting with a Na I line width (b = 0.4 km s^-1) indicative of a cloud temperature below 220 K. It is likely that the μ Cru variations are sampling a widespread pattern of small-scale structure in the cold diffuse interstellar gas that could easily have been missed in previous observations.