Dust and Atomic Hydrogen near ρ Ophiuchi

We discuss the distribution of dust and atomic hydrogen in and around a small dark cloud nearρ Ophiuchi. Although the obscuration reaches values as high as 8 magnitudes, the values ofN _H derived inside the cloud do not differ markedly from those found at nearby, less obscured points. At low values of the obscuration, dust and hydrogen isophotes show satisfactory agreement, i.e.,N _H increases withN _G. However, we find thatN _H starts decreasing for sufficiently large values ofN _G. In the entire range of optical extinctions investigated our observations are satisfied by an expression of the typeN _H=AN _G exp (-BN _G). Interpretation of this relationship suggests that an appreciable part of the hydrogen has been converted to molecular form inside the cloud. A tentative method for calculating the time elapsed since the conversion process began is proposed.