Constraining MHD disk winds with ALMA. Apparent rotation signatures and application to HH212

Abstract

Large millimeter interferometers are revealing a growing number of rotating outflows, which are suggested to trace magneto-centrifugal disk winds (MHD DWs). However, their impact on disk accretion is not yet well quantified. Here we identify systematic biases in retrieving the true launch zone, magnetic lever arm, and angular momentum flux of an MHD DW from apparent rotation signatures. Synthetic position-velocity cuts are constructed from self-similar MHD DWs over a broad range of parameters, and three different methods are applied for estimating the specific angular momentum. We find that the launch radius inferred using the well-known relation from Anderson et al. (2006) can markedly differ from the true outermost launch radius $r_{out}$ of the DW. The "double-peak separation" and "flow width" methods provide only a strict lower limit to $r_{out}$. This bias is independent of angular resolution and can reach a factor ten. In contrast, the "rotation curve" method gives a good estimate of $r_{out}$ when the flow is well resolved, and an upper limit otherwise. The magnetic lever arm is always underestimated. Only comparison with synthetic predictions can take into account properly all observational effects. As an application, we present a comparison with ALMA observations of HH212 at resolutions from 250 au to 16 au, which represents the most stringent observational test of MHD DW to date. This comparison confirms our predicted biases for the double-peak separation method, and the large $r_{out}\sim 40$au and small magnetic lever arm first suggested by Tabone et al. (2017). We also derive the first accurate analytical expression for the fraction of disk angular momentum extracted by an MHD disk wind of given radial extent, magnetic lever arm, and mass flux. Application to HH212 confirms that MHD DWs are serious candidates for the steady angular momentum extraction process in young disks.

Accepted for publication in A&A

References in corpus (17)

• Which jet launching mechanism(s) in TTauri stars?
• The Gould's Belt Distances Survey (GOBELINS) II. Distances and structure towards the Orion Molecular Clouds
• Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics
• Resolved images of a protostellar outflow launched by an extended disk wind
• Disk-Driven Rotating Bipolar Outflow in Orion Source I
• ALMA discovery of a rotating SO/SO${}_2$ flow in HH212. A possible MHD disk wind?
• Formation and Atmosphere of Complex Organic Molecules of the HH 212 Protostellar Disk
• ALMA Results of the Pseudodisk, Rotating disk, and Jet in Continuum and HCO+ in the Protostellar System HH 212
• ALMA reveals a large structured disk and nested rotating outflows in DG Tau B
• Rotating molecular outflows: the young T Tauri star in CB26
• Jet Motion, Internal Working Surfaces, and Nested Shells in the Protostellar System HH 212
• Magnetically-driven jets and winds from weakly magnetized accretion disks
• Kinematics of the Outflow From The Young Star DG Tau B: Rotation in the vicinities of an optical jet
• Deuterated methanol on Solar System scale around the HH212 protostar
• A Search for Consistent Jet and Disk Rotation Signatures in RY Tau
• HST-Scale 3D simulations of MHD disc winds : A rotating two-component jet structure
• ALMA observations of the Th 28 protostellar disk - A new example of counter-rotation between disk and optical jet

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• Rotating Motion of the Outflow of IRAS 16293-2422 A1 at its Origin Point near the Protostar