The Ratio of Atomic Hydrogen to Dust in the Direction of the Omega Nebula

The 21-cm line of atomic hydrogen was observed in absorption against the continuum background of the thermal source known as the Omega Nebula (M17, NGC 6618). The absorption profile was observed in directions where there is relatively little dust and also in the direction of a dense dust cloud. A de- crease in the optical depth of the 21-cm line in the latter direction indicated that in the presence of dust part of the hydrogen may become molecular. Because of the size of the beam it is not possible to make a precise quantitative determination of the decrease in the number of hydrogen atoms. The observations were made with the 30-rn dish of the I.A.R.-Carnegie Radio Astronomy Station in Argentina