We investigate the evolution of molecular clouds through the kinematics of their atomic hydrogen (HI) envelopes, using $^{ 12}\mathrm{ CO}$ and 21-cm emission to trace the molecular and atomic gas, respectively. We measure the large-scale gradients, $\Omega$, in the velocity fields of 22 molecular clouds and their HI envelopes, then calculate their specific angular momenta, $j\propto \Omega R^2$. The molecular clouds have a median velocity gradient of $9.6\times 10^{ -2}\ \mathrm{ km\ s^{ -1}\ pc^{ -1}}$, and a typical specific angular momentum of $2.7 \times 10^{ 24}\ \mathrm{ cm^2\ s^{ -1}}$. The HI envelopes have smaller velocity gradients than their respective molecular clouds, with an average of $\Omega_\mathrm{ HI} = 0.03\ \mathrm{ km\ s^{ -1}\ pc^{ -1}}$, and a median angular momentum of $j_\mathrm{ HI} \approx 5.7 \times 10^{ 24}\ \mathrm{ cm^2\ s^{ -1}}$. For a majority of the systems, $j_\mathrm{ HI} > j_\mathrm{ H_2}$, with an average of $j_\mathrm{ HI}/j_\mathrm{ H_2} = 4$. 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: $j_\mathrm{ H_2} \propto R^{ 1.67\pm 0.22}$, and $j_\mathrm{ HI} \propto R^{ 1.71\pm 0.27}$. Combining our measurements with previous observations, we demonstrate a scaling of $j \propto R^{ 1.50\pm 0.02}$ in star-forming regions spanning 5 dex in size, $R\in (10^{ -3},\ 10^2) \ \mathrm{ pc}$. 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 HI envelope free-fall times.
Categories: astro-ph.GA