Kinematics of HI Envelopes Associated with Molecular Clouds

Abstract

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 $9.6\times 10^{-2}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^{-1}$, and a typical specific angular momentum of $2.7\times 10^{24}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$. The HI envelopes have smaller velocity gradients than their respective molecular clouds, with an average of ${\Omega }_{\mathrm{HI}}=0.03\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^{-1}$, and a median angular momentum of $j_{\mathrm{HI}}\approx 5.7\times 10^{24}\mathrm{c}{\mathrm{m}}^2{\mathrm{s}}^{-1}$. 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.

24 pages, 14 figures, 2 Tables. Accepted to ApJ, comments welcome