There is no magnetic braking catastrophe: Low-mass star cluster and protostellar disc formation with non-ideal magnetohydrodynamics (1908.03241v1)

Abstract: We present results from the first radiation non-ideal magnetohydrodynamics (MHD) simulations of low-mass star cluster formation that resolve the fragmentation process down to the opacity limit. We model 50~M$\odot$ turbulent clouds initially threaded by a uniform magnetic field with strengths of 3, 5 10 and 20 times the critical mass-to-magnetic flux ratio, and at each strength, we model both an ideal and non-ideal (including Ohmic resistivity, ambipolar diffusion and the Hall effect) MHD cloud. Turbulence and magnetic fields shape the large-scale structure of the cloud, and similar structures form regardless of whether ideal or non-ideal MHD is employed. At high densities ($10^6 \lesssim n{\rm H} \lesssim 10^{11}$~cm$^{-3}$), all models have a similar magnetic field strength versus density relation, suggesting that the field strength in dense cores is independent of the large-scale environment. Albeit with limited statistics, we find no evidence for the dependence of the initial mass function on the initial magnetic field strength, however, the star formation rate decreases for models with increasing initial field strengths; the exception is the strongest field case where collapse occurs primarily along field lines. Protostellar discs with radii $\gtrsim 20$~au form in all models, suggesting that disc formation is dependent on the gas turbulence rather than on magnetic field strength. We find no evidence for the magnetic braking catastrophe, and find that magnetic fields do not hinder the formation of protostellar discs.

• The paper challenges the magnetic braking catastrophe by showing that turbulent environments enable robust protostellar disc formation.
• The paper employs radiation non-ideal MHD simulations to model low-mass star cluster evolution and resolve fragmentation processes with high accuracy.
• The paper finds that while stronger magnetic fields reduce star formation rates, they do not significantly alter the initial mass function, highlighting turbulence's dominant role.

Review of Star Cluster Formation in Non-Ideal MHD Context

This paper presents a detailed paper of star cluster formation using radiation non-ideal magnetohydrodynamics (MHD) simulations, shedding light on the intricacies of low-mass star and protostellar disc formation. The research leverages a series of simulations that model the evolution of a 50 M$_\odot$ turbulent molecular cloud, incorporating both ideal and non-ideal MHD effects. The investigation notably challenges the long-standing concept of the magnetic braking catastrophe by demonstrating the viability of protostellar disc formation under various initial magnetic field conditions.

Simulation Setup and Methodology

The authors conduct their paper by solving the equations underlying self-gravitating, radiation non-ideal MHD within simulated molecular cloud cores. The starting point is a cloud with a 50 M$_\odot$ mass filled with turbulent gas, threaded by magnetic fields with varying strengths between three and twenty times the critical mass-to-flux ratio. Both ideal and non-ideal scenarios are examined, with non-ideal MHD incorporating Ohmic resistivity, ambipolar diffusion, and the Hall effect. Key to their methodology is resolving fragmentation processes down to the opacity limit for realistic stellar casting.

Key Findings

1. Large-Scale Structure Evolution:
   • Magnetic fields significantly influence the macroscopic evolution of molecular clouds. Higher field strengths tend to smooth out structures, delaying the formation of high-density clumps due to enhanced magnetic support. Interestingly, collapse may happen along field lines in very strong magnetic fields, defying typical trends related to field strength.
2. Star Formation Rates and IMF:
   • The simulations reveal a reduction in star formation rates correlating with increased magnetic field strength, except at extreme field values where unique collapse dynamics are observed. The initial mass function (IMF) shows no strong dependency on the initial magnetic field strength, aligning with findings that other factors such as radiative feedback play a dominant role in shaping the IMF.
3. Protostellar Disc Formation:
   • In stark contrast to isolated core collapse scenarios, protostellar discs with radii greater than 20 au manifest robustly across all simulated environments, regardless of the initial magnetic field configurations. This directly contradicts the concept of a magnetic braking catastrophe, suggesting that disc formation is driven by turbulence instead of being inhibited by magnetic fields. Non-ideal MHD effects further stabilize disc formation by moderating magnetic field strengths in dense regions.
4. Multiplicity and Stellar Dynamics:
   • Binary and higher-order star systems manifest through capture processes, underscoring the dynamic nature of star cluster environments. Further computational resolution did not qualitatively change these findings, indicating robustness against numerical artifacts.

Implications and Future Directions

The paper's insights substantiate the assertion that turbulent motion within molecular clouds imposes a more significant influence on disc formation than previously acknowledged, challenging classical models of protostellar evolution. While non-ideal MHD processes demonstrate significant effects on small scales, particularly in fostering disc stability, more extensive parameter space exploration might be warranted to fully delineate environmental dependencies on star formation.

The results suggest reviewing the conceptual framework of magnetic braking in star formation simulations to incorporate dynamic environmental factors realistically. Future studies could leverage these findings to refine models of stellar evolution, potentially integrating them into broader galaxy formation and evolution simulations. Moreover, further exploration into the universality of magnetic influence—or lack thereof—in various cosmic scenarios remains a pertinent area for advancement.