CMR

The Collision-induced Magnetic Reconnection mechanism

I am interested in molecular cloud and star formation. Most recently, I am focusing on understanding the role of magnetic fields in the interstellar medium (ISM) through a combination of observational and theoretical approaches. The ISM, a major constituent of the Galaxy and an active participant in the baryon cycle, is essentially a plasma that is threaded with magnetic fields. As such, understanding the interplay between magnetic fields and the ISM is crucial. Traditional studies have often emphasized the resistance of magnetic fields to gas condensation, portraying them as playing a primarily passive role. This view is part of the reason that magnetic fields were thought to be secondary in star formation and galaxy evolution. To expand this perspective, I take a different approach by exploring an active role of magnetic fields. Specifically, by modeling the novel Collision-induced Magnetic Reconnection (CMR) mechanism, I show that magnetic fields can actively condense interstellar gas into star-forming clouds. The new model, firmly based on observational motivations, suggests that magnetic fields can be dynamically decisive, offering a missing piece in our understanding of the interplay between magnetic fields and the ISM.

Research Figure — Initial condition for CMR from the fiducial model simulation MRCOL from Kong et al. (2021, ApJ, 906, 80). Two spherical clouds collide (or any other shape, here I use spheres for simplicity) at the interface between reverse magnetic fields. Black arrows indicate initial magnetic fields (with only z-components): B_1,z > 0 (for x<0) and B_2,z < 0 (for x>0). The cartesian coordinate system axes are illustrated at lower-left. The point of collision is at the origin (the colliding clouds can be separated initially). The idea is to have colliding flows/clouds bring reverse fields from afar to trigger magnetic reconnection. Collision velocities along the x-axis are v_1,x > 0 and v_2,x < 0. The CMR filament forms along the y-axis through the origin (see movie on the right).

Volume rendering of dense gas as a function of simulation time. The streamlines show magnetic fields near the collision midplane (x=0). Initially a pancake structure is formed in the middle. Soon, reconnection of fields on two sides of the pancake happens, forming field loops that encircle the pancake. Due to magnetic tension, the field loops pinch the pancake, pulling gas toward the central axis. As a result, a filament forms along the central axis. (If the video does not play, you can download it or find "CMRfield.mpg" from Dataverse.)

A movie showing the filament formation via CMR. The simulation data is from the fiducial model MRCOL from Kong et al. (2021, ApJ, 906, 80) in which I proposed CMR for the first time to explain the formation of the Stick filament in the Orion A giant molecular cloud. As the collision continues along x-axis, more dense gas forms in the pancake (y-z plane or the collision midplane) which is continuously transported to the central filament. At the end, helical fields wrap around the filament with confining surface pressure. Formation of dense cores and stars shall follow in the filament.

The interactive plot (made by Dr. Griselda Arroyo-Chávez) shows the filament at snapshot 0.6 Myr from the fiducial model MRCOL in the original paper Kong et al. (2021, ApJ, 906, 80).

CMR streamlines — Magnetic field streamlines around the filament.

While going through the CARMA-NRO survey C¹⁸O data cube, I noticed a special filament (the Stick) at the southern end of the integral-shaped filament. It shows ring-like fibers at different velocity channels (see left column in the figure below). These ring-like fibers resemble those structures in magnetic reconnection simulations, which motivated me to model the filament formation in the context of magnetic reconnection. To my surprise, the 15-year-long effort by Dr. Carl Heiles who observed the HI Zeeman effect (Heiles (1997), ApJS, 111, 245) showed a reverse magnetic field on two sides of the Orion A cloud. This field-reversal is a necessary condition for CMR. Considering other observational facts such as the double-velocity spectral feature and the perpendicular plane-of-the-sky magnetic fields (e.g., Soler (2019), A&A, 629, 96), the only initial condition for the model is a setup like the right panel in the figure below. So I used the Athena++ code to set up a collision simulation between two spherical diffuse clouds (we always start with spheres:) with reverse magnetic fields (see the left panel in the top figure).

Two C¹⁸O channel maps (channel velocity at lower-right) from the CARMA-NRO survey C¹⁸O data cube. The two rows show the same channels, except that the top row uses blue dashed curves to highlight the fiber-like structures that were referred to as the rings and forks in Kong et al. (2021, ApJ, 906, 80). They resemble those structures in magnetic reconnection simulations. The resemblance is the motivation for CMR.

CMR Initial Conditions — A schematic diagram for the initial condition for the simulation. The two diffuse clouds collide at the interface of the reverse field. We (the observer) view the system with an inclination, so that we see both the line-of-sight reverse field and the plane-of-the-sky perpendicular field. To match the observation, the filament should form along the y-axis (the red circle in the figure shows the filament cross-section).

Surprisingly, instead of forming a compressed “pancake” structure, the collision resulted in the formation of a filament. This outcome contrasts with the intuitive expectation that a head-on collision between two spherical bodies would simply create a flattened structure at the center. However, a closer examination of a simulation slice revealed that a pancake did initially form. Soon after, it became wrapped by magnetic field loops, which exerted strong tension and gradually squeezed the pancake material toward the central axis, forming the filament. In other words, the filament formation does not need gravity! Although, gravity will take over with the growth of the filament mass, as long as the cloud collision continues.

The figure below illustrates the formation of these magnetic loops. The collision triggered magnetic reconnection on both sides of the pancake, generating two half-loops and a full loop. The half-loops pulled gas outward, while the full loop compressed the pancake inward. As the collision progressed, ongoing reconnection continuously funneled dense gas into the central filament. This dynamic process is shown in the movie in the top figure.

CMR 2D — A slice of the x-z plane in the CMR simulation. It shows how the collision compresses the reverse field and triggers magnetic reconnection. The process results in a pattern of half-loop, loop, half-loop of the field in the collision midplane (x=0). Magnetic tension of the reconnected field squeezes the pancake to z=0, resulting in a filament along the y-axis.

Using the model filament, I performed synthetic observations and compared the resulting radiative transfer images with actual observational data. The model showed strong agreement with observed features, including the column density distribution, the position-velocity diagram, and overall morphological characteristics. Notably, it provided a natural explanation for the observed reversed magnetic field. An intriguing implication emerged from this analysis: the structures within the CMR filament appear to be confined by surface magnetic pressure. This finding aligns with the conclusion of Kirk et al. (2017), which suggests that dense cores in Orion are primarily pressure-confined.

To investigate whether a filament like the Stick can form stars, we used the Arepo code with sink formation to repeat the above Athena++ simulation. First, the filament formation was reproduced in the Arepo simulation, so that CMR was independently confirmed with a different code. Second, the filament collapsed and formed a star cluster, but only after the filament became massive enough. An intriguing finding was that the star formation rate in CMR was a factor of two lower than a control model without CMR. The lower rate was due to the surface magnetic field that acted like a shield and hindered gas inflow. The result was reported in Kong et al. (2022), MNRAS, 517, 4679.

In Kong et al. (2023), ApJ, 265, 58, I led a brief parameter exploration of CMR to see how different initial conditions affect the filament property. Again, I used Athena++ for the simulations, following the 2021 Stick paper. A notable finding was that a curved filament would form if the initial colliding clouds have different radii or different density.

CMR filament different view — Synthetic images of CMR filament from different angles.

Using the exploration data, Dr. Duo Xu utilized machine-learning techniques to capture the morphological features of CMR filaments. Then, we applied the machine-learning models to the Herschel far infrared images of several filamentary clouds, including the Taurus cloud, the Orion cloud, and the Auriga–California Cloud. Several filamentary structures of these clouds were identified as CMR filaments. The findings were reported in Xu et al. (2023). Notably, these clouds were reported to have reverse magnetic fields (Tahani (2022) and private communication).

The MRCOL simulation began with molecular gas collision. A natural follow-up question is whether collisions between atomic gas can also lead to molecular cloud formation. In Kong et al. (2024), ApJ, 975, 97, we used Arepo again to model CMR within the cold neutral medium (CNM). The results showed that a filamentary molecular cloud successfully formed. This cloud exhibited numerous fiber-like substructures, with sinks actively accreting material through streamers. Recall from Kong et al. (2022), that sink accretion was only possible through streamers, as surface magnetic fields inhibited free gas inflow. Remarkably, this behavior persisted at a much larger scale (20 pc), with all sinks in the molecular cloud accreting via streamers. Most notably, the molecular cloud was naturally turbulent, driven by reconnected magnetic fields that funneled gas into the filament at supersonic (Alfvénic) speeds. This process compressed and transported shocked gas into the filament in a piece-by-piece manner. Consequently, filament formation in this scenario follows a bottom-up process -- distinct from the traditional top-down view of star formation, where gravity collects diffuse gas into a molecular cloud, which then collapses and fragments into clumps and cores. Remember, CMR filament formation proceeds without the need for gravity. In Kong (2022), ApJ, 933, 40, I also explored CMR in warm neutral medium in a disk galaxy.

CMR CNM 1 — Results from Kong et al. (2024). Note the supersonic gas motion due to Alfvénic magnetic transportation.