A team of astronomers solve a 30-year-old stellar mystery

An international team of astronomers have captured the first direct, high-resolution images of a magnetic field wrapped tightly around the outflow of gas streaming away from a forming star — evidence that solves a decades-old puzzle about how young stars sculpt the powerful jets that form them.

The team, in which the Institute of Space Sciences participates, used the Atacama Large Millimeter/submillimeter Array (ALMA), of which the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO) is a partner. 

The findings, published by Nature Communications, from a research team led by Tao-Chung Ching, a former NSF NRAO Jansky Fellow under the mentorship of scientist Emmanuel Momjian, focus on NGC 1333 IRAS 4A, a young double-star system embedded in the Perseus molecular cloud, roughly 960 light-years from Earth.

Newborn stars grow by pulling in gas and dust from a surrounding disk of material. As they do, they also blast some of that material back out into space in fast, narrow jets and wider, slower outflows — a process astronomers have long suspected is shaped and powered by magnetic fields twisted into a funnel-like, doughnut shape around the jet.

“For the first time, these ALMA observations have captured this invisible funnel of magnetic fields,” said Ching. “This is exciting because it proves a decades-old theory about how stars, like our own Sun, are born and fire off powerful cosmic jets”, he adds. 

The team used ALMA’s exceptional resolving power, roughly 30 times sharper than that of earlier telescopes, to measure the faint polarization of carbon monoxide gas radiating from the outflow around IRAS 4A. That polarization signal let the researchers trace the morphology and strength of the magnetic field threading through the outflow at distances of only a few hundred astronomical units (the average distance between the Earth to the Sun) from the young star.

The team found the magnetic field measured a few thousandths of a gauss (modest compared to a household magnet, but immense on the scale of interstellar space) and that it wrapped around the outflow like a coil, running perpendicular to the direction the gas was flowing and matching the outflow’s rotation. That geometry is the signature of a “toroidal” (or ring-shaped) magnetic field, exactly what theoretical models have predicted for decades — but never directly confirmed at this level of detail.

“This study represents the first and most high-resolution observation of milligauss-strength toroidal magnetic fields at a scale of several hundred astronomical units from a protostar,” adds Ching.

“We knew that IRAS 4A was a textbook case: 20 years ago, in a work published in Science in 2006, we found that this region followed the theoretically expected magnetically driven collapse”, says Josep Miquel Girart, co-author and researcher at the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC). 

The team also uncovered an unexpected bonus: a straightforward mathematical relationship, based on the physics principle known as Ampère’s law, linking the twisting of the magnetic field to the electric currents flowing through the gas. Because that relationship follows a predictable, linear pattern, it gives astronomers a new and more direct way to work out the direction of magnetic fields in the clouds of gas and dust where stars are born — a notoriously difficult measurement to make.

Understanding how magnetic fields shape stellar outflows helps astronomers explain a fundamental step in star formation, for how young stars shed excess material and angular momentum so they can continue growing, rather than spinning themselves apart. The same physical process is thought to play out at vastly different scales throughout the Universe, from newborn stars like the one studied here, to the supermassive black holes that power distant galaxies.