To the Point
- An international team, including MPE researchers, has found the first observational signature of ambipolar diffusion in a prestellar core.
- The result is based on high-resolution spectral observations of the dense core L1544 with the IRAM 30-meter telescope.
- The team detected a small but systematic velocity offset between an ion and a neutral molecule that traces nearly the same gas.
- The finding offers a new way to test theories of how magnetic fields regulate the earliest stages of star formation.
- Future observations could test whether similar signatures appear in other prestellar cores and help constrain magnetic fields, geometry, chemistry, and dust growth.
An international team with major contributions from the Max Planck Institute for Extraterrestrial Physics (MPE) has found the first observational signature consistent with ambipolar diffusion in a prestellar core. Using high spectral resolution observations with the IRAM 30-meter telescope, the researchers detected a small but systematic velocity difference between ionized and neutral gas in L1544, a prototypical dense core in the Taurus molecular cloud.
The result addresses one of the central questions in star-formation research: how gravity and magnetic fields interact in the earliest phases of collapse. In dense molecular gas, ions remain coupled to magnetic fields, while neutral molecules can move more freely. Ambipolar diffusion describes the gradual decoupling of these components, allowing neutral gas to drift inward while charged particles remain coupled to the magnetic field. Until now, this process had been predicted by theory and simulations but had not been directly identified in a prestellar core.
“L1544 gives us a rare opportunity to study the interplay between magnetic fields and collapsing gas at a very early stage,” says Tommaso Grassi of MPE. “The data reveal a small but systematic relative motion between ions and neutrals — exactly the kind of signature expected if ambipolar diffusion is at work.” How the team detected the effect
L1544 is a prototypical prestellar core: cold, dense, gravitationally bound, and still without a protostar. That makes it an ideal laboratory for studying the physical conditions just before star birth. At such low temperatures, however, many common molecular tracers freeze onto dust grains, making them difficult to observe. The team therefore selected two molecules that probe similar dense regions of the core: the molecular ion N2D+ and the neutral molecule para-NH2D.
“This is a powerful example of what becomes possible when the right target is observed with very high spectral resolution and carefully matched tracers,” says Silvia Spezzano, Max Planck Research Group Leader at MPE. “It provides a direct observational probe of a process that has long been central to star-formation theory.”
The result addresses one of the central questions in star-formation research: how gravity and magnetic fields interact in the earliest phases of collapse. In dense molecular gas, ions remain coupled to magnetic fields, while neutral molecules can move more freely. Ambipolar diffusion describes the gradual decoupling of these components, allowing neutral gas to drift inward while charged particles remain coupled to the magnetic field. Until now, this process had been predicted by theory and simulations but had not been directly identified in a prestellar core.
“L1544 gives us a rare opportunity to study the interplay between magnetic fields and collapsing gas at a very early stage,” says Tommaso Grassi of MPE. “The data reveal a small but systematic relative motion between ions and neutrals — exactly the kind of signature expected if ambipolar diffusion is at work.” How the team detected the effect
L1544 is a prototypical prestellar core: cold, dense, gravitationally bound, and still without a protostar. That makes it an ideal laboratory for studying the physical conditions just before star birth. At such low temperatures, however, many common molecular tracers freeze onto dust grains, making them difficult to observe. The team therefore selected two molecules that probe similar dense regions of the core: the molecular ion N2D+ and the neutral molecule para-NH2D.
“This is a powerful example of what becomes possible when the right target is observed with very high spectral resolution and carefully matched tracers,” says Silvia Spezzano, Max Planck Research Group Leader at MPE. “It provides a direct observational probe of a process that has long been central to star-formation theory.”
What the observations reveal
The observations reveal a mean ion-neutral velocity offset of about 0.05 km/s. In a cold, slowly evolving prestellar core, this is a very small number — but one that is physically meaningful and consistent with the drift expected when ions and neutrals begin to decouple during gravitational collapse. The analysis also shows that the two tracers have similar spatial distributions, strengthening the case that they sample nearly the same gas and that the measured velocity difference reflects a real physical effect rather than a difference in the layers being traced.
At the same time, the authors are careful not to overstate the result. Geometry, projection effects, and the internal structure of the core all influence what can be seen along the line of sight. The study also does not detect a significant difference in linewidth between the ion and neutral species. For that reason, the result should be seen as strong evidence for ambipolar diffusion, not as final proof.
At the same time, the authors are careful not to overstate the result. Geometry, projection effects, and the internal structure of the core all influence what can be seen along the line of sight. The study also does not detect a significant difference in linewidth between the ion and neutral species. For that reason, the result should be seen as strong evidence for ambipolar diffusion, not as final proof.
What comes next
Future observations with higher spatial and spectral resolution could test whether similar signatures appear in other prestellar cores and how they vary across a core’s structure. They could also help disentangle the roles of ambipolar diffusion, geometry, chemistry, and dust growth in shaping the collapse process. If ion-neutral drift velocities can be measured more broadly, they may become a new diagnostic for magnetic field strength and the physical conditions that regulate star formation.
For MPE, the study highlights the institute’s role in precision astrophysics at the frontier of star-formation research. For the field more broadly, it provides one of the clearest observational tests of a process that has shaped theory for decades but has been difficult to capture directly.
“This study highlights the remarkable synergy between astronomy, astrochemistry, and laboratory spectroscopy," says Paola Caselli, Director at the Max Planck Institute for Extraterrestrial Physics. "Detecting velocity differences of only a few tens of meters per second is possible only because laboratory measurements have established molecular transition frequencies with extraordinary precision. These advances now allow us to probe subtle physical processes, such as ambipolar diffusion, that govern the earliest stages of star formation, just before a new star is born.”
For MPE, the study highlights the institute’s role in precision astrophysics at the frontier of star-formation research. For the field more broadly, it provides one of the clearest observational tests of a process that has shaped theory for decades but has been difficult to capture directly.
“This study highlights the remarkable synergy between astronomy, astrochemistry, and laboratory spectroscopy," says Paola Caselli, Director at the Max Planck Institute for Extraterrestrial Physics. "Detecting velocity differences of only a few tens of meters per second is possible only because laboratory measurements have established molecular transition frequencies with extraordinary precision. These advances now allow us to probe subtle physical processes, such as ambipolar diffusion, that govern the earliest stages of star formation, just before a new star is born.”
Contacts:
Dr. Silvia Spezzano
Max Planck Research Group Leader - CAS
Tel: +49 89 30000-3513
Email: spezzano@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics, Garching
Dr. Tommaso Grassi
Scientist
Tel: +49 89 30000-3639
Email: tgrassi@mpe.mpg.de
Center for Astrochemical Studies
Prof. Dr. Paola Caselli
Director of the Center for Astrochemical Studies (CAS)
Tel: +49 89 30000-3400
Fax: +49 89 30000-3399
Email: caselli@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics, Garching
Original Publication
1. Arzoumanian, D., S. Spezzano, T. Grassi, P.Caselli, Y. Tsukamoto, H. Fukihara, Y. Misugi, F. Alves, J. Pineda, S.Jensen, E. Redaelli, and A. Ivlev
Probing the ion-neutral drift velocity towards the L1544
prestellar core: Detection of ambipolar diffusion using N2D+ and para-NH2D
A & A
Source | DOI
2. Grassi, T. , J.E. Pineda, S. Spezzano, D. Arzoumanian, F. Lique, Y. Misugi, E. Redaelli, S. S. Jensen, P. Caselli
A differentiable and optimizable 3D model for interpretation of observed spectral data cubes
A & A
Source | DOI
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