Do lognormal column-density distributions in molecular clouds imply supersonic turbulence?

K. Tassis, D. A. Christie, A. Urban, J. L. Pineda, T. Ch Mouschovias, H. W. Yorke, H. Martel

Research output: Contribution to journalArticlepeer-review


Recent observations of column densities in molecular clouds find lognormal distributions with power-law high-density tails. These results are often interpreted as indications that supersonic turbulence dominates the dynamics of the observed clouds. We calculate and present the column-density distributions of three clouds, modelled with very different techniques, none of which is dominated by supersonic turbulence. The first star-forming cloud is simulated using smoothed particle hydrodynamics; in this case gravity, opposed only by thermal-pressure forces, drives the evolution. The second cloud is magnetically subcritical with subsonic turbulence, simulated using non-ideal magnetohydrodynamics; in this case the evolution is due to gravitationally-driven ambipolar diffusion. The third cloud is isothermal, self-gravitating and has a smooth density distribution analytically approximated with a uniform inner region and an r-2 profile at larger radii. We show that in all three cases the column-density distributions are lognormal. Power-law tails develop only at late times (or, in the case of the smooth analytic profile, for strongly centrally concentrated configurations), when gravity dominates all opposing forces. It therefore follows that lognormal column-density distributions are generic features of diverse model clouds, and should not be interpreted as being a consequence of supersonic turbulence.

Original languageEnglish (US)
Pages (from-to)1089-1094
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - Oct 2010


  • ISM: clouds
  • ISM: structure
  • Methods: numerical
  • Methods: statistical
  • Stars: formation
  • Turbulence

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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