The formation of massive star multiples through disk fragmentation

André Oliva
Postdoc - Observatory of Geneva, Department of Astronomy, University of Geneva

We present the highest-resolution 3D radiation-gravito-hydrodynamical simulations to date of a fragmenting accretion disk in the context of massive star formation: from the gravitational collapse of a 200 $\mathrm{M_\odot}$ cloud, to the early stages of companion formation.

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Physics and grid

The simulations consider the following physical effects:

Initial conditions and overview

We start from the gravitational collapse of a $200\,\mathrm{M_\odot}$ cloud core with initial conditions as indicated in the figure. After 4 kyr, enough angular momentum is transferred to the center of the cloud to form an accretion disk with spiral arms; they in turn quickly start forming fragments. Fragmentation continues up to $\sim$15 kyr of evolution. We use a grid in spherical coordinates with the radial coordinate $r$ increasing logarithmically. This choice allows us to strictly conserve angular momentum around the central massive protostar and reach sub-au spatial resolutions in the midplane for $r < 100 \mathrm{\,au}$.


The fragments produced have masses of $\sim 1\mathrm{\,M_\odot}$ (they grow over time) and typical radii of a few up to 40 au. They interact gravitationally, migrate, merge, get sheared, develop secondary disks on their own and get accreted by the central massive protostar. The fragments are hydrostatically supported and form First Larson cores (see video). By monitoring their central temperature, we are able to determine when the fragments reach hydrogen-dissociation temperatures (2000 K) and continue their further evolution as second Larson cores.

By estimating how many fragments evolve further into second cores, we estimate a number of companions formed in our simulation. At the end of the simulated time, the simulation yields ~6 companions: 3 close (possible spectroscopic) companions, and 3 in the middle and outer disk (at distances of the order of 1000 au from the primary). The fragments that never reach the right central temperature and are accreted by the primary will produce accretion bursts.

Grids and codes: Pluto vs Ramses

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To test the robustness of our results, we performed a thorough convergence study (five different grid resolutions) using Pluto, and then we tried to replicate the results as closely as possible using the adaptive-mesh-refinement-based code Ramses. We found similar results overall, but also some differences in the early stages of the fragmentation epoch and limitations of each setup.

We also study the launching of magnetically-driven outflows with (2D) axisymmetric magnetohydrodynamical simulations, including the effects of self-gravity, radiation transport and Ohmic dissipation. We built a simulation catalog considering different cloud initial conditions.

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Two kinds of magnetic outflows are obtained: a magneto-centrifugally launched fast ($v \gtrsim 100 \mathrm{km/s}$) jet, and a magnetic-pressure-driven tower flow ($v\sim 10 \mathrm{km/s}$). A comparison of outflow streamlines from the simulation and streamlines traced by water maser observations of IRAS 21078+5211 (see figure) yielded a remarkable match, proving the magneto-centrifugal origin of early protostellar outflows.

Stellar rotation

We measure the magnetic lever arm of the jet to be $\sim 18$ and the ejection-to-accretion ratio as $\sim 0.01$ close to the protostar. A simple analysis with the formula by Matt & Pudritz (2005) reveals that the protostar should rotate below critical speed ($\Omega/\Omega_c \sim 0.5$) independently of the existence or not of an intrinsic stellar magnetic field.
(preliminary result)

Zoom into observations of the star-forming region IRAS 21078+5211, from the molecular cloud down to the launching region of the protostellar jet with this video.

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