ALMA observations and modeling of the rotating outflow in Orion Source I

TL;DR

This study uses ALMA observations and modeling to analyze the rotating outflow in Orion Source I, revealing velocity asymmetries, rotation, and expansion dynamics, and comparing these with theoretical models to understand outflow mechanisms.

Contribution

It provides detailed ALMA observations of the outflow, compares them with a shell interaction model, and highlights the need to consider disk wind angular momentum for explaining rotation.

Findings

Rotation velocity decreases with height from the disk.

Expansion and axial velocities increase with height.

Ulrich flow model underestimates observed rotation velocities.

Abstract

We present $^{29}$SiO(J=8--7) $\nu$=0, SiS (J=19--18) $\nu$=0, and $^{28}$SiO (J=8--7) $\nu$=1 molecular line archive observations made with the Atacama Large Millimeter/Submillimeter Array (ALMA) of the molecular outflow associated with Orion Source I. The observations show velocity asymmetries about the flow axis which are interpreted as outflow rotation. We find that the rotation velocity ($\sim$4--8 km s$^{-1}$) decreases with the vertical distance to the disk. In contrast, the cylindrical radius ($\sim$100--300 au), the expansion velocity ($\sim$2--15 km s$^{-1}$), and the axial velocity $v_{\rm z}$ ($\sim$-1--10 km s$^{-1}$) increase with the vertical distance. The mass estimated of the molecular outflow $\mathrm{M}_{\rm outflow}\sim$0.66--1.3 M$_\odot$. Given a kinematic time $\sim$130 yr, this implies a mass loss rate $\dot{\mathrm{M}}_{\rm outflow} \sim 5.1-10 \times 10^{-3}$ M$_\odot$ yr$^{-1}$. This massive outflow sets important contraints on disk wind models. We compare the observations with a model of a shell produced by the interaction between an anisotropic stellar wind and an Ulrich accretion flow that corresponds to a rotating molecular envelope in collapse. We find that the model cylindrical radii are consistent with the $^{29}$SiO(J=8--7) $\nu$=0 data. The expansion velocities and the axial velocities of the model are similar the observed values, except close to the disk ($z\sim\pm$150 au) for the expansion velocity. Nevertheless, the rotation velocities of the model are a factor $\sim$3--10 lower than the observed values. We conclude that the Ulrich flow alone cannot explain the rotation observed and other possibilities should be explored, like the inclusion of the angular momentum of a disk wind.