High resolution observations of interstellar Ca II and Na I in the 30 Doradus nebula

High-dispersion optical observations (FWHM ~ 5–15 km/s) of interstellar gas external to our own Galaxy are presented. Interstellar Ca II has been studied in three sight-lines associated with 30 Dor in the Large Magellanic Cloud; for the brightest source in the nebula, namely R136, complementary Na Iions have been made. All sight-lines show complex interstellar profiles with three principal components occurring between +239 and +300 km/s; the gas exhibits a complicated distribution with a relatively high velocity spread. Total LMC column densities are in the range: N(Ca II) =$$6-8\times\,{10}^{12}\,\text{cm}^{-2}$;$ for R136, N(Na I) =$$\,7.3\,\times\,{10}^{12}\,\text{cm}^{-2}.$$ In this direction one interstellar cloud exhibits an unusually large Ca II/Na I ratio (~ 11) whereas the other two components have values typical of low-velocity galactic gas (≲ 1). Reddening, E (B– V), in 30Dor is estimated to be 0.24mag. A low-velocity (+18km/s) galactic cloud occurs in this direction. Its Na I column density is consistent with a foreground reddening of 0.08 mag. There is no evidence of strongly absorbing interstellar clouds in the outer halo of the Galaxy, although high-velocity, low-density clouds may exist.