Kinematics of HI Envelopes Associated with Molecular Clouds

TL;DR

This study examines the kinematics and angular momentum of atomic hydrogen envelopes around molecular clouds, revealing that angular momentum is not conserved during cloud formation and that turbulence influences their scaling relations.

Contribution

It provides new measurements of velocity gradients and angular momenta in HI envelopes and molecular clouds, and models angular momentum transport during cloud formation.

Findings

HI envelopes have smaller velocity gradients than molecular clouds.

Angular momentum in HI envelopes is typically higher than in molecular clouds.

Angular momentum scales with size as a power law consistent with turbulence models.

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 $9.6\times 10^{-2}\ \mathrm{km\ s^{-1}\ pc^{-1}}$, 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.