Volume 624, April 2019
|Number of page(s)||10|
|Section||Letters to the Editor|
|Published online||10 April 2019|
Letter to the Editor
Stringent limits on the magnetic field strength in the disc of TW Hya
ALMA observations of CN polarisation
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
2 Max-Planck-Insitut für Extraterrestrische Physik, Gießenbachstrasse 1, 85748, Garching, Germany
3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
4 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
5 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
6 Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching bei München, Germany
7 Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
Accepted: 26 March 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 |Bz| < 0.8 mG (1σ limit). We also provide a 1σ 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 efficient 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.
Key words: magnetic fields / accretion, accretion disks / stars: pre-main sequence / stars: individual: TW Hya
© ESO 2019
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