Collapse of turbulent massive cores with ambipolar diffusion and hybrid radiative transfer

Abstract

Context. Massive stars form in magnetized and turbulent environments and are often located in stellar clusters. The accretion and outflows mechanisms associated with forming massive stars and the origin of the stellar multiplicity of their system are poorly understood. Aims. We study the effect of magnetic fields and turbulence on the accretion mechanism of massive protostars and their multiplicity. We also focus on disk formation as a prerequisite for outflow launching. Methods. We present a series of four radiation-magnetohydrodynamical simulations of the collapse of a massive magnetized, turbulent core of 100 M⊙ with the adaptive-mesh-refinement code RAMSES, including a hybrid radiative transfer method for stellar irradiation and ambipolar diffusion. We varied the Mach and Alfvénic Mach numbers to probe sub- and super-Alfvénic turbulence and sub- and supersonic turbulence regimes. Results. Sub-Alfvénic turbulence leads to single stellar systems, and super-Alfvénic turbulence leads to binary formation from disk fragmentation following the collision of spiral arms, with mass ratios of 1.1–1.6 and a separation of several hundred AU that increases with initial turbulent support and with time. In these runs, infalling gas reaches the individual disks through a transient circumbinary structure. Magnetically regulated, thermally dominated (plasma beta β > 1) Keplerian disks form in all runs, with sizes 100–200 AU and masses 1–8 M⊙. The disks around primary and secondary sink particles have similar properties. We obtain mass accretion rates of ~10−4 M⊙ yr−1 onto the protostars and observe higher accretion rates onto the secondary stars than onto their primary star companion. The primary disk orientation is found to be set by the initial angular momentum carried by turbulence rather than by magnetic fields. Even without turbulence, axisymmetry and north–south symmetry with respect to the disk plane are broken by the interchange instability and thermally dominated streamers, respectively. Conclusions. Small (≲300 AU) massive protostellar disks such as those that are frequently observed today can so far only be reproduced in the presence of (moderate) magnetic fields with ambipolar diffusion, even in a turbulent medium. The interplay between magnetic fields and turbulence sets the multiplicity of stellar clusters. A plasma beta β > 1 is a good indicator for distinguishing streamers and individual disks from their surroundings.

Digging into the Interior of Hot Cores with ALMA (DIHCA). II. Exploring the Inner Binary (Multiple) System Embedded in G335 MM1 ALMA1

“…The observations presented here show this type of structure at smaller scales (<1000 au), which have been observed in disks of single systems (e.g., Maud et al 2019;Johnston et al 2020b). Simulations show that gravitational instabilities of the circumstellar disk can produce spirallike arms that can feed the protostar in episodic bursts of accretion (e.g., Meyer et al 2018;Mignon-Risse et al 2021;Riaz et al 2021). These simulations show overdensities with shapes similar to that of the continuum observations presented here and are predicted to be observable by ALMA at high resolution (Ahmadi et al 2019;Meyer et al 2019).…”

Discovery of a 500 au Protobinary in the Massive Prestellar Core G11.92-0.61 MM2

“…Riaz et al 2014, for ∼equal-mass low-mass binaries) and disk fragmentation (e.g. Mignon-Risse et al 2021, in which disk fragmentation is precipitated by the collision of extended spiral arms).…”

“…Other recent magnetohydronamic (e.g. Mignon-Risse et al 2021) and hydrodynamic (e.g. Oliva & Kuiper 2020) models of the collapse of massive cores indicate that binaries form via disk rather than core fragmentation, with disk spiral arms playing an important role.…”

“…Rosen et al 2016Rosen et al , 2019Rosen & Krumholz 2020;Meyer et al 2018) instead predict the formation of hierarchical systems with a single high-mass member. Notable recent exceptions are the super-Alfvénic cases of Mignon-Risse et al (2021), which form stable binary systems with mass ratios of ≈1.1-1.6 and separations of a few hundred au.…”

A Massive Star is Born: How Feedback from Stellar Winds, Radiation Pressure, and Collimated Outflows Limits Accretion onto Massive Stars

“…In contrast, the influence of magnetic fields reduces disk formation due to magnetic braking as shown in Figure 8, which shows thin density projections of the dense circumstellar material, as a function of primary stellar mass, that surround the primary star along its equatorial plane for runs ROFµ φ 2 (top row) and ROFWµ φ 2 (bottom row). Rosen & Krumholz (2020) showed that magnetic braking, which removes angular momentum from the infalling material as the core collapses inhibits the formation of a discernible accretion disk around the massive star, however higher resolution and non-ideal MHD effects such as ambipolar diffusion and Ohmic re-sistivity may reduce how much angular momentum is removed leading to smaller accretion disks than those produced when magnetic fields are not included (e.g., Zhao et al 2020;Mignon-Risse et al 2021;Commerçon et al 2021). Such effects are not explored in this work.…”