Average density along interstellar lines of sight

Recent studies indicate that the depletion of certain elements in the interstellar gas is well correlated with the mean particle density of hydrogen along each line of sight. It is shown that these results can be understood in terms of a simple theoretical model, based on random distributions of the two types of cold clouds which fit the observed distribution of color excess, E(B-V), together with a more uniform warm H I gas of lower density. With increasing mean particle density of hydrogen, the relative contributions of these three constituents to NH change, with the low-density warm gas predominant at low mean particle density of hydrogen (less than 0.2 per cu cm), the 'standard' diffuse clouds at intermediate mean particle density of hydrogen (about 0.7 per cu cm), and the more absorbing 'large' clouds or cloud complexes at the highest mean particle density of hydrogen (greater than 3 per cu cm). Hence, any correlations of physical properties with mean particle density of hydrogen may be attributed in part to differences of these properties between such constituents of the interstellar gas.