The “Turbulent” Relationship between Stellar Feedback and Magnetic Fields

Title: The role of initial magnetic field structure in the launching of protostellar jets

Authors: I. A. Gerrard, C. Federrath, R. Kuruwita

First Author’s Institution: Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

Status: Accepted in MNRAS; open access on arXiv

 

Building a star is not an easy task. We now know that a number of factors come into play, such as turbulence in the surrounding material, stellar feedback, and one of the most notoriously tricky things in astronomy – magnetic fields. Turbulence has a complicated role in the process, creating regions of high-density gas that gravitationally collapse to form stars while also preventing gas from collapsing too much on large scales. Magnetic fields are also believed to support the gas against large scale collapse and can prevent stable accretion disks from forming around protostars. Today’s paper discusses the relationship between all three aforementioned components to see how the degree of turbulence in magnetic field lines affects the young star. Let’s step back first, though, and talk about what is meant by stellar feedback.

 

What is stellar feedback?

Broadly defined, stellar feedback includes all of the ways that a star transfers momentum and energy back into its surroundings. The light that stars give off transfers both energy and momentum when it is absorbed by the surrounding gas, blowing away material while raising its temperature. What most people don’t know is that stars also eject matter. This occurs both during their formation and throughout the rest of their lives. Almost paradoxically, this ejection of material during the formation of the star is crucial for its stability.

This ejection of matter comes in two forms: collimated bipolar outflows and isotropic (spherical) winds. During the low-mass star formation phase, the bipolar outflows dominate the momentum injection (see Figure 1). Why are they so collimated? This is primarily due to the presence of the accretion disk, relegating the ejected material to the openings on the top and bottom. However, this work also shows that the orientation of the surrounding magnetic field plays a large role in the structure of these bipolar outflows.

Figure 1: Left – Artist’s conception of an accreting protostar. The material in the disk loses energy due to friction and falls into the protostar. Some of this material is ejected in the bipolar outflow. (reproduced from “Protostars” by Tom Greene, Scientific American 2001)   Right – Hubble image of one lobe of a bipolar outflow from a cluster of three stars. The stars are enshrouded in the dusty accretion disk at the bottom. (taken from COMPLETE Survey website)

 

Making stars on a computer

In order to investigate the role of magnetic fields in protostellar outflows, the grid-based, adaptive mesh refinement code FLASH was used to perform three simulations of the collapse of a 1 solar mass molecular cloud core: one with a uniform magnetic field, one with a partially turbulent field, and one with a fully turbulent field. Figure 2 shows what the magnetic field lines look like for each of these simulations. The magnitude of the magnetic field remains constant across all simulations, so the orientation of the field is the only variable. In the uniform case, the field lines are parallel to the rotation axis of the cloud. It is worth noting that these are only initial conditions; the field lines will change as the simulation progresses.

Figure 2: Schematic of the magnetic field lines in each of the simulations from the paper. Though the orientation of the field lines changes, the magnitude of the magnetic field remains constant. (Taken from Fig 1 in the paper)

As the simulation progresses, gas will begin to gravitationally collapse in certain regions of the cloud. These regions will eventually acquire enough mass to turn into sink particles. In modern simulations, sink particles are used to reduce computational cost and increase accuracy when studying star formation.  Instead of trying to calculate all of the interactions that occur within a particularly dense region of gas (which will introduce numerical artifacts due to the limited resolution of the simulation), we can replace this dense region with a single sink particle of the same mass. These sink particles then attract other gas in the same way as the prior region of dense gas, so these sink particles can be thought of as protostars that will continue accreting material.

 

How does the magnetic field orientation impact this?

By comparing three different simulations in which magnetic field orientation is the only variable, the effects of this factor on star formation can be isolated. First, the authors consider how quickly these sink particles (protostars) grow in each of the three different cases. Figure 3 shows the growth of the sink particles as a function of time. A sink particle appears to grow the fastest in the uniform magnetic field, but many more sink particles are produced in the fully turbulent case.

Figure 3: Growth of sink particles as a function of time in each of the three simulations. Each dashed line represents a single sink particle. The fully turbulent case produced five, and the solid green line is the sum of all five. The x-axis is scaled by a Star Formation Efficiency (SFE) cutoff, measuring the time since 1% of the total initial mass of the molecular cloud was turned into sink particles (Taken from Figure 2 in the paper).

Next, the role of magnetic field orientation in the launching of bipolar protostellar outflows is addressed (see Figure 4). A bipolar outflow is clearly present in both the uniform and partially turbulent simulations, though the outflow in the partially turbulent case expands slower and has larger lobes than the outflow in the uniform case. While this work finds that the outflow in the partially turbulent case carries the same amount of mass as the outflow in the uniform case, it moves slower and only injects 71% of the momentum that is injected by the outflow in the uniform magnetic field. A bipolar outflow is completely absent in the fully turbulent simulation.

 

Figure 4: Outflows launched from protostars in each of the three simulations after 20% of the total initial mass in the cloud has been accreted by sink particles. From left to right, this plot shows the uniform case, the partially turbulent case, and the fully turbulent case. Purple lines indicate the magnetic field lines. The arrows represent the direction of the velocity field, and the lighter blue colors represent regions of higher gas density. Black crossed spheres indicate sink particles. (Taken from Figure 3 in the paper)

 

The authors conclude that this is because a fully turbulent magnetic field would apply pressure to the protostar isotropically, meaning that the protostar would feel the same magnetic pressure from all sides. In the uniform and partially turbulent case, there is a gradient in the magnetic pressure that could launch these outflows. Consequently, it appears that the orientation of the magnetic field can alter how quickly stars grow in mass and how much of an impact their feedback will have. To untangle the mysteries of star formation, perhaps the degree of entanglement possessed by the magnetic field will play a crucial role!

 

About Michael Foley

I'm a graduate student studying Astrophysics at Harvard University. My research focuses on using simulations and observations to study stellar feedback - the effects of the light and matter ejected by stars into their surroundings. I'm interested in learning how these effects can influence further star and galaxy formation and evolution. Outside of research, I'm really passionate about education, music, and free food.

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