Kinematics of HI Envelopes Associated with Molecular Clouds

TL;DR

The paper addresses how angular momentum evolves during molecular cloud formation by comparing kinematics of molecular gas and surrounding HI envelopes in 22 Solar Neighborhood clouds. It measures large-scale velocity gradients using $^{12}\mathrm{CO}$ and 21-cm emission, computes specific angular momenta via $j \propto \Omega R^2$ with a geometric factor $c = 0.4$, and finds that $j_{\mathrm{HI}}$ is typically larger than $j_{\mathrm{H_2}}$ by a factor of about 4 with frequent misalignment of rotation axes. Across five orders of magnitude in size, the data yield a power-law scaling $j \propto R^{1.50\pm 0.02}$, consistent with a supersonic turbulent cascade and extending to broader star-forming regions when combined with previous work. A simple angular-momentum-transport model estimates the redistribution timescale to be $\Delta t_{\mathrm{diss}} \sim 13$ Myr, comparable to the HI envelope free-fall time, supporting efficient braking mechanisms during cloud assembly and challenging purely angular-momentum-conserving formation scenarios.

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, $Ω$, in the velocity fields of 22 molecular clouds and their HI envelopes, then calculate their specific angular momenta, $j\propto ΩR^2$. 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 $Ω_\mathrm{HI} = 0.03\ \mathrm{km\ s^{-1}\ pc^{-1}}$, 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.

Figures (11)

• Figure 1: The Solar Neighborhood molecular clouds and their associated HI envelopes studied in this work (Table \ref{['tab:intro']}). The background map shows the surface density of HI with contours of $\Sigma_\mathrm{HI} \in (5-40)\ M_\odot\ \mathrm{pc^{-2}}$, with an even spacing of $5\ M_\odot\ \mathrm{pc^{-2}}$, as well as at $[50,75,100]\ M_\odot\ \mathrm{pc^{-2}}$. The white squares show the spatial extent of the HI envelope associated with each molecular cloud. (For ease of viewing, molecular gas not analyzed in this work is not included in the map.)
• Figure 2: Velocity spectra of select molecular clouds (blue) and corresponding HI envelopes (red). The vertical lines indicate velocity bounds used to define the HI envelope. Spectra of the remaining molecular clouds and HI envelopes can be found in Appendix \ref{['sec:appndx_specs']}.
• Figure 3: Left: Column density map of the HI envelope around the Perseus molecular cloud (black outline). The overlaid blue contours are at $(0.25,\ 0.5,\ 0.75)$ times the peak $\mathrm{H_2}$ surface density, which is indicated in lower left corner. Middle: Velocity field maps of the Perseus molecular cloud and (right) HI envelope, derived from the first moments of $^{12}\mathrm{CO}$ and 21-cm emission at each pixel (Equation \ref{['eq:first_mom']}). The blue (red) arrow in the lower right corner of the middle panel indicates the velocity gradient direction $\theta_\mathrm{H_2}$ ($\theta_\mathrm{HI}$). Maps of the remaining molecular clouds and HI envelopes can be found in Appendix \ref{['sec:appndx_maps']}.
• Figure 4: Distribution of angular separation between the rotation axis of molecular clouds and their corresponding HI envelopes.
• Figure 5: Intensity-weighted velocity centroid, $\langle v_\mathrm{LSR} \rangle$, as a function of the perpendicular displacement, $r_\perp$, from the rotation axis of the California molecular cloud (left) and its HI envelope (right). The dashed lines indicate the planar model, and the shaded regions show the $\pm 1\sigma$ and $\pm 2\sigma$ scatter of the velocity field map at each radial bin. The Pearson correlation coefficient is in the upper left corner of each plot. Velocity profiles of the remaining molecular clouds and HI envelopes can be found available in the Appendix \ref{['sec:appndx_vvsr']}.