Equivalent hydrogen column density NH versus extinction in the V-band AV. For 90% of sources the uncertainties are lower than those represented by the grey error bars in the lower right corner of the figure. Median values of errors are indicated by the black error bars. The solid black, orange, and blue lines are linear fits to the entire sample, IR-excess sources only, and Class III objects only, respectively. The shaded area and dotted orange or blue lines indicate the 1σ confidence band of the fits. The dashed grey line corresponds to the NH/AV ratio of the diffuse ISM in the Milky Way, NH = 2 × 1021 cm-2 mag-1·AV.
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