Study Supports Role of Magnetic Fields in Star Formation

A recent study published in Nature, co-authored by researchers at the Harvard-Smithsonian Center for Astrophysics, found mounting evidence for the role of magnetic fields in star formation. By measuring the orientation of magnetic field lines on a variety of scales in the Cat’s Paw Nebula, scientists found that magnetic field direction is well preserved through different spatial scales.

Stars form within dense regions that exist inside giant molecular clouds. Due to the large mass of the gas molecules, gravity exerts a massive force to pull the particles together, leading to an increase in density and an eventual collapse. The rising temperature that results from the release of gravitational potential energy ends up facilitating fusion of hydrogen atoms into heavier elements within the cloud. This process builds pressure that counteracts the pull of gravity.

“This is how hydrostatic equilibrium state is reached, and that’s what stars are. But once that energy’s out, it could further collapse, leading to neutron stars and black holes,” said T.K. Sridharan, one of the co-authors of the study and associate of the Harvard College Observatory. “So why aren’t they all collapsing?”

Scientists believe that the answer lies in turbulence and magnetic fields. According to Qizhou Zhang, another co-author of the study and an astrophysicist at the Smithsonian Astrophysical Observatory, turbulence is thermal motion of particles caused by interstellar energy that “cascades down to very small spatial scales in the clouds into dense cores, acting like a pressure to stop gas from actually condensing.”

Magnetic fields, on the other hand, restrict the flow of gas and oppose the cloud’s tendency to collapse.

“Magnetic fields couple with electrons, anything that’s ionized,” said Zhang. “The ions will collide with neutral gas and produce resistance if the gas wants to move across the magnetic field.”

The impact of magnetic fields has been more challenging to assess due to the difficulty of measuring their strength and orientation. However, thanks to the Smithsonian’s Submillimeter Array, a collection of eight radio telescopes located atop Mauna Kea, researchers have been able to detect the direction of magnetic field lines.

“The effect of the turbulence on the magnetic field is to pull it around everywhere and mix it up,” said Sridharan. “If turbulence is very strong, then it will get completely tangled up. If magnetic field is strong, it will maintain its orientation and guide turbulence.”

Using the Submillimeter Array and other facilities, researchers measured the polarization of light emitted by spinning dust particles in dense cores. Polarization reveals the orientation of the dust particles, which in turn is aligned with the orientation of the magnetic field lines.

Since the field orientation was conserved across varying scales in the Cat’s Paw Nebula, it seems that the magnetic field is strong enough to resist being disturbed by turbulence. The study is part of an ongoing effort to determine the impact of magnetic fields on the collapse of molecular clouds to create stars.

“The picture that is emerging is that magnetic fields are playing a very important role in star formation,” Sridharan said.

—Staff writer Jessica Kim can be reached at jessica.kim@thecrimson.com.