Kinematics of H I Envelopes Associated with Molecular Clouds

Abstract We investigate the evolution of molecular clouds through the kinematics of their atomic hydrogen (H I ) envelopes, using 12 CO and 21 cm emission to trace the molecular and atomic gas, respectively. We measure the large-scale gradients, Ω, in the velocity fields of 22 molecular clouds and their H I envelopes, then calculate their specific angular momenta, j ∝ Ω R 2 . The molecular clouds have a median velocity gradient of 9.6 × 10 −2 km s −1 pc −1 , and a typical specific angular momentum of 2.7 × 10 24 cm 2 s −1 . The H I envelopes have smaller velocity gradients than their respective molecular clouds, with an average of Ω H I = 0.03 km s −1 pc −1 , and a median angular momentum of of j H I ≈ 5.7 × 10 24 cm 2 s −1 . For a majority of the systems, j HI > j H 2 , with an average of j HI / j H 2 = 4 . Their velocity gradient directions tend to be misaligned, indicating that angular momentum is not conserved during molecular cloud formation. Both populations exhibit a j − R scaling consistent with that expected of supersonic turbulence: j H 2 ∝ R 1.67 ± 0.22 , and j H I ∝ R 1.71±0.27 . Combining our measurements with previous observations, we demonstrate a scaling of j ∝ R 1.50±0.02 in star-forming regions spanning 5 dex in size, R ∈ (10 −3 , 10 2 ) pc. We construct a model of angular momentum transport during molecular cloud formation, and derive the angular momenta of the progenitors to the present-day systems. We calculate a typical angular momentum redistribution timescale of 13 Myr, comparable to the H I envelope free-fall times.