Astronomers have detected for the first time a phenomenon known as ambipolar diffusion in a pre-stellar core, providing direct observational evidence of a process that is critical to the formation of stars. The discovery, published on July 10 in the journal Astronomy & Astrophysics, focuses on the core L1544 in the constellation Taurus and sheds new light on the earliest stages of star formation, long before a protostar even begins to take shape.
The study, led by Doris Arzoumanian of Kyushu University, with Silvia Spezzano of the Max Planck Institute for Extraterrestrial Physics as second author, reveals how neutral molecules and ions move at different speeds within the dense gas and dust of the core. This differential motion, called ambipolar diffusion, occurs when the magnetic field that permeates the core weakens its grip on neutral particles, allowing them to drift inward under gravity while ions remain tied to the magnetic field lines.
Spezzano, an astrochemist who grew up in Corigliano Calabro, Italy, and earned her degree in industrial chemistry before specializing in astrochemistry, explained that the team focused on two specific molecules: diazenylium (N2H+) and para-monodeuterated ammonia (NH2D). These nitrogen-rich molecules were chosen because their chemistry in space is slow, making them excellent tracers of the dense inner regions of pre-stellar cores. By observing how these molecules move, the researchers could directly measure the decoupling of neutral species from the magnetic field.
«We discovered that diazenylium and ammonia move at different speeds,» Spezzano said. «The reason for this difference is the interaction with magnetic fields. Diazenylium, being an ion, feels the magnetic field and is slowed down by it, while ammonia, being a neutral molecule, does not feel the magnetic field and is not affected.»
The observations were made using the IRAM telescope in the Sierra Nevada mountains of Spain, which provided the sensitivity needed to detect the very small velocity differences between the two molecular species. The team also relied on the fact that L1544 is a pre-stellar core very close to the point of forming a star, making it an ideal laboratory for studying the transition from a quiescent cloud to a collapsing protostar.
Pre-stellar cores are dense clumps of gas and dust that contain all the material needed to form a star like the Sun, but have not yet begun the collapse that leads to a protostar. Spezzano likened them to seeds from which a flower or plant will grow. Understanding the dynamics within these cores is essential for building a complete picture of star formation, a process that has been studied for decades but still holds many mysteries.
The detection of ambipolar diffusion is significant because it confirms a key theoretical prediction about how magnetic fields regulate star formation. In the outer regions of a core, both ions and neutral molecules are coupled to the magnetic field, which supports the core against gravity. As the density increases, the core becomes shielded from radiation, ionization decreases, and the coupling weakens. Eventually, neutral molecules drift inward faster than ions, initiating gravitational collapse. Without this process, magnetic fields would prevent cores from collapsing, and star formation would be severely inhibited.
«The effect we observed is expected, but very difficult to see,» Spezzano noted. «Many factors were important in this discovery: the choice of molecules to study, the instruments available at the telescope that allow us to see very small velocity differences, and finally the choice of the source itself.»
The findings have implications for understanding not only how individual stars form, but also how planetary systems like our own solar system emerge from the same processes. By tracing the chemical and physical evolution of pre-stellar cores, astronomers can piece together the sequence of events that leads from a diffuse cloud of gas and dust to a fully formed star with orbiting planets.
The study was published in Astronomy & Astrophysics and is available online. The research team included scientists from Kyushu University, the Max Planck Institute for Extraterrestrial Physics, and other institutions. Further observations of L1544 and other pre-stellar cores are planned to refine the understanding of ambipolar diffusion and its role in star formation.
