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We revisit the problem of the star formation timescale and the ages ofmolecular clouds. The apparent overabundance of star-forming molecular cloudsover clouds without active star formation has been thought to indicate thatmolecular clouds are "short-lived" and that star formation is "rapid". We showthat this statistical argument lacks self-consistency and, even within therapid star-formation scenario, implies cloud lifetimes of approximately 10 Myr.We discuss additional observational evidence from external galaxies thatindicate lifetimes of molecular clouds and a timescale of star formation ofapproximately 10 Myr . These long cloud lifetimes in conjunction with the rapid(approximately 1 Myr) decay of supersonic turbulence present severedifficulties for the scenario of turbulence-controlled star formation. Bycontrast, we show that all 31 existing observations of objects for which thelinewidth, the size, and the magnetic field strength have been reliablymeasured are in excellent quantitative agreement with the predictions of theambipolar-diffusion theory. Within the ambipolar-diffusion-controlled starformation theory the linewidths may be attributed to large-scale non-radialcloud oscillations (essentially standing large-amplitude, long-wavelengthAlfven waves), and the predicted relation between the linewidth, the size, andthe magnetic field is a natural consequence of magnetic support ofself-gravitating clouds.Comment: 7 pages, 2 figures, uses emulateapj; accepted for publication in Ap

astrophysical journal/the astrophysical journal

Observational Constraints on the Ages of Molecular Clouds and the Star Formation Timescale: Ambipolar‐Diffusion–controlled or Turbulence‐induced Star Formation?