[Paper Review] Discs and outflows in the early phases of massive star formation: influence of magnetic fields and ambipolar diffusion

B. Commerçon, M. González+3 more|arXiv (Cornell University)|Sep 22, 2021

Astrophysics and Star Formation StudiesPhysics and Astronomy139 references47 citations

TL;DR

This study uses high-resolution 3D adaptive-mesh-refinement simulations of massive star formation to investigate the roles of magnetic fields and ambipolar diffusion (AD) in shaping accretion, ejection, and disc formation. It finds that magnetic processes dominate early evolution, with AD reversing disc magnetic topology: ideal MHD produces toroidal-dominated, magnetically pressurized discs (β < 1), while resistive MHD yields thermally pressurized discs with vertical fields in the inner regions (R < 100–200 au), fundamentally altering disc structure and stability.

ABSTRACT

We study mass accretion and ejection in the vicinity of massive star forming cores using high-resolution (5 au) 3D AMR numerical simulations. We investigate the mechanisms at the origin of outflows and characterise the properties of the disc forming around massive protostars. We include both protostellar radiative feedback via PMS evolutionary tracks and magnetic ambipolar diffusion. We studied 3 different cases: purely hydrodynamical, ideal MHD, and ambipolar diffusion. In the resistive models, we investigate the effects the initial amplitude of both magnetic field and rotation have on the properties of the massive protostellar system. We use simple criteria to identify the outflow and disc material and follow their evolution as the central star accretes mass up to 20 solar mass. The outflow is completely different when magnetic fields are introduced, so that magnetic processes are the main driver of the outflow up to stellar masses of ~20 solar mass. The disc properties depend on the physics included. The disc formed in the ideal and resistive runs show opposite properties in terms of plasma beta and of magnetic fields topology. While the disc in the ideal case is dominated by the magnetic pressure and the toroidal magnetic fields, the one formed in the resistive runs is dominated by the thermal pressure and has essentially vertical magnetic fields in the inner regions (R<200 au). We find that magnetic processes dominate the early evolution of massive protostellar systems (<20 solar mass) and shapes the accretion/ejection as well as the disc formation. Ambipolar diffusion is mainly at work at disc scales and regulates its properties. Our finding for the outflow and disc properties are reminiscent of low-mass star formation, suggesting that accretion and ejection in young massive and low-mass protostars are regulated by the same physical processes at the early stages.

Motivation & Objective

• To understand the influence of magnetic fields and ambipolar diffusion on mass accretion and ejection in early massive star formation.
• To resolve the debate on whether radiative or magnetic forces drive outflows in massive protostars.
• To characterize disc properties—mass, radius, magnetic topology—under varying physical conditions including ideal vs. resistive MHD.
• To test how initial magnetic field strength and solid-body rotation affect the final protostellar system.
• To provide testable predictions for high-resolution observations of discs and outflows in massive young stellar objects.

Proposed method

• Conducted 3D adaptive-mesh-refinement (AMR) simulations of massive dense core collapse using resistive grey radiation-MHD equations.
• Integrated protostellar evolution via pre-main sequence evolutionary tracks to model radiative feedback self-consistently.
• Implemented sink particles to track mass accretion and ejection over time up to 20 M⊙.
• Performed three main simulations: pure hydrodynamics, ideal MHD (perfect coupling), and resistive MHD (including ambipolar diffusion).
• Varied initial magnetic field amplitude and rotation rate to probe parameter space effects.
• Used physical criteria to identify and track disc and outflow material, measuring their radiative, magnetic, and hydrodynamic properties.

Experimental results

Research questions

• RQ1 What is the dominant mechanism driving outflows in massive protostars: radiative force or magnetic acceleration?
• RQ2 How do magnetic fields and ambipolar diffusion alter disc formation and structure in massive star-forming cores?
• RQ3 What is the impact of initial magnetic field strength and rotation on the final disc and outflow properties?
• RQ4 How does the plasma beta (β) and magnetic field topology differ between ideal and resistive MHD regimes in massive protostellar discs?
• RQ5 To what extent do the accretion and ejection mechanisms in massive protostars resemble those in low-mass protostars?

Key findings

• Magnetic fields dominate the early evolution of massive protostellar systems (M⋆ < 20 M⊙), with magneto-centrifugal processes being the primary driver of outflows.
• In the resistive MHD case, the inner disc (R < 100–200 au) is threaded by a vertical magnetic field and thermally pressurized (β > 1), contrasting sharply with the toroidal-dominated, magnetically pressurized (β < 1) discs in the ideal MHD case.
• The resistive MHD discs exhibit gravitational instability in the inner regions due to resistive effects, while outer regions remain magnetically coupled and stable.
• Disc mass and radius depend strongly on the inclusion of non-ideal MHD physics, with resistive models producing discs with distinct magnetic topology and thermal dominance.
• The outflow and disc properties in massive protostars are reminiscent of low-mass star formation, suggesting that accretion and ejection are regulated by the same physical processes across the mass spectrum at early stages.
• The flashlight effect—where outflows clear channels for radiation escape—remains valid when outflows are self-consistently launched in magnetized models.