Stringent limits on the magnetic field strength in the disc of TW Hya: ALMA observations of CN polarisation
Journal article, 2019

Despite their importance in the star formation process, measurements of magnetic field strength in proto-planetary discs remain rare. While linear polarisation of dust and molecular lines can give insight into the magnetic field structure, only observations of the circular polarisation produced by Zeeman splitting provide a direct measurement of magnetic field strenghts. One of the most promising probes of magnetic field strengths is the paramagnetic radical CN. Here we present the first Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Zeeman splitting of CN in the disc of TW Hya. The observations indicate an excellent polarisation performance of ALMA, but fail to detect significant polarisation. An analysis of eight individual CN hyperfine components as well as a stacking analysis of the strongest (non-blended) hyperfine components yields the most stringent limits obtained so far on the magnetic field strength in a proto-planetary disc. We find that the vertical component of the magnetic field vertical bar B-z vertical bar < 0.8 mG (1 sigma limit). We also provide a 1 sigma toroidal field strength limit of < 30 mG. These limits rule out some of the earlier accretion disc models, but remain consistent with the most recent detailed models with e ffi cient advection. We detect marginal linear polarisation from the dust continuum, but the almost purely toroidal geometry of the polarisation vectors implies that his is due to radiatively aligned grains.

magnetic fields

stars: individual: TW Hya

accretion, accretion disks

stars: pre-main sequence

Author

Wouter Vlemmings

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

Boy Lankhaar

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

P. Cazzoletti

Max Planck Society

Chiara Ceccobello

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

Daria Dall` Olio

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

E. F. van Dishoeck

Max Planck Society

Leiden University

S. Facchini

European Southern Observatory Santiago

E. M. L. Humphreys

European Southern Observatory Santiago

Magnus V. Persson

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

L. Testi

European Southern Observatory Santiago

Arcetri Astrophysical Observatory

Ludwig Maximilian University of Munich (LMU)

J. P. Williams

University of Hawaii

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 624 L7

Subject Categories

Astronomy, Astrophysics and Cosmology

DOI

10.1051/0004-6361/201935459

More information

Latest update

4/5/2022 7