Birth of magnetized low-mass protostars and circumstellar disks

¹ Université Paris Cité, Université Paris-Saclay, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France
² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
³ Univ Lyon, Ens de Lyon, Univ Lyon 1, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69007 Lyon, France

^⋆ Corresponding author.

Received: 2 January 2025
Accepted: 9 March 2025

Abstract

Context. Providing a comprehensive description of the birth of protostars and circumstellar disks, and how these two evolve over time, are among the goals of stellar formation theory. Although the two objects are often studied separately owing to numerical and observational challenges, breakthroughs in recent years have highlighted the need to study both objects in concert. The role of magnetic fields in this regard must also be investigated, and current observational surveys broadly report ∼kG field strengths in young stellar objects.

Aims. Our aim is to describe the birth of the protostar and of its circumstellar disk, as well as their early joint evolution following the second collapse. We wanted to study the structure of the nascent star-disk system, and that of its magnetic fields, while focusing on the innermost sub-AU region.

Methods. We carried out very high-resolution 3D radiative magnetohydrodynamics simulations (MHD), describing the collapse of turbulent dense cloud cores to stellar densities, both under the ideal and non-ideal approximation in which ambipolar diffusion is accounted for. The calculations were integrated as far as possible in time, reaching ≈2.3 yr after protostellar birth. Our simulations were also compared to their hydrodynamical counterparts to better isolate the role of magnetic fields.

Results. In line with previous results, we find that the ideal MHD run yields extremely efficient magnetic braking, which suppresses the formation of circumstellar disks and produces a central spherical protostar. In addition, this run predicts a magnetic field strength of ∼10⁵ G within the protostar at birth. In the non-ideal run, the efficiency of magnetic braking is drastically reduced by ambipolar diffusion and the nascent protostar reaches breakup velocity, thus forming a rotationally supported circumstellar disk. The diffusion of the magnetic field also allows the implantation of a ∼kG field in the protostar, which is thereafter maintained. The magnetic field is mainly toroidal in the star-disk system, although a notable vertical component threads it. No outflows or jets are reported owing to our use of turbulent initial conditions, which reduces the coherence of the magnetic field, although we report that conditions are being set in place for it to occur at later times. We also show that the nascent circumstellar disk is prone to the magneto-rotational instability, although our resolution is inadequate to capture the mechanism. We note a sensitivity of the nascent disk’s properties with regard to the angular momentum inherited prior to the dissociation of H₂ molecules, as well as the magnetic field strength, thus emphasizing the need for better constraints on dust resistivities throughout the collapse.

Conclusions. These calculations illustrate the role of magnetic fields in dictating the behavior of the gas throughout the collapse. They carry multiple implications on several issues in stellar formation theory, and offer perspectives for future modeling of the innermost regions of the star-disk system. Most notably, should the fossil field hypothesis used to explain the origins of magnetic fields in young stellar objects hold, we show that a ∼kG field strength may be implanted and maintained in the protostar at birth.