A very small interstellar neutral hydrogen cloud observed with VLBI techniques

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Abstract

The letter reports VLBI observations of neutral hydrogen absorption in front of quasar 3C 147. The results show a variation in the optical depth of interstellar H I with a scale no larger than 0.16 arcsec. Assuming that a cloud of higher density superposed on a slowly varying background is being observed, a column density of 10 to the 20th power atoms/sq cm is estimated for the H I cloud along with a maximum probable distance of 600 pc, a linear diameter of 70 AU, a density of 100,000 atoms/cu cm, and a mass of 3 ten-millionths of a solar mass. It is noted that this cloud is smaller by several orders of magnitude than any H I cloud previously observed and that its size and density are more similar to those associated with dense molecular clouds.

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