Abstract

In order to understand the origin of observed molecular cloud properties, it is critical to understand how clouds interact with their environments during their formation, growth, and collapse. It has been suggested that accretion-driven turbulence can maintain clouds in a highly turbulent state, preventing runaway collapse, and explaining the observed non-thermal velocity dispersions. We present 3D, AMR, MHD simulations of a kiloparsec-scale, stratified, supernova-driven, self-gravitating, interstellar medium, including diffuse heating and radiative cooling. These simulations model the formation and evolution of a molecular cloud population in the turbulent interstellar medium. We use zoom-in techniques to focus on the dynamics of the mass accretion and its history for individual molecular clouds. We find that mass accretion onto molecular clouds proceeds as a combination of turbulent and near free-fall accretion of a gravitationally bound envelope. Nearby supernova explosions have a dual role, compressing the envelope, boosting accreted mass, but also disrupting parts of the envelope and eroding mass from the cloud's surface. It appears that the inflow rate of kinetic energy onto clouds from supernova explosions is insufficient to explain the net rate of charge of the cloud kinetic energy. In the absence of self-consistent star formation, conversion of gravitational potential into kinetic energy during contraction seems to be the main driver of non-thermal motions within clouds. We conclude that although clouds interact strongly with their environments, bound clouds are always in a state of gravitational contraction, close to runaway, and their properties are a natural result of this collapse.

Figure 1. Animation

10 Myr evolution of the zoomed-in clouds, from the moment self-gravity is turned on. The global evolution of the simulation is 240 Myr, during this time, SN explosions have stirred the interstellar medium, driving significant amounts of turbulence, and the gas has divided into a multi-phase ISM. In this figure, we zoom-in on three low mass clouds, which have formed self-consistently out of the turbulent ISM. We maintain the background ISM at a coarse resolution in order to provide live boundary conditions for the evolution of the high resolution molecular clouds.

Figure 9 Animation

Evolution of randomly selected tracer particles eventually accreted by clouds M3, M4 and M8. Left: Column density projection along lines of sight (top) perpendicular and (bottom) parallel to the midplane. The solid black contour follows the projected surface of the cloud. Both panels include the location of the ten particles at the current time and their trajectories from the moment of injection up to the current time. Right: Four panels showing the dynamical properties sampled by each of the ten particles along their trajectories. Panels show: (top) Local number density traced by the particles. The horizontal dashed red line shows the cloud density threshold. (second and third) Velocity of the particles perpendicular and parallel to the local density gradient. (bottom) Mach number of the particles calculated using the local adiabatic sound speed of the gas and with respect to the center of mass of the cloud. A dashed red line shows the sound speed, while the transition from black to grey shows the current time.

Figures 10 Animation

Evolution of randomly selected tracer particles eventually accreted by cloud M4 using same diagnostics and notation as Figure 9.

Figures 11 Animation

Evolution of randomly selected tracer particles eventually accreted by cloud M4 using same diagnostics and notation as Figure 9.

Figures 15 Animation

Distribution of the total energy density of cloud M3 and it's surroundings, with blue showing bound regions and red showing unbound regions. The figure shows a slice through the center of mass of the cloud, while an online animation shows every slice. (Top left) Cloud surface at a time of 4 Myr (defined as connected structure and gravitationally bound fragments above a number density of n = 100 cm−3 ). On top, an arrow indicates the normal to the slice plots, while the height of the current cut is indicated by the square surrounding the cloud. (Bottom, left to right) Slices of the total energy density at times of 2, 4 and 6 Myr after self-gravity. Density contours are shown at (solid line) 100 cm−3 and (dotted line) 10 cm −3 . In-plane velocity vectors are given with scales at upper right. Black arrows show velocities below |v| < 100 km s −1 , while white arrows show velocities above.

Figures 16 Animation

Distribution of the total energy density of cloud M4 and it's surroundings. Using same diagnostics and notation as Figure 15.

Figures 17 Animation

Distribution of the total energy density of cloud M8 and it's surroundings. Using same diagnostics and notation as Figure 15.