Japanese and German scientists studied prestellar cores — relatively small concentrations of matter from which stars later form. They concluded that matter drifts inside these cores, changing their structure and affecting magnetic fields.
Studying the magnetic fields of prestellar cores
Publishing their results in the journal Astronomy & Astrophysics, researchers from Kyushu University and the Max Planck Institute for Extraterrestrial Physics detected for the first time a phenomenon known as ambipolar diffusion occurring inside a prestellar core. This phenomenon weakens the magnetic support of the core, leading to gravitational collapse and the formation of a young star, known as a protostar. These results make it possible to better understand the key processes of early star formation and, as a result, how stellar systems are formed. This was reported by phys.org.
Stars similar to our Sun form as a result of the collapse of stellar objects known as prestellar cores — cold, dense accumulations of gas and dust held together by gravity. Although many questions remain about the exact mechanisms of star formation, modern radio telescopes have given scientists new insights into the internal processes occurring in young stars.
“Prestellar cores are fascinating stellar structures. They are dense and cold and are the source of numerous complex chemical processes. The cold environment allows molecules to combine into more complex compounds, such as the 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 studied by the researchers was the role of magnetic fields in star formation. Strong magnetic fields penetrate prestellar cores. If this field is too strong, it can delay gravitational collapse and therefore delay star formation. Astronomers wanted to study how prestellar cores reduce the strength of their magnetic field.
Using the 30-meter telescope of the Institute for Radio Astronomy in the Millimeter Range (IRAM), the research team focused on L1544, a prestellar core in the Taurus molecular cloud, one of the star-forming regions closest to Earth.
Ions and neutral gas as indicators of the magnetic field
In molecular clouds, the gas is partially ionized, meaning that ions are closely connected 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. The team therefore had to identify a new set of molecules for tracking.
“We chose the diazenylium-d1 ion (N₂D⁺) and para-monodeuterated ammonia (para-NH₂D) — a neutral molecule — as our tracers because they are usually found in similar high-density regions inside prestellar cores,” explains the paper’s second author, Silvia Spezzano, a group leader at the Max Planck Institute for Extraterrestrial Physics. “We therefore collected spectral data from the core and modeled the velocities of these two molecules.”
Particle drift inside the core
The scientists found a clear velocity difference between the molecules, amounting to about 0.05 km/s, which they interpreted as evidence of drift between ions and neutral particles. As the density of a prestellar core increases, it becomes shielded from radiation, and the ionization level decreases. This weakens the interaction between the molecules and the magnetic field. As a result, neutral particles detach from the magnetic-field lines and drift inward under the influence of gravity, while the ions remain tied to the magnetic field.
As neutral particles fall toward the center of the core, they accelerate, while the ions remain connected to the magnetic field, producing the observed velocity difference.
“This process is known as ambipolar diffusion. Until now, observing this phenomenon in a prestellar core has been a major challenge,” Arzoumanian notes. “As ambipolar diffusion continues, the strength of the magnetic field decreases. Eventually, gravity becomes the main driving force in the core, leading to its gravitational collapse into a protostar.”
More detailed maps will help verify the drift
The team hopes to further confirm its findings by observing other prestellar cores and obtaining data with higher angular resolution in order to map the velocity drift of ions and neutral molecules more precisely.
“These results were made possible by interdisciplinary collaboration between experienced observers and theorists in the fields of gas dynamics, astrochemistry, and dust physics,” Arzoumanian concludes. “Understanding the process of star formation allows us to answer the fundamental question of the origin of life in planetary systems and helps us better understand the universe as a whole.”
