On the origin and structure of isolated dark globules

Abstract

High resolution carbon monoxide observations of over a dozen isolated dark globules indicate that a power-law correlation exists between the internal velocity dispersion and cloud size. The power-law relation implies the presence of supersonic turbulence that is similar to that found in other diffuse clouds and molecular cloud complexes. Thus the observed motions in globules may be all part of a common hierarchy of interstellar tubular motions. Most of the gobules are found to be gravitationally bound and in virial equilibrium. The average gas density seems to decrease with increasing cloud size, suggesting that gravitational contraction may account for the difference in gas density. The effects of gravitational contraction, which tends to increase the central gas density, coupled with the more rapid dissipation of turbulent motions in high density regions may account for the observed difference in velocity dispersion in different parts of a cloud. The results are consistent with the scenario that isolated dark globules represent condensed fragments from nearby filamentary cloud complexes, the formation of which depends strongly on the properties of interstellar turbulence.