Figure 1: Young disks observed with ALMA at a wavelength of 3 mm. These disks clearly display substructure and the early presence of companion stars. from: Maureira et al., 2025, A&A, 705, A96
Figure 2: Zoom simulations of proto-stellar disk formation inside a molecular cloud (large image). The colour scale indicates the the gas column densities. The insets show multiple zoom regions, in which several dense cores have formed. Six cores (labels and orange borders) were studied in more detail. © MPA
Figure 3: High resolution views of the resulting proto-stellar disks for various simulations of Core 1: the ‘control’ case without magnetic fields called ‘hydro’ (left), the absence of a disk with ‘ideal’ MHD (middle), and the disk forming in the most realistic ‘non-ideal’ MHD simulation (right), including ambipolar diffusion. © MPA
Sun-like stars form within turbulent molecular clouds, encircled by disks of gas and dust - the birthplaces of planets. While the earliest phases of the disk assembly process are obscured by the surrounding dense gas, ALMA can observe proto-stellar disks shortly after their formation. In a project supported by the Excellence Cluster ORIGINS, researchers from MPA, MPE, Harvard, and the University of Cologne performed high-resolution non-ideal magneto-hydrodynamical simulations that self-consistently follow proto-stellar disk formation from their parental turbulent molecular clouds down to stellar scales, spanning over 10 orders of magnitude. The study uncovers the complex paths by which disks assemble and demonstrates that magnetic fields play a central role in their formation and early evolution.
The interstellar medium (ISM), the site of star formation in galaxies, is a very complex environment. Diffuse hot regions (with temperatures of several million Kelvin) often exist in close proximity to cold, dense molecular clouds (with temperatures below a few hundred Kelvin). ‘Stellar feedback’, e.g. the explosion of massive stars as supernovae, creates the hot gas and drives turbulent gas motion in the ISM. This turbulence also causes cooling and can lead to gravitational collapse in certain regions, which form molecular clouds. Stars and their proto-stellar disks form in these molecular clouds from dense cores.
This process covers a large range of spatial scales: using the distance between earth and sun, an ‘astronomical unit’ or AU, as a ruler, the scales range from several 10 million AU for the size of molecular clouds, to a million AU large ‘bubbles’ created by supernovae, to regions smaller than a per cent of an AU for a newly forming proto-star. Specific numerical techniques are required to simulate such a system, as using equally high resolution everywhere would overwhelm even supercomputers. Most previous studies of disk formation simplify the problem and focus on the final disk formation phase after the collapse of dense cloud cores with uniform densities and turbulent velocities imposed by hand. This, however, misses the self-consistent formation of the cloud core structure, kinematics, and magnetic fields from its the large-scale environment.
How important are magnetic fields in this picture? It is well established observationally that clouds cores are strongly magnetized, which impacts their evolution. ‘Ideal’ magneto-hydrodynamical (MHD) models assume that magnetic fields are carried along with the gas. They back-react on the gas through the Lorentz force and provide support against gravitational collapse. The Lorentz force also works against the rotational twisting of magnetic field lines - a situation encountered where a rotating disk surrounds a young star. This resistance slows down gas so much that it falls onto the star, while fast-rotating material leaves the system in a proto-stellar wind - leaving no disks behind. However, extended proto-stellar disks are regularly observed around young stars (Figure 1) – inconsistent with ‘ideal’ MHD models.
This problem can be solved with more realistic ‘non-ideal’ MHD models where neutral and ionized particles move differently (ambipolar diffusion).
With this process, magnetic fields in collapsing cores are reduced and proto-stellar disks are able to form. Numerical simulations of this process are expensive but essential to understand proto-stellar disk formation.
In a project supported by the DFG Excellence Cluster ‘ORIGINS’ researchers from the Max Planck Institute for Astrophysics (MPA,), the Max Planck Institute for Extraterrestrial Physics (MPE,), Harvard, and the University of Cologne performed high-resolution non-ideal MHD ‘zoom’ simulations to self-consistently follow proto-stellar disk formation from their parent turbulent, multi-phase molecular clouds down to stellar sub-AU scales. The unprecedented ‘non-ideal’ MHD simulations span over 10 orders of magnitude in spatial scales.
In this setup with a realistic large-scale turbulent environment (Figure 2), no extended proto-stellar disks can form with ‘ideal’ MHD, while ‘non-ideal’ MHD allows for the early formation of a disk, similar to what is seen in the ‘hydro’ model without any magnetic field (Figure 3). However, the substructures of the disks formed in these different models are clearly distinct from each other. The study indicates that magnetic fields, along with non-ideal MHD effects, and the large-scale, multi-phase and turbulent environment play a central role for proto-stellar disk formation.
Ongoing work building on this study will focus on the evolution of these disks formed in realistic environments over a longer time-span. This will also allow the researchers to study how early stellar companions form.
The interstellar medium (ISM), the site of star formation in galaxies, is a very complex environment. Diffuse hot regions (with temperatures of several million Kelvin) often exist in close proximity to cold, dense molecular clouds (with temperatures below a few hundred Kelvin). ‘Stellar feedback’, e.g. the explosion of massive stars as supernovae, creates the hot gas and drives turbulent gas motion in the ISM. This turbulence also causes cooling and can lead to gravitational collapse in certain regions, which form molecular clouds. Stars and their proto-stellar disks form in these molecular clouds from dense cores.
This process covers a large range of spatial scales: using the distance between earth and sun, an ‘astronomical unit’ or AU, as a ruler, the scales range from several 10 million AU for the size of molecular clouds, to a million AU large ‘bubbles’ created by supernovae, to regions smaller than a per cent of an AU for a newly forming proto-star. Specific numerical techniques are required to simulate such a system, as using equally high resolution everywhere would overwhelm even supercomputers. Most previous studies of disk formation simplify the problem and focus on the final disk formation phase after the collapse of dense cloud cores with uniform densities and turbulent velocities imposed by hand. This, however, misses the self-consistent formation of the cloud core structure, kinematics, and magnetic fields from its the large-scale environment.
How important are magnetic fields in this picture? It is well established observationally that clouds cores are strongly magnetized, which impacts their evolution. ‘Ideal’ magneto-hydrodynamical (MHD) models assume that magnetic fields are carried along with the gas. They back-react on the gas through the Lorentz force and provide support against gravitational collapse. The Lorentz force also works against the rotational twisting of magnetic field lines - a situation encountered where a rotating disk surrounds a young star. This resistance slows down gas so much that it falls onto the star, while fast-rotating material leaves the system in a proto-stellar wind - leaving no disks behind. However, extended proto-stellar disks are regularly observed around young stars (Figure 1) – inconsistent with ‘ideal’ MHD models.
This problem can be solved with more realistic ‘non-ideal’ MHD models where neutral and ionized particles move differently (ambipolar diffusion).
With this process, magnetic fields in collapsing cores are reduced and proto-stellar disks are able to form. Numerical simulations of this process are expensive but essential to understand proto-stellar disk formation.
In a project supported by the DFG Excellence Cluster ‘ORIGINS’ researchers from the Max Planck Institute for Astrophysics (MPA,), the Max Planck Institute for Extraterrestrial Physics (MPE,), Harvard, and the University of Cologne performed high-resolution non-ideal MHD ‘zoom’ simulations to self-consistently follow proto-stellar disk formation from their parent turbulent, multi-phase molecular clouds down to stellar sub-AU scales. The unprecedented ‘non-ideal’ MHD simulations span over 10 orders of magnitude in spatial scales.
In this setup with a realistic large-scale turbulent environment (Figure 2), no extended proto-stellar disks can form with ‘ideal’ MHD, while ‘non-ideal’ MHD allows for the early formation of a disk, similar to what is seen in the ‘hydro’ model without any magnetic field (Figure 3). However, the substructures of the disks formed in these different models are clearly distinct from each other. The study indicates that magnetic fields, along with non-ideal MHD effects, and the large-scale, multi-phase and turbulent environment play a central role for proto-stellar disk formation.
Ongoing work building on this study will focus on the evolution of these disks formed in realistic environments over a longer time-span. This will also allow the researchers to study how early stellar companions form.
Authors:
Alexander Mayer
PhD student
Tel: 2042
amayer@mpa-garching.mpg.de
Thorsten Naab
Scientific Staff
tnaab@mpa-garching.mpg.de
Original publication
Mayer, Alexander C.; Naab, Thorsten; Caselli, Paola; et al.
Protostellar discs in their natural habitat ─ the formation of protostars and their accretion discs in the turbulent and magnetized interstellar medium
