Capturing the cosmic ‘drift’ before a star is born

Stars like our sun are formed from the collapse of stellar objects called prestellar cores, cold and dense concentrations of gas and dust held together by gravity. While many questions remain about the exact mechanisms of star formation, advanced radio telescopes have given researchers new insights into the inner workings of infant stars.

Now, publishing in Astronomy & Astrophysics, researchers from Kyushu University and Max Planck Institute for Extraterrestrial Physics have, for the first time, detected a phenomenon known as ambipolar diffusion occurring in a prestellar core. This phenomenon weakens the magnetic support of the core, leading to gravitational collapse to form an infant star called a protostar.

These findings provide further insight into the key processes of early star formation and, by extension, how stellar systems are created.

Magnetic fields can stall collapse

“Prestellar cores are fascinating stellar bodies. They are dense and cold, and a source of a lot of complex chemistry. The cold environment allows molecules to assemble into more complex ones, like precursors of prebiotic organic molecules,” explains first author Doris Arzoumanian, an associate professor at Kyushu University’s Institute for Advanced Study.

“One of the questions we are investigating is the role of magnetic fields in star formation. Strong magnetic fields permeate prestellar cores. If that field is too strong, it can delay gravitational collapse and therefore star formation. We wanted to investigate how prestellar cores reduce the strength of their magnetic field.”

Using the Institute for Radio Astronomy in the Millimeter Range (IRAM) 30-meter telescope, the research team turned its sights to L1544, a prestellar core in the Taurus molecular cloud, one of the nearest star-forming regions to Earth.

In molecular clouds, gas is partially ionized, meaning ions are strongly coupled to magnetic fields, while neutral particles interact with the field indirectly through collisions. Studying these molecules is key to understanding the state of the core’s magnetic field.

However, because prestellar cores are so cold, the most common molecular tracers freeze onto dust grains, making them invisible. Therefore, the team had to identify a new set of molecules to trace.

Tracing ions and neutral gas

“We selected Diazenylium‑d₁ (N₂D⁺), an ion, and para‑monodeuterated ammonia (para‑NH₂D), a neutral molecule, as our tracers because they are generally located in similar high-density regions within prestellar cores,” explains second author Silvia Spezzano, group leader at the Max Planck Institute for Extraterrestrial Physics.

“We therefore collected spectral data of the core and modeled the velocity of the two molecules.”

They found a clear velocity difference between the molecules of about 0.05 km/s, which was interpreted as evidence of ion-neutral drift. As the density of the prestellar core increases, it becomes shielded from radiation, and ionization decreases. This weakens the coupling between the molecules and magnetic fields, and eventually neutral particles decouple and drift inward because of gravity, while ions remain tied to the magnetic field.

As the neutral particles fall toward the core center, they speed up while the ions remain coupled to the magnetic field, causing the velocity difference.

“This process is known as ambipolar diffusion. Until now, observing this phenomenon in a prestellar core was a major challenge,” continues Arzoumanian. “As ambipolar diffusion continues, the strength of the magnetic field decreases. Eventually, gravity becomes the primary driving force in the core, resulting in its gravitational collapse into a protostar.”

Sharper maps could test the drift

The team hopes to further confirm its findings by observing additional prestellar cores and obtaining higher-angular-resolution observations to better map the velocity drift of ion and neutral molecules.

“These results were possible thanks to an interdisciplinary collaboration of expert observers and theorists in the fields of gas dynamics, astrochemistry and dust physics,” concludes Arzoumanian. “Understanding star formation addresses a fundamental question about the origin of life in planetary systems and helps us better understand the universe as a whole.”

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Sadie Harley

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Andrew Zinin

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