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
Boy Lankhaar
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
P. Cazzoletti
Max Planck Society
Chiara Ceccobello
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Daria Dall` Olio
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
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
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 L7Subject Categories
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/201935459