Jets and magnetic outflows from forming massive stars

André Oliva1,3 & Rolf Kuiper2
1 Observatory of Geneva, University of Geneva, CH
2 Faculty for Physics, University of Duisburg-Essen, DE
3 Space Research Center (Cinespa), School of Physics, University of Costa Rica, CR

We performed detailed simulations of a disk-jet system surrounding a forming massive star, starting from the collapse of a cloud core. The high spatial resolution in the central region enabled us to study the processes involved in the launching, acceleration, propagation and termination of the magnetically driven outflows. We compared our results to new observations of water masers in the star forming region IRAS 21078+5211 (which trace individual streamlines of the jet), confirming that the high-speed protostellar outflow is launched as a magnetohydrodynamical disk wind.

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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.

This video shows the density, velocity and magnetic field of the highest-resolution run in the simulation series.

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Setup and Physics

The simulations start from the gravitational collapse of a cloud core threaded by an initially-uniform magnetic field and with a given density and rotation profiles (see figure). We built a catalog of 31 runs exploring different values of initial conditions (i.e., natal environments). We modeled the weakly ionized gas with magnetohydrodynamics (PLUTO code, Mignone et al. 2007), Ohmic dissipation (Machida et al. 2007), self-gravity (Haumea: Kuiper, Klahr et al. 2010), diffuse thermal radiation transport by the dust and gas (Makemake: Kuiper, Yorke & Mignone 2020) and stellar evolution (evolutionary tracks from Hoskawa & Omukai 2009). We used a time-independent grid in spherical coordinates assuming axial and midplane symmetry, which allowed us to reach high spatial resolution close to the protostar (up to 0.03 au).

Magneto-centrifugal jet

After the initial collapse, an accretion disk is formed due to angular momentum conservation. A high-speed ($\gtrsim 100 \,\mathrm{km\,s^{-1}}$) jet is launched by the magneto-centrifugal mechanism at $t\sim 5\,\mathrm{kyr}$. Incoming material from the disk reaches the jet cavity (see figure), where both the centrifugal force dominates over gravity and the flow becomes sub-Alfvénic (i.e., it is forced outwards following the mostly poloidal magnetic field lines). At distances of $\sim 1000\,\mathrm{au}$ away from the protostar, magnetic hoop stress re-collimates the fast outflow.

Magnetic tower flow

A slower ($\sim 10\,\mathrm{km\,s^{-1}}$) tower flow is observed at larger distances in the cloud core ($\gtrsim 10^4\,\mathrm{au}$). As the magnetic field is wound by rotation, it creates a vertical magnetic pressure gradient that overcomes gravity and drives the flow (see figure). Magnetic pressure is also responsible for the initial formation of the jet cavity, producing an initial bow shock that propagates outwards. Once the cavity is created, the magneto-centrifugal mechanism is able to start. A slower molecular outflow and a bow shock have been observed in IRAS 21078+5211 (Moscadelli et al. 2021).

Comparison of both kinds of protostellar outflows

The figure shows a schematic view of both processes. In the case of the magneto-centrifugal mechanism, the material in the cavity is sub-Alfvénic in the co-rotaing frame (low density, strong magnetic field), which means that the flow has to follow the magnetic field lines. In contrast, the material in the tower flow and the infalling envelope is super-Alvénic, that is, the magnetic field lines are dragged by the flow. In the magneto-centrifugal jet, material in the cavity experieces a larger centrifugal acceleration than the component of gravity in the cylindrical-radial direction (horizontal, in the illustration), which in absence of magnetic fields would mean that the material should move to a larger orbit. However, given that the flow is sub-Alfvénic, the magnetic field guides the flow towards the spherical-radial direction. The magnetic tower flow overcomes gravity using a completely different mechanism: rotation winds the magnetic field lines on top of the accretion disk, which creates a magnetic pressure gradient, eventually becoming stronger than gravity and pushing the material outwards.


See Moscadelli et al. 2022

Novel observations of water maser emission in the star-forming region IRAS 21078+5211 were performed using very long baseline interferometry (VLBI). With a resolution of 0.05 au, the spatial paths traced by the masers reveal individual streamlines of the jet emerging from the accretion disk. Both a kinematical analysis using the line-of-sight velocities and the comparison with the simulations indicate that the flow is originated by a magnetohydrodynamical disk wind.

See Moscadelli et al. 2023

The magnetic field strength and direction have been observationally obtained and compared against the simulations, finding 100-700 mG close to the protostar. Using a more detailed comparison of the streamlines from the simulation and the water maser observations, we were able to estimate the launching radius of the material.