EPoS Contribution
EPoS Contribution
Magnetic field and gas, a sticky couple: observations and models to quantify magnetic braking

Nacho Anez-Lopez
CEA, Paris, FR
The role of magnetic braking in regulating gravitational collapse and circumstellar disc during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disc sizes and the launching of high-velocity collimated jets, the work in our team aims at more direct tests of the magnetic braking in observations, and a characterisation of the efficiency of magnetic field coupling to the gas. The study of polarised dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process. In the present work we have used both synthetic observations from the radiative transfer of non-ideal MHD models of protostellar formation, and observations of the molecular line emission in the B335 protostar, to put constraints on the magnetically-regulated disc formation scenario. We find that the inner envelope of B335 is presenting high levels of ionised gas, which origin seems to boil down in local cosmic rays production at the protostar surface: these results suggest that the magnetic field may be highly coupled and very efficient at braking the inner envelope and e.g. set the disk size. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetised model has a higher angular momentum dissipation above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O (2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetised model similar to the characteristics found in B335. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from infall motions.
Caption: Dust continuum emission at 360GHz (colour scale) overlapped with magnetic field (LIC maps) inferred from linear polarisation. Contours show first moment map, derived from C18O (2-1) line for μ=3 and μ=10 models. Red contours range 0.04 to 0.08 by 0.02 times maximum velocity. Blue contours range fro -0.1 to -0.05 by -0.01 times maximum velocity.
Collaborators:
V. Cabedo, CEA, FR
P. Hennebelle, CEA, FR
A. Maury, CEA, FR
J.M. Girart, ICE, ES
U. Lebreuilly, CEA, FR
Suggested Session: Magnetic Fields